I wouldn't be surprised if the hardware filtering of 8-bit components operated at less than 32 bit precision, at least on some cards.

Also worth mentioning that the order of your floating point operations is probably being aggressively modified by your shader compiler in its attempts to optimize. Those last two bits of code you included are probably compiling into the same result.

On Fri, Oct 1, 2010 at 3:45 AM, Stefan Sandberg <keffo.sandberg@gmail.com> wrote:
Assuming you're after precision, what's wrong with doing it manually? :)
If performance is what you're after, and you're working on textures as they were intended(ie, game textures or video or something like that, not 'data'), you could separate contrast & color separately, keeping high contrast resolution, and downsampled color, and 
you'd save both bandwidth and instr.
If you simply want to know 'why', I'm guessing loss of precision in the tex units?
You've already ruled out shader precision from your own manual filtering, so doesn't leave much else, imo..
Other than manipulating the data you're working on, which is the only thing you -can- change I guess, I cant really see a solution,
but far greater minds linger here than mine, so hold on for what I assume will be a lengthy description of floating point math as 
it is implemented in modern gpu's :)



On Fri, Oct 1, 2010 at 9:57 AM, Andreas Brinck <andreas.brinck@gmail.com> wrote:
Hi,

I have a texture in which I use the R, G and B channel to store a
value in the [0, 1] range with very high precision. The value is
extracted like this in the (Cg) shader:

float
extractValue(float2 pos) {
   float4 temp = tex2D(buffer, pos);
   return (temp.x * 16711680.0 + temp.y * 65280.0 + temp.z * 255.0) *
(1.0 / 16777215.0);
}

I now want to sample this value with bilinear filtering but when I do
this I don't get a correct result. If I do the filtering manually like
this:

float
sampleValue(float2 pos) {
       float2 ipos = floor(pos);
       float2 fracs = pos - ipos;
       float d0 = extractValue(ipos);
       float d1 = extractValue(ipos + float2(1, 0));
       float d2 = extractValue(ipos + float2(0, 1));
       float d3 = extractValue(ipos + float2(1, 1));
       return lerp(lerp(d0, d1, fracs.x), lerp(d2, d3, fracs.x), fracs.y);
}

everything works as expected. The values in the buffer can be seen as
a linear combination of three constants:

value = (C0 * r + C1 * g + C2 * b)

If we use the built in texture filtering we should get the following
if we sample somewhere between two texels: {r0, g0, b0} and {r1, g1,
b1}. For simplicity we just look at filtering along one axis:

filtered value = lerp(r0, r1, t) * C0 + lerp(g0, g1, t) * C1 +
lerp(b0, b1, t) * C2;

Doing the filtering manually:

filtered value = lerp(r0 * C0 + b0 * C1 + g0 * C2, r1 * C0 + g1 * C1 +
b1 * C2, t)  =
                  = (r0 * C0 + b0 * C1 + g0 * C2) * (1 - t) + (r1 *
C0 + g1 * C1 + b1 * C2) * t =
                  = (r0 * C0) * (1 - t) + (r1 * C0) * t + ... =
                  = lerp(r0, r1, t) * C0 + ...

So in the world of non floating point numbers these two should be
equivalent right?

My theory is that the error is caused by an unfortunate order of
floating point operations. I've tried variations like:

(temp.x * (16711680.0 / 16777215.0) + temp.y * (65280.0/16777215.0) +
temp.z * (255.0/16777215.0))

and

(((temp.x * 256.0 + temp.y) * 256.0 + temp.z) * 255.0) * (1.0 / 16777215.0)

but all exhibit the same problem. What do you think; is it possible to
solve this problem?

Regards Andreas

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