Re: [Algorithms] Filtering
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Re: [Algorithms] Filtering
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From: Jeff R. <je...@8m...> - 2010-10-01 18:04:59
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I'm thinking maybe older gpu's did even courser filtering than this - 6-bit
or something instead of 8. Maybe this was the recent improvement?
On Fri, Oct 1, 2010 at 12:59 PM, Nathaniel Hoffman <na...@io...> wrote:
> Didn't the newer NVIDIA GPUs fix this?
>
> > You guessed right. The loss of precision is in the texture units.
> > Unfortunately, 8 bit components are filtered to 8 bit results (even
> though
> > they show up as floating point values in the shader). This is true for
> > nvidia gpus for sure and probably all other gpus.
> >
> > -mike
> > ----- Original Message -----
> > From: Stefan Sandberg
> > To: Game Development Algorithms
> > Sent: Friday, October 01, 2010 1:45 AM
> > Subject: Re: [Algorithms] Filtering
> >
> >
> > 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
> > <and...@gm...> 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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> >
> >
> >
> >
> >
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> >
> >
> >
> ------------------------------------------------------------------------------
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> >
> >
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> >
> >
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>
>
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> and start using them to simplify application deployment and
> accelerate your shift to cloud computing.
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--
Jeff Russell
Engineer, 8monkey Labs
www.8monkeylabs.com
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