Encoding floats to RGBA, again
Hey, it looks like the quest for encoding floats to RGBA textures (part 1, part 2) did not end yet.
Here’s the “best available” code that I have now:
inline float4 EncodeFloatRGBA( float v ) {
return frac( float4(1.0, 255.0, 65025.0, 160581375.0) * v ) + bias;
}
inline float DecodeFloatRGBA( float4 rgba ) {
return dot( rgba, float4(1.0, 1/255.0, 1/65025.0, 1/160581375.0) );
}
Before I thought that bias should be +0.5/255.0 normally, except it had to be around -0.55/255.0 on Radeon cards (older than Radeon HD series). Well, turns out I was wrong, the bias mostly has to be around -0.5/255.0.
Here’s the list (same bias on Windows/D3D9 and OS X/OpenGL, so it seems to be hardware dependent, and not something in API/drivers):
- Radeon 9500 to X850: -0.61/255
- Radeon X1300 to X1900: -0.66/255
- Radeon HD 2xxx/3xxx: -0.49/255
- GeForce FX, 6, 7, 8: -0.48/255
- Intel 915, 945, 965: -0.5/255
Those are the best bias values I could find. Still, every once in a while (rarely) encoding the value to RGBA texture and reading it back would produce something where one channel is half a bit off. Not a problem if you were encoding numbers were originally 0..1 range, but for example if you were encoding something that spans over whole range of the camera, then 0..1 range gets expanded into 0..FarPlane…
And all of a sudden there are huge precision errors, up to the point of being unusable. I just tried doing a quick’n'dirty depth of field and soft particles implementation using depth encoded this way… not good.
Oh well. Has anyone successfully used encoding of high precision number into RGBA channels before?
This looks like it’s related to the internal float representation of the card. I’m pulling numbers out of air, but I would guess that the early Radeons used FP16, x13-x19 used FP24, and HD series finally went to FP32. That would explain the large discrepancies between the early Radeons and the HD/GeForce/Intel cards.
Hmm, come to think of it, I don’t think you could get enough texture addressing with FP16, so scratch that theory.
Hi, I had a go at this problem in my own special way. I’d be very interested in your opinions and results with the following (my post):
http://www.gamedev.net/community/forums/topic.asp?topic_id=485186
It’s certainly less elegant than the frac encode!
@Pat: I think the difference is caused by the texture sampler differences. Even if GPUs do have floating point precision internally, regular 8 bit/channel textures are still sampled (and filtered!) at 8 bit precision, I guess for backwards compatibility reasons. Now, if there’s a very slight change in how the sampling and filtering works, then differences like we have here might surface. That’s my theory at least :)
@Alex: not sure, I haven’t actually tried that approach.