Since shaders are more or less “assets”, having two different languages to deal with is not very nice. What, I’m supposed to write my shader twice just to support both (for example) D3D and iPad? You would think in 2010, almost a decade since high level realtime shader languages have appeared, this problem would be solved… but it isn’t!

In upcoming Unity 3.0, we’re going to have OpenGL ES 2.0 for mobile platforms, where GLSL ES is the only option to write shaders in. However, almost all other platforms (Windows, 360, PS3) need HLSL/Cg.

I tried a bit making Cg spit out GLSL code. In theory it can, and I read somewhere that id uses it for OpenGL backend for Rage… But I just couldn’t make it work. What’s possible for John apparently is not possible for mere mortals.

Then I looked at ATI’s HLSL2GLSL. That did produce GLSL shaders that were not absolutely horrible. So I started using it, and (surprise!) quickly ran into small issues here and there. Too bad development of the library stopped around 2006… on the plus side, it’s open source!

So I just forked it. Here it is: http://code.google.com/p/hlsl2glslfork/ (commit log here). There are no prebuilt binaries or source drops right now, just a Mercurial repository. BSD license. Patches welcome.

Note on the codebase: I don’t particularly like the codebase. It seems somewhat over-engineered code, that was probably taken from reference GLSL parser that 3DLabs once did, and adapted to parse HLSL and spit out GLSL. There are pieces of code that are unused, unfinished or duplicated. Judging from comments, some pieces of code have been in the hands of 3DLabs, ATI and NVIDIA (what good can come out of that?!). However, it works, and that’s the most important trait any code can have.

Note on the preprocessor: I bumped into some preprocessor issues that couldn’t be easily fixed without first understanding someone else’s ancient code and then changing it significantly. Fortunately, Ryan Gordon’s project, MojoShader, happens to have preprocessor that very closely emulates HLSL’s one (including various quirks). So I’m using that to preprocess any source before passing it down to HLSL2GLSL. Kudos to Ryan!

Side note on MojoShader: Ryan is also working on HLSL->GLSL cross compiler in MojoShader. I like that codebase much more; will certainly try it out once it’s somewhat ready.

You can never have enough notes: Google’s ANGLE project (running OpenGL ES 2.0 on top of Direct3D runtime+drivers) seems to be working on the opposite tool. For obvious reasons, they need to take GLSL ES shaders and produce D3D compatible shaders (HLSL or shader assembly/bytecode). The project seems to be moving fast; and if one day we’ll decide to default to GLSL as shader language in Unity, I’ll know where to look for a translator into HLSL :)

In a deferred rendering setup (see Game Angst for a good discussion of deferred shading & lighting), lights are applied using data from screen-space buffers. Position, normal and other things are reconstructed from buffers and lighting is computed “in screen space”.

Because each light is applied to a portion of the screen, the pixels it computes can belong to different objects. If in any place of lighting computation you use textures with mipmaps, be careful. Most common use for mipmapped light textures is light “cookies” (aka Gobo).

Let’s say we have a very simple scene with a spot light:

Light’s angular attenuation comes from a texture like this:

If the texture has mipmaps and you sample it using the “obvious” way (e.g. tex2Dproj), you can get something like this:

Black stuff around the sphere is no good! It’s not the infamous half-texel offset in D3D9, not a driver bug, not a shader compiler bug and not the nature trying to prevent you from writing a deferred renderer.

It’s the mipmapping.

Mipmaps of your cookie texture look like this (128×128, 16×16, 8×8, 4×4 shown):

Now, take two adjacent pixels, where one belongs to the edge of the sphere, and the other belongs to the background object (technically you take a 2×2 block of pixels, but just two are enough to illustrate the point). When the light is applied, cookie texture coordinates for those pixels are computed. It can happen that the coordinates are very different, especially when pixels “belong” to entirely different surfaces that are quite far away from each other.

What the GPU does when texture coordinates of adjacent pixels are very different? Chooses a lower mipmap level so that texel to pixel density roughly matches 1:1. On the edges of this “wrong” screenshot, it happens that very small mipmap level is sampled, which is either black or white color (see 4×4 mip level).

What to do here? You could disable mip-mapping (which is not good for performance and not good for image quality). You could drop some smallest mip levels which might be enough and not that bad for performance. Another option is to manually supply LOD level or derivatives to sampling instructions, using something else than cookie texture coordinates. For example, derivative in view space position, or something like that. This might not be possible on lower shader models though.

Well, finally I implemented it for upcoming Unity 2.6. I wrote some sort of a technical report here.

In short, I’m combining assembly fragments and doing simple temporary register allocation, which seems to work quite well. Performance is very similar to using fixed function (I know it’s implemented as vertex shaders internally by the runtime/driver) on several different cards I tried (Radeon HD 3xxx, GeForce 8xxx, Intel GMA 950).

What was unexpected: the most complex piece is not the vertex lighting! Most complexity is in how to route/generate texture coordinates and transform them. Huge combination explosion there.

Otherwise – I like! Here’s a link to the article again.

Back in 2002, I wrote something that would be roughly translated like “C++ amazes me more and more”. In a positive sense! And I was talking about what is Boost.Spirit now.

A reply on local game development forums I wrote today (again, rough translation): “C++ is very hard and quite a horrible language, maybe you should not use it unless there are no alternatives”.

That’s quite a change in attitude we have here!

I feel like much of C++ horrors are a consequence of “it just somehow happened” (the whole template metaprogramming thing) or as a backwards compatibility with C requirement. Or maybe not, but I do agree with what ryg says here. Let’s play the internet memes:

I don’t really know why that happens, because it seems that most modern cards don’t have fixed function units, so internally they are running shaders anyway. DX9 runtime on Vista’s WDDM also seems to be only handling shaders to the driver internally. Still, for some reason somewhere the precision does not match…

How such a task should be approached?

My requirements are:

• Should handle any possible state combination in D3D fixed function T&L.
• D3D 9.0c, using vertex shader 2.0 is ok. For now I don’t care about OpenGL.
• No HLSL at runtime. I don’t want to add a megabyte or more to Unity web player just for HLSL. DX9 shader assembly is ok, because we already have the assembler code.
• Should work as fast (or close to) as the regular fixed function pipeline.

I looked at ATI’s FixedFuncShader sample. It’s an ubershader approach; one large (230 instructions or so) shader with static VS2.0 branching. It had some obvious places to optimize, I could get it down to 190 or so instructions, kill some rcp‘s and reduce the amount of constant storage by 2x.

Still, it did not handle some things in the D3D T&L or had some issues:

• It assumes one input UV, one output UV and no texture matrices. This place in T&L gets quite convoluted – any input UVs or a texgen mode can be transformed by matrices of various sizes, and routed into any output UVs.
• It was not using full T&L lighting model. No biggie here.
• I haven’t checked with NVShaderPerf or AMD ShaderAnalyzer yet, but last time I checked the static branch instruction was taking two clocks on some NV architecture. So ubershader approach does not come for free.

Another thing I’m considering, is to combine final shader(s) from assembly fragments, with some simple register allocation.

In T&L shader code, there’s only limited set of could-be-redundant computations, mostly computing world space position, camera space normal, view vector and so on (those could be used lighting, texgen or fog). Those computations can be explicitly put into separate fragments, and later fragments could just use their result.

What is left then is some register allocation. A shader assembly fragment could want some temporary registers for internal use (this is simple, just give it a bunch of unused registers), also want some registers as input (from previous fragments), and save some output in registers.

Again, I haven’t checked with shader performance tools, but I think, guess and hope that the drivers do additional register allocation, liveness analysis etc. when converting D3D shader bytecode into hardware format. This would mean that I can be quite sloppy with it, i.e. don’t have to implement some super smart allocation scheme.

I wrote some experimental code for the shader assembly combiner and so far it looks like a reasonable approach (and not too hard either).

Does that make sense? Or did everyone solve those problems eons ago already?

Edit: half a year later, I wrote a technical report on how I implemented all this: http://aras-p.info/texts/VertexShaderTnL.html

void GUIView::MakeVistaDWMHappyDance()
{
    // Looks like Vista has some bug in DWM. Whenever we maximize or dock
    // a view, we must do something magic, otherwise
    // white stuff appears in place of the view.
    // See http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=4208117&SiteID=1

    bool earlierThanVista = systeminfo::GetOperatingSystemNumeric() < 600;
    if( earlierThanVista )
        return;

    // What seems to work is drawing one pixel via GDI.
    // We draw it at (1,1) with usual background color.
    int grayColor = 0.61f * 255.0f;
    PAINTSTRUCT ps;
    BeginPaint(m_View, &ps);
    SetPixel(ps.hdc, 1, 1, RGB(grayColor,grayColor,grayColor));
    EndPaint(m_View, &ps);
}

I know. Reading from screen when Aero is on is slow, bad and wrong. But then, what do you do? It’s better than users staring an all-white window just because Vista decided to draw it white, no matter what you think you’re drawing into it.

…still, MakeVistaDWMHappyDance is not nearly as cool as

internal interface ICanHazCustomMenu { … }

that Nicholas added a while ago.

for( int y = 0; y < height/2; ++y ) {
    memswap( img+y*width, img+(height-y-1)*width, width*img(arr[0]) );
}

memswap function was done this way:

// why isnt this in the std lib?
// using XOR to avoid tmp var
void memswap( void* m1, void* m2, size_t n )
{
    char *p = (char*)m1; char *q = (char*)m2;
    while ( n-- ) {
        *p ^= *q; *q ^= *p; *p ^= *q;
        p++; q++;
    }
}

The comment above the function was what triggered my interest. I just added:

// because it can be slower (local variable is likely in register;
// whereas using XOR involves reads/writes to memory)

But then I got interested in this, I just had to check what happens in one or another case.

Using Apple's gcc 4.0.1 on Core 2 Duo, the above memory swapping code takes about 12.5 clock cycles per swapped image pixel (pixel = 4 bytes). The inner loop is this:

movzx  eax,BYTE PTR [edx-0x1]
xor    al,BYTE PTR [ecx-0x1]
mov    BYTE PTR [edx-0x1],al
xor    al,BYTE PTR [ecx-0x1]
mov    BYTE PTR [ecx-0x1],al
xor    BYTE PTR [edx-0x1],al
dec    ebx
inc    edx
inc    ecx
cmp    ebx,0xffffffff
jne    loopstart

So the loop is three memory reads, three writes and some increments of the pointers / loop counter. Visual C++ 2008 compiles it very similarly, just uses more complex addressing mode to save one loop counter:

movzx       edx,byte ptr [ecx+eax]
xor         byte ptr [eax],dl
mov         dl,byte ptr [eax]
xor         byte ptr [ecx+eax],dl
mov         dl,byte ptr [ecx+eax]
xor         byte ptr [eax],dl
dec         esi
inc         eax
test        esi,esi
jne         loopstart

What if we don't do this "XOR trick", and just swap the contents using a temporary variable?

// ...
char t = *p; *p = *q; *q = t;
// ...

Lo and behold, now it runs at 7 cycles / pixel (almost twice as fast), and the inner loop is two memory reads and two writes:

movzx  edx,BYTE PTR [ebx-0x1]
movzx  eax,BYTE PTR [ecx-0x1]
mov    BYTE PTR [ebx-0x1],al
mov    BYTE PTR [ecx-0x1],dl
// ... incrementing pointers / counter here, like in previous case

So yeah. The XOR trick is pretty much useless here - it's twice as slow. Hey, it can even be slower as images get larger - if tested on a 2048x2048 image, regular swap still takes 7 cycles/pixel, but XOR trick takes 55 cycles/pixel!

I guess XOR trick is useful only in quite rare situations, for example when you're inside of some inner loop and want to swap register values without spilling them to memory or using an additional register. Heh, Wikipedia has info on this, so I'm not saying anything new :)

Now of course, if we happen to know that our pixels are 32 bits in size, there's no good reason to keep the loop in bytes. We can operate on integers instead:

void memswapI( void* m1, void* m2, size_t n )
{
    size_t nn = n/sizeof(int);
    int *p = (int*)m1; int *q = (int*)m2;
    while ( nn-- ) {
        int t = *p; *p = *q; *q = t;
        p++; q++;
    }
}

This runs at 1.5 cycles/pixel (XOR variant at 2.5 cycles/pixel). The assembly is pretty much the same, just with 32 bit registers.

Another option? If you use STL, just use:

std::swap_ranges(p, p+n, q);

on the pixel datatype. On 32 bit pixels, this also runs at 1.5 cycles/pixel.

So yeah. Don't try to outsmart the compiler without measuring it.

For the sake of the nation,
this operator must die!

Seriously. Suppose there is some class, let’s say ColorRGBAf. That has four floats inside. Now, someone at some point decided to add this operator to it:

operator float* () { /**/ }
operator const float* () const { /**/ }

Probably because it’s easier to pass color to OpenGL this way, or something like that.

This is evil. Like, really evil. Especially if that class did not have comparison operators defined, and some totally unrelated code four years later does:

if (color != oldColor) { /* … */ }

Ouch! Sounds like someone will spend four hours debugging something that looks like an event routing issue that only happens on Windows and only with optimizations on (yes, I just did that…).

What happens here? The compiler takes pointers to two colors and compares the pointers. If for some reason both colors are temporary objects, then it can even happen that both get folded into the same variable/register/whatnot. The pointers are the same. Ouch!

Implicit “nice” operators are just disguised evil. Remove that operator, add something like GetPointer() to class if someone really wants to use that, and better even make the comparison operators private and without implementations. Yes. Much better.

static void WatchdogFunc()
{
    while( true )
    {
        time_t now = time(NULL);
        Mutex::AutoLock lock(g_WatchdogMutex);
        if( now - g_StartTime > kWatchdogTimeout )
            ComplainLoudlyAndDoSomething();
        Thread::Sleep( 0.1f );
    }
}

Mutex is taken because g_StartTime can be occasionally updated by the same tool. Yes, possibly a mutex is an overkill here, and aligned variable + some memory fences should be enough (or just nothing), but hey, this is some random offline tool code.

What is horribly wrong with it?

Mutex is held locked for the whole duration of Sleep! That is, almost all the time; and other thread(s) barely have a chance to ever update g_StartTime.

And this is the code I’ve written. Oh stupid me.

Look again. And again. Eventually you’ll find where your code is broken.

(Of course, in some cases quite often the compiler is broken… GLSL, anyone?)

Pimp my code, part 15: The Greatest Bug of All says the above in a much nicer way:

Maybe the problem was there was some huge bug in Apple’s Mach, where if you open too many files in a short period of time, the filesystem tried to, like, cache the results, and the cache blew up, and as a result the filesystem incorrectly just would fail to open any more files, instead of flushing the cache.

…

I’ve also been around long enough to know that whenever I know the operating system must be bugged, since my code is correct, I should take a damn close look at my code. The old adage (not mine) is that 99% of the time operating system bugs are actually bugs in your program, and the other 1% of the time they are still bugs in your program, so look harder, dammit.

A post well worth reading… about the process of investigating tricky bugs. And sincere as well. It’s so good that I’ll just quote it again:

It’s a bug we should have caught. We should have spent the time to get the images in the 10,000 item file. I messed up.

Software is written by humans. Humans get tired. Humans become discouraged. They aren’t perfect beings. As developers, we want to pretend this isn’t so, that our software springs from our head whole and immaculate like the goddess Athena. Customers don’t want to hear us admit that we fail.

The measure of a man cannot be whether he ever makes mistakes, because he will make mistakes. It’s what he does in response to his mistakes. The same is true of companies.

We have to apologize, we have to fix the problem, and we have to learn from our mistakes.

So very true.

And herein lies the problem: most often precision of vertex transformations is not the same in fixed function versus programmable vertex pipelines. If you’d just draw some objects in multiple passes, mixing fixed function and programmable paths, this is roughly what you will get (excuse my programmer’s art):

Not pretty at all! This should have looked like this:

So what do we do to make it look like this? We “pull” (bias) some rendering passes slighly towards the camera, so there is no depth fighting.

Now, at the moment Unity editor runs only on the Macs, which use OpenGL. In there, most of hardware configurations do not need this depth bias at all – they are able to generate same results in fixed function and programmable pipelines. Only Intel cards do need the depth bias on Mac OS X (on Windows, AMD and Intel cards need depth bias). So people author their games using OpenGL, where it does not need depth bias in most cases.

How do you apply depth bias in OpenGL? Enable GL_POLYGON_OFFSET_FILL and set glPolygonOffset to something like -1, -1. This works.

How do you apply depth bias in Direct3D 9? Conceptually, you do the same. There are DEPTHBIAS and SLOPESCALEDEPTHBIAS render states that do just that. And so we did use them.

And people complained about funky results on Windows.

And I’d look at their projects, see that they are using something like 0.01 for camera’s near plane and 1000.0 for the far plane, and tell them something along the lines of “increase your near plane, stupid!” (well ok, without the “stupid” part). And I’d explain all the above about mixing fixed function and vertex shaders, and how we do depth bias in that case, and how on OpenGL it’s often not needed but on Direct3D it’s pretty much always needed. And yes, how sometimes that can produce “double lighting” artifacts on close or intersecting geometry, and how the only solution is to increase the near plane and/or avoid close or intersecting geometry.

Sometimes this helped! I was so convinced that their too-low-near-plane was always the culprit.

And then one day I decided to check. This is what I’ve got on Direct3D:

Ok, this scene is intentionally using a low near plane, but let me stress this again. This is what I’ve got:

Not good at all.

What happened? It happened in roughly this way:

1. First, depth bias documentation on Direct3D is wrong. Depth bias is not in 0..16 range, it is in 0..1 range which corresponds to entire range of depth buffer.
2. Back then, our code was always using 16 bit depth buffers, so the equivalent of -1,-1 depth bias in OpenGL was multiplied with something like 1.0/65535.0, and that was fed into Direct3D. Hey, it seemed to work!
3. Later on, the device setup code was modified to do proper format selection, so most often it ended up using 24 bit depth buffer. Of course no one I never modified the depth bias code to account for this change…
4. And it stayed there. And I kept deceiving myself that the content of the users is to blame, and not some stupid code of mine.

It’s good to check your assumptions once in a while.

So yeah, the proper multiplier for depth bias on Direct3D with 24 bit depth buffer should be not 1.0/65535.0, but something like 1.0/(2^24-1). Except that this value is really small, so something like 4.8e-7 should be used instead (see Lengyel’s GDC2007 talk). Oh, but for some reason it’s not really enough in practice, so something like 2.0*4.8e-7 should be used instead (tested so far on GeForce 8600, Radeon HD 3850, Radeon 9600, Intel 945, reference rasterizer). Oh, and the same value should be used even when a 16 bit depth buffer is used; using 1.0/65535.0 multiplier with 16 bit depth buffer produces way too large bias.

With proper bias values the image is good on Direct3D again. Yay for that (fix is coming in Unity 2.1 soon).

…and yes, I know that real men fudge projection matrix instead of using depth bias… someday maybe.

CAsyncMonikerFile is derived from CMonikerFile, which in turn is derived from COleStreamFile. A COleStreamFile object represents a stream of data; a CMonikerFile object uses an IMoniker to obtain the data, and a CAsyncMonikerFile object does so asynchronously.

So yeah, I am dealing with downloading something from the internet inside an ActiveX control that is written in MFC. A seemingly simple task – I give you an URL, you give me back the bytes. But no! That would not be a proper architecture, so instead it has asynchronous monikers which are based on monikers which are based on stream files which use some interfaces and whatnot. And for ActiveX controls the docs suggest using CDataPathProperty or CCachedDataPathProperty, which are abstractions build on top of the above crap. And I don’t even know what “a moniker” is!

Of course all this complexity fails spectacularly in some quite common situations. For example, try downloading something when the web server serves gzip compressed html output. Good luck trying to figure out why everything seemingly works, you are notified of downloading progress, but never get the actual downloaded bytes.

Turns out the solution is to change downloading behaviour of the above pile of abstractions to use “pull data” model, instead of default “push data” model. The default behaviour just seems to be broken (though it is not broken in that pile of abstractions, instead it is broken somewhere deeper in Windows code). Is this mentioned anywhere in the docs? Of course not!

This is pretty much how a code comment looks like for this:

We don’t use CCachedDataPathProperty because it’s awfully slow, doing data reallocations for each 1KB received. For 8MB file it’s 8000 reallocations and 32 GB (!) of data copied for no good reason!

While we’re at it, we don’t use CDataPathProperty either, because it’s a useless wrapper over CAsyncMonikerFile.

Oh, and we don’t use CAsyncMonikerFile either, because it has bugs in VS2003′ MFC where it never notifies the container that it is done with download, making IE still display “X items remaining” indefinitely. Some smart coder was converting information message and returning “out of memory” error if result was NULL, even if input message was NULL (which it often was). So we use our own “fixed” version of CAsyncMonikerFile instead.

Oh MFC, how we love thee.

Some N years ago I would have liked a title with “Architect” or “Analyst” or something like that. I would have called myself “developer” instead of “programmer” because hey, a developer thinks up things, whereas a programmer is a mere “code monkey”. More on code monkeys below.

But wait! Back then I also believed that knowing and using Design Patterns is essential for a programmer! In one place when I was interviewing new hires, design pattern knowledge was something I would look for… how stupid! Nowadays my view of patterns is more along the lines of “yeah, whatever”. I don’t exactly think of them as things from hell, but they could have caused more harm than good already.

Back to job titles. Code monkey is actually the key employee. A software product is largely defined by the code, heck, it is code. Sure, it also has the user interface, the fancy icons, the documentation, the website, the support, the roadmap and whatnot, but the code is the product, whereas everything else is more or less addons (possibly excluding UI… UI also defines the product).

Code design? Design patterns? Who cares about that.

It’s the final result that matters. Futurist programming for the win.

On the other hand, Memento Observer is probably very cool.

Fix FBX animation import crash once more. When exported symbols are not listed for a dylib, it seems to link back to calling executable (?!), making them share function impls with the same name. And because Keyframe is actually different in editor vs ImportFBX, this is wrong. Apparently this is OS X Leopard only, or something. Argh.

The code change in question was just telling the compiler “here’s the list of the functions that are exported from this dynamic library”. The list was already there, just the compiler was never told about existence of it.

The bug manifested itself as a crash when importing animations. But it would not happen when importer was run from a small unit test application. There were no memory corruptions happening, it was not running out of memory, yet the code was crashing with access violation, usually because STL’s vector was returning it’s wrong size (but the actual data of the vector was correct; it was just returning bogus size). And it was doing that only on OS X Leopard, and not on OS X Tiger. Huh?

Turns out what did happen – and I’m not sure if that’s a bug in OS X or a feature – is that the calling application did contain a class called Keyframe. And the shared library (where the crash was happening) also contained a class called Keyframe. But those classes were slightly different; first was 20 bytes in size, and second one was 16 bytes.

Now, somehow when the shared library was calling vector<Keyframe>::size(), the function from the calling application was used. I have no idea at all how or why this was happening, but it sure was! I could see from tracing the assembly code, that it was doing difference of two pointers, and then doing something that for sure was not division by 16.

What was the code doing? Turns out it was calculating division by 20 in a cunning way:

mov  edx,esi   # edx = end()
sub  edx,eax   # edx -= begin()
mov  eax,edx   # eax = edx
sar  eax,0x2   # eax >>= 2
imul eax,eax,0xcccccccd # eax *= 0xcccccccd

In other words, the compiler was replacing division by constant (as used in vector’s size()) by a shift and multiplication with a magic number. You can read more about the technique here or here.

But of course the code above only works if the number was actually divisible by 20; otherwise it returns totally wrong result. This is perfectly fine for computing the difference in two pointers to structures of known size… Except that inside the shared library the Keyframe structures are 16 bytes, and not 20!

So yeah. Watch out for peculiarities of dynamic linking on your platform.

So it’s a small very small 2D game without any next-gen bells and whistles. It can probably be done casually on the side, by allocating an hour here and there. We’ll see how it goes. Hey, I never actually done any game in Unity, I only make or break some underlying parts…

Of course, first I start with no game, just imported graphics. Hey look, I can do sprites!

Then cook up some base things: define the game grid, throw in some basic user interface on the right hand side, and make it actually do something. This wasn’t so hard; that already gets me an almost working level building functionality. It does not have fancy block building delay or block deletion yet; that will come later.

Next come basic physics. Danc’s design calls for simple arcade-like physics (things moving at constant speeds, bouncing off at equal angles, and so on), but in Unity I have a fully fledged physics engine just waiting to be used. Let’s use that.

The design has sloped ramp pieces, which are hard to approximate using any primitive colliders, so instead I’ll use convex mesh colliders for them. Now, on this machine I only have Blender, which I totally don’t know how to use; and I was too lazy to go to PC and use 3ds Max there. What a coder does? Of course, just type in the mesh file in ASCII FBX format. Excerpt:

; scaled 2x in Z, by 0.85 in Y
Vertices: -0.5,-0.425,-1.0, 0.5,-0.425,-1.0, -0.5,-0.425,1.0, 0.5,-0.425,1.0, -0.5,0.425,-1.0, -0.5,0.425,1.0
PolygonVertexIndex: 0,1,-3,2,1,-4,1,0,-5,2,3,-6,0,2,-5,2,5,-5,3,1,-5,5,3,-5

It’s a left ramp mesh! So much for fancy asset auto-importing functionality, when you don’t know how to use those 3D apps :)

After a while I’ve got peas being controlled by physics, colliding with level and so on. Physics is very bad for productivity, as I ended up just playing around with pea-stacks!

So far there’s no game yet… Next up: implement some AI for the peas, so they can wander around, climb the walls, fall down and bounce around. I guess that will be more work and less playing around… We’ll see.

For some reason though, I could not find a decent tool that would look at a Visual Studio compiled executable or a DLL, and report an overview of how large are the functions, classes, object files and whatnot. .kkrunchy executable packer does have a very nice size report, but it’s not exactly suitable for large executables…

Anyway, ryg of farbrausch fame was kind enough to donate the size reporting code, I did some modifications, and here it is: Sizer – executable symbol size reporting utility.

Enjoy. Oh, and the source code looks messy mostly because ryg and I use different indentation, and I never cared to format everything with a single style. Noone cares about the source code anyway, as long as it works. I’m not claiming that this code works, of course!

Here it is.

Here is how we do it at work. This took quite some time to set up, but I think it’s very worth it.

First you need hardware to test things on. For a start just a couple of graphics cards that you can swap in and out might do the trick. A larger problem is integrated graphics cards – it’s quite hard to swap them in and out, so we bit the bullet and bought a machine for each integrated card that we care about. The same machines are then used to test discrete cards (we have several shelves of those by now, going all the way back to… does ATI Rage, Matrox G45 or S3 ProSavage say anything to you?).

Then you make the unit tests (or perhaps these should be called the functional tests). Build a small scene for every possible thing that you can imagine. Some examples:

• Do all blend modes work?
• Do light cookies work?
• Does automatic texture coordinate generation and texture transforms work?
• Does rendering of particles work?
• Does glow image postprocessing effect work?
• Does mesh skinning work?
• Do shadows from point lights work?

This will result in a lot of tests, with each test hopefully testing a small, isolated feature. Make some setup that can load all defined tests in succession and take screenshots of the results. Make sure time always progresses at fixed rate (for the case where a test does not produce a constant image… like particle or animation tests), and take a screenshot of, for example, frame 5 for each test (so that some tests have some data to warm up… for example motion blur test).

By this time you have something that you can run and it spits out lots of screenshots. This is already very useful. Get a new graphics card, upgrade to new OS or install a new shiny driver? Run the tests, and obvious errors (if any) can be found just by quickly flipping through the shots. Same with the changes that are made in rendering related code – run the tests, see if anything became broken.

The testing process can be further automated. Here we have a small set of Perl scripts that can either produce a suite of test images for the current hardware, or run all the tests and compare the results with “known to be correct” suite of images. As graphics cards are different from each other, the “correct” results will be somewhat different (because of different capabilities, internal precision etc.). So we keep a set of test results for each graphics card.

Then these scripts can be run for various driver versions on every graphics card. They compare results for each test case, and for failed tests copy out the resulting screenshot, the correct screenshot, log the failures into a wiki-compatible format (to be posted on some internal wiki), etc.

I’ve heard that some folks even go a step further – fully automate the testing of all driver versions. Install one driver in silent mode, reboot the machine, after reboot runs another script that launches the tests and proceeds with the next driver version. I don’t know if that is only an urban legend or if someone actually does this^*, but that would be an interesting thing to try. The testing per card then would be: 1) install a card, 2) run the test script, 3) coffee break, happiness and profit!

^{* My impression is that at least with the big games it works the other way around – you don’t test with the hardware; instead the hardware guys test with your game. That’s how it looks for a clueless observer like me at least.}

So far this unit test suite was really helpful in a couple of ways: making of the just-announced Direct3D renderer and discovering new & exciting graphics card/driver workarounds that we have to do. Making of the suite did take a lot of time, but I’m happy with it!

Is it insane or the only sane thing to do? It’s insane amount of work, but it looks like they know what they’re doing. STL is broken in many ways, especially on memory limited systems… Now they could release it as open source with a decent license!

From time to time I hit some nasty debugging situation, and it always takes ages to figure out, and the path to the solution is always different. This is an example of such a debugging story.

While developing shadow mapping I implemented a “screen space shadows” thing (where cascaded shadow maps are gathered into a screen-space texture and shadow receiver rendering later uses only that texture). Then while being in the editor and maximizing/restoring the window a few times, everything locks up for 3 or 5 seconds, then resumes normally.

So there’s a problem: a complete freeze after editor window is being resized after a couple of times (not immediately!), but otherwise everything just works. Where is the bug? What caused it?

Since shadows were working fine before, and I never noticed such lock-ups – it must be the screen-space shadow gathering thing that I just implemented, right? (Fast-forward answer: no) So I try to figure out where the lock-up is happening. Profiling does not give any insights – the lock-up is not even in my process, instead “somewhere”. Hm… I insert lots of manual timing code around various code blocks (that deal with shadows). They say the lock-up most often happens when activating a new render texture (an OpenGL p-buffer), specifically, calling a glFlush(). But not always, sometimes it’s still somewhere else.

After some head-scratching, a session with OpenGL Driver Profiler reveals what is actually happening – video memory is leaked! Apparently Mac OS X “virtualizes” VRAM, and when it runs out, the OS will still happily create p-buffers and so on, it will just start swapping VRAM contents to AGP/PCIe area. This swapping causes the lock-up. Ok, so now I know what is happening, I just need to find out why.

I look at all the code that deals with render textures – it looks ok. And it would be pretty strange if a VRAM leak would be unnoticed for two years since Unity is out in the wild… So that must be the depth render textures that are causing a leak (since they are a new type for the shadows), right? (Answer: no)

I build a test case that allocates and deallocates a bunch of depth render textures each frame. No leaks… Huh.

I change my original code so that it gathers screen-space shadows onto the screen directly, instead of the screen-sized texture. No leaks… Hm… So it must be the depth render texture followed by screen-size render texture, that is causing the leaks, right? (Answer: no) Because when I have just the depth render texture, I have no leaks; and when I have no depth render texture, instead I gather shadows “from nothing” into a screen-size texture, I also have no leaks. So it must be the combination!

So far, the theory is that rendering into a depth texture followed by creation of screen-size texture will cause a video memory leak (Answer: no). It looks like it leaks the amount that should be taken by depth texture (I say “it looks” because in OpenGL you never know… it’s all abstracted to make my life easier, hurray!). Looks like a fine bug report, time to build a small repro application that is completely separate from Unity.

So I grab some p-buffer sample code from Apple’s developer site, change it to also use depth textures and rectangle textures, remove all unused cruft, code the expected bug pattern (render into depth texture followed by rectangle p-buffer creation) and… it does not leak. D’oh.

Ok, another attempt: I take the p-buffer related code out of Unity, build a small application with just that code, code the expected bug pattern and… it does not leak! Huh?

Now what?

I compare the OpenGL call traces of Unity-in-test-case (leaks) and Unity-code-in-a-separate-app (does not leak). Of course, the Unity case does a lot more; setting up various state, shaders, textures, rendering actual objects with actual shaders, filtering out redundant state changes and whatnot. So I try to bring in bits of stuff that Unity does into my test application.

After a while I made my test app leak video memory (now that’s an achievement)! Turns out the leak happens when doing this:

1. Create depth p-buffer
2. Draw to depth p-buffer
3. Copy it’s contents into a depth texture
4. Create a screen-sized p-buffer
5. Draw something into it using the depth texture
6. Release the depth texture and p-buffer
7. Release the screen-sized p-buffer

My initial test app was not doing step 5… Now, why the leaks happens? Is it a bug or something I am doing wrong? And more importantly: how to get rid of it?

My suspicion was that OpenGL context sharing was somehow to blame here (finally, a correct suspicion). We share OpenGL contexts, because, well, it’s the only sane thing to do – if you have a texture, mesh or shader somewhere, you really want to have it available both to the screen and when rendering into something else. The documentation on sharing of OpenGL contexts is extremely spartan, however. Like: “yeah, when they are shared, then the resources are shared” – great. Well, the actual text is like this (Apple’s QA1248):

All sharing is peer to peer and developers can assume that shared resources are reference counted and thus will be
maintained until explicitly released or when the last context sharing resources is itself released. It is helpful to think of this in the simplest terms possible and not to assume excess complication.

Ok, I am thinking of this in the simplest terms possible… and it leaks video memory! The docs do not have a single word on how the resources are reference counted and what happens when a context is deleted.

Anyway, armed with my suspicion of context sharing being The Bad Guy here, I tried random things in my small test app. Turns out that unbinding any active textures from a context before switching to new one got rid of the leak. It looks like objects are refcounted by contexts, and they are not actually deleted while they are bound in some context (that is what I expect to happen). However, when a context itself is deleted, it seems as if it does not decrease refcounts of these objects (that is definitely what I don’t expect to happen). I am not sure if that’s a bug, or just undocumented “feature”…

All happy, I bring in my changes to the full codebase (“unbind any active textures before switching to a new context!”)… and the leak is still there. Huh?

After some head-scratching and randomly experimenting with whatever, turns out that you have to unbind any active “things” before switching to a new context. Even leaving a vertex buffer object bound can make a depth texture memory be leaked when another context is destroyed. Funky, eh?

So that was some 4 days wasted on chasing the bug that started out as “mysterious 5 second lock-ups”, went through “screen-space shadows leak video memory”, then through “depth textures followed by screen-size textures leak video memory” and through “unbind textures before switching contexts” to “unbind everything before switching contexts”. Would I have guessed it would end up like this? Not at all. I am still not sure if that’s the intended behavior or a bug; it looks more like a bug to me.

The take-away for OpenGL developers: when using shared contexts, unbind active textures, VBOs, shader programs etc. before switching OpenGL contexts. Otherwise at least on Mac OS X you will hit video memory leaks.

It’s somewhat sad that I find myself fighting issues like that most of my development time – not actually implementing some cool new stuff, but making stuff actually work. Oh well, I guess that is the difference between making (tech)demos and an actual software product.

I used stb_image library from Sean Barrett – and it just rocks! Here’s the code to load a PNG image from file:

int width, height, bpp;
unsigned char* rgb = stbi_load( "myimage.png", &width, &height, &bpp, 3 );
// rgb is now three bytes per pixel, width*height size. Or NULL if load failed.
// Do something with it...
stbi_image_free( rgb );

That’s it! Basically a single line to load the image (and of course the library has similar functions to load from a block of memory, etc.). And the whole “library” is a single file – just add to your project and there it is. In comparison, loading a PNG file using de-facto libpng takes more than 100 lines of code (and some time to read the docs).

Small is beautiful.

…and the way we do graphics related unit/functional/compatibility testing deserves a separate article. Sometime in the future!

inline int SecondsToEnergy( float time )
{
  return FastFloorfToInt( time * (float)(1 << kEnergyFixedPoint) );
}

It’s used in the particle system, and converts particle lifetime to an internal fixed point representation (10 bits for fractional part, i.e. kEnergyFixedPoint=10).

Some of the emitted particles are okay on a Mac, but completely not visible on Windows. This function is to blame.

Of course, what’s wrong is the possible overflow in float-to-int conversion. Whenever someone tries to use lifetime longer than about 2097151, the conversion to signed 32 bit integer is undefined. It seems to clamp result in gcc and produce something like -1 in msvc.

Using multiple compilers can be hard, but it can also help in finding obscure bugs. Ha!

It sure feels nice to work on an actual software product. I think it’s probably the first time in my carreer that I know people are using my work and I do care about that. Having worked on projects before, it’s very different – a project just comes and goes, and once it’s finished you never think about it again. And most of the time you don’t care about “the clients” that much either. Working on a product is much more rewarding (especially if the users seem to like it).

Another interesting here is that we are a very small software shop. So everyone has to be a one-man-army (the others certainly are, not sure about myself). Design, program, fix bugs, decide on features, do support, write docs and even do html tweaks for the website. Of course, it could be Jack of all trades, master of none (*), but somehow I feel that we are managing pretty well. And I like to be involved in various aspects of making a product.

(*) though wikipedia says that the full saying is Jack of all trades, master of none, though ofttimes better than master of one – which looks like a positive thing to me.

A completely different theme: when programming, it’s always good to massage the code you’re working with a bit. Remove unneccessary #includes. Write a comment on tricky code block. Fix warnings. Do small refactorings. Remove unused code paths. It does not take much time and helps to keep the codebase clean. Removing unused code is especially good – for some reason I love removing code. Could do that all day long; probably I’m some kind of anti-programmer :)

Maybe it’s because shaders are such a short piece of code, without too complex dependencies… I’m sure anyone who knows graphics hardware will corect me here, but let’s oversimplify and pretend that shaders actually execute in a simple way… So when you make a shader shorter, you pretty much know it’s going to be faster. When you make it “look better”, it almost certainly will look better. Try doing that in your regular big codebase – by optimizing something you may break something else; and in general you have no clue what to optimize unless you do your profiling homework. So, my take is that shaders are much simpler, so the joys of looking at assembly output actually make sense.

So, yeah, I’m back to some shader programming.

Yeah, I split this into separate files, removed this and made these classes to make it actually work.
Ok, but don’t go too fancy with objects here.
Sure! I think it’s the only place where I actually use inheritance!

Heh. I’d imagine how that would have looked back some 5 years ago. “What design patterns did you use here?” etc. Funny how things change.

I think I’ve got it by now – took me way too much time for such a trivial thing – there is no silver bullet. OOP or any other buzzword is just a means to do something; sometimes it fits, sometimes it does not. Regarding OOP, I highly recommend Execution in the Kingdom of Nouns essay – it’s way exaggerated, but has the point. The best part:

advocating Object-Oriented Programming is like advocating Pants-Oriented Clothing

There is one thing about the codebase that we have at work that I love: it does not use any particular design/programming technique. A bit of OO, a bit of metaprogramming, a bit of plain C style, a bit of preprocessor macros, etc. I like to think that we’re using the best of those worlds, of course :)

The topic I and Joe tried it on was a pretty hard one – related to the core animation system. Now, of course I don’t know anything about animation systems, but my impression is that there is just no “universal” way of designing it. The ones that are floating around inside free/open engines/libraries (cal3d, nebula2) are quite fine, but not much more impressive than my own very simplistic attempts at doing animations. There is nothing wrong with that of course – its simple, it gets the job done, and its okay in most of the cases when you’re doing simple stuff.

But then, if you want something more advanced, you either have to go and get the big serious libraries, or just… well… not do it.

So, back to pair programming – making the core animation system that would have transitions, continuous blends, animation layers, bone masks and whatnot (and the kitchen sink of course!) is just not very easy. We paired basically on writing the pseudocode of the system, or some sort of outline; changed the implementation several times along the way, and in the end we’re left with a really nice and fast system and the code is actually quite simple. Much of the credit for that goes to Joe, as he found out some really cool ways to optimize the expensive things away (at that time we were not doing pair programming anymore – I went to do some research on shadows!)

But to reiterate – pairing can definitely work. I guess mostly because the other person just keeps asking “why you’re doing this?” or “this is wrong” or “we’re in a deep shit now” or “that’s awesome” :) ]]> http://aras-p.info/blog/2006/02/15/pair-programming-animations/feed/ 5 http://aras-p.info/blog/2005/12/23/64k-coding-continued/ http://aras-p.info/blog/2005/12/23/64k-coding-continued/#comments Fri, 23 Dec 2005 16:58:00 +0000 Aras Pranckevičius http://aras-p.info/blog/?p=83 I’m making a steady, but very slow progress on “my” 64k intro. Over the last week I couldn’t get over 13 kilobytes, so you can see that the progress is really slow. Not because I don’t code anything, but all code increase was cancelled by data size optimizations.

So far coding and data design for small sizes is not that much pain at all. Just, well, code and, well, keep your data small :) We’re only talking about the size of initial data, not the runtime size though.

A few obvious or new notes:

• Code to construct a cylinder is more complex than the one to construct a sphere. That’s what I expected. However, code to construct a box with multiple segments per side is the most complex of all!
• Dropping last byte from floats is usually okay. And instant 25% save! For some of the numbers, I plan to switch to half-style float (2 bytes) if space becomes a concern.
• Storing quaternions in 4 bytes (byte per component) is good. Actually, now that I think of it, it makes more sense to store three components at 10 bits each, and just store the sign of 4th component – better precision for the same size.
• This intro literally has the most complex and most automated “art pipeline” of any demo/game I (directly) worked on! I’ve got maxscripts generating C++ sources, custom commandline tools preprocessing C++ sources (mostly floats packing – due to lack of maxscript functionality), lua scripts for batch-compiling HLSL shaders, “development code” generating .obj models for import back into max, etc. It’s wacky, weird and cool!
• Compiling HLSL in two steps (HLSL->asm and asm->bytecode) instead of direct (HLSL->bytecode) gets rid of the constant table, some copyright strings and hence is good. (thanks blackpawn!)
• Getting FFD code to behave remotely similar to how 3dsmax does FFD is hard :)

Nothing helped.

Then I noticed that in the pixel shader, I wrote

sample = tex2D( s0, uv + vSmpOffsets[i] )

instead of

sample += tex2D( s0, uv + vSmpOffsets[i] )

Aaargh. So much for a plus sign.

How to deal with such bugs? Why some bugs are trivial to find, and some are hard? Why sometimes (often?) the time required to find the bug does not correlate with bug’s “trickiness”? Why sometimes I can find a tricky bug in big unknown codebase in a couple of minutes; yet spend two hours on the plus sign in my own small code?

I’ve got no answers to the above.

By the way: PIX is a great tool, but D3D guys should really polish the UI :) ]]> http://aras-p.info/blog/2005/10/26/debugging-plus/feed/ 4 http://aras-p.info/blog/2005/09/19/c-clunkiness/ http://aras-p.info/blog/2005/09/19/c-clunkiness/#comments Mon, 19 Sep 2005 17:42:00 +0000 Aras Pranckevičius http://aras-p.info/blog/?p=66 Here I am coding various things and it suddenly struck me: C++ is pretty clunky. Now, I knew this to some extent for quite a time already, but the more I code C++ the more clunky it feels.

I admire it as a low-level language; it’s very powerful and there’s lots of unbelievable things you can do with it (think templates).

But still, it feels like a low-level one. I really want to code by “next project” (whatever that might be) in Lua, for example (especially now that LuaJIT is out). ]]> http://aras-p.info/blog/2005/09/19/c-clunkiness/feed/ 2 http://aras-p.info/blog/2005/07/08/immediate-mode-gui/ http://aras-p.info/blog/2005/07/08/immediate-mode-gui/#comments Fri, 08 Jul 2005 06:59:00 +0000 Aras Pranckevičius http://aras-p.info/blog/?p=54 It’s already advertised somewhere else, but I’ll add it as well: Casey Muratori has an amazing lecture on Immediate Mode GUI. I like the idea, and while I think IMGUI mostly applies to realtime UIs (in-game, editors etc.), this is a good read (er… watch) for any UI programmer.

I was subconsciously heading towards that UI style as well; drawing and processing some UI “immediately”, though not at the level Casey does.