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Re: [Public WebGL] Weird texture problems.



On Tue, Jun 29, 2010 at 6:15 PM, Steve Baker <steve@sjbaker.org> wrote:
> I'm testing my WebGL app on some ancient hardware - probing to see where
> the compatibility envelope is - and I'm getting some really strange
> texture mapping errors on simple RGB textures on an nVidia GeForce GO
> 6400 with WinXP and Minefield.
>
> The texture is a 16x1024 texels .PNG - (ultimately, it's a lookup table
> for another calculation - but it goes wrong even when I don't use it
> that way).  Because this is really a lookup table, I'm only reading the
> ".rg" components of the map and only using one axis of the texture - so
> in the minimal test case, my shader is something like:
>
>   gl_FragColor = vec4 ( texture2D ( myMap, vec2(0.5, texCoord.y ) ).rg,
> 0, 1 ) ;
>
> If the '.b' component of the map is all zeroes, the colors come out
> perfectly...but if I put non-zero data in blue, the red and green go
> nuts...nothing like the data I put in (48,51,128)==>(22,65,128) and
> (16,64,0)==>(0,79,0) !   It doesn't seem to be an addressing issue
> because the colors I'm getting don't have the value of any of the texels
> in the original map...and it doesn't look like a MIPmapping issue either
> because the colors that I see are nowhere near the average of my texels.
>
> The program/shader/texture works great on all manner of other hardware,
> OS's, etc).
>
> Any ideas?   Is it possible that some kind of lossy compression might be
> happening under the hood?  I've seen this kind of thing with DXT
> compression before...but this is uncompressed .PNG.

Could undesired color space conversion be occurring?

Does the same thing happen in the Chromium continuous builds for Windows?

-Ken

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