Re: [Algorithms] Filtering
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Re: [Algorithms] Filtering
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From: Tibor K. <tib...@gm...> - 2010-10-01 20:42:26
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Just wanted to point something out - now that you know it won't work
it probably makes sense to switch to a float32 texture and still do the
filtering manually... you'll at least save the pack/unpack instructions :)
- Tibor
On 10/1/2010 9:51 PM, Andreas Brinck wrote:
> Hi Ola,
>
> unfortunately the floating point textures on the platform I'm working
> on don't support bilinear filtering (this is the reason why I stored
> the value in an RGB-texture to begin with).
>
> /Andreas
>
> P.S. Got your Ph.D yet?
>
> On Fri, Oct 1, 2010 at 8:12 PM, Ola Olsson<ola...@gm...> wrote:
>> While we're throwing ideas around, why not try using a float texture and see
>> if it produces the same error?
>>
>> .ola
>>
>> P.S.
>> Hi Andreas :)
>>
>> ----- Original Message -----
>> From: "Nathaniel Hoffman"<na...@io...>
>> To: "Game Development Algorithms"<gda...@li...>
>> Sent: Friday, October 01, 2010 7:59 PM
>> Subject: Re: [Algorithms] Filtering
>>
>>
>>> 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
>>>>
>>>> ------------------------------------------------------------------------------
>>>> Start uncovering the many advantages of virtual appliances
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>>>>
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>> ------------------------------------------------------------------------------
>>>>
>>>>
>>>> ------------------------------------------------------------------------------
>>>> Start uncovering the many advantages of virtual appliances
>>>> and start using them to simplify application deployment and
>>>> accelerate your shift to cloud computing.
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>>>>
>>>>
>>>> ------------------------------------------------------------------------------
>>>>
>>>>
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>>>> Start uncovering the many advantages of virtual appliances
>>>> and start using them to simplify application deployment and
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>>>> http://p.sf.net/sfu/novell-sfdev2dev_______________________________________________
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>>>
>>>
>>> ------------------------------------------------------------------------------
>>> Start uncovering the many advantages of virtual appliances
>>> and start using them to simplify application deployment and
>>> accelerate your shift to cloud computing.
>>> http://p.sf.net/sfu/novell-sfdev2dev
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>>
>> ------------------------------------------------------------------------------
>> Start uncovering the many advantages of virtual appliances
>> and start using them to simplify application deployment and
>> accelerate your shift to cloud computing.
>> http://p.sf.net/sfu/novell-sfdev2dev
>> _______________________________________________
>> GDAlgorithms-list mailing list
>> GDA...@li...
>> https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list
>> Archives:
>> http://sourceforge.net/mailarchive/forum.php?forum_name=gdalgorithms-list
>>
> ------------------------------------------------------------------------------
> Start uncovering the many advantages of virtual appliances
> and start using them to simplify application deployment and
> accelerate your shift to cloud computing.
> http://p.sf.net/sfu/novell-sfdev2dev
> _______________________________________________
> GDAlgorithms-list mailing list
> GDA...@li...
> https://lists.sourceforge.net/lists/listinfo/gdalgorithms-list
> Archives:
> http://sourceforge.net/mailarchive/forum.php?forum_name=gdalgorithms-list
>
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