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Re: [Public WebGL] NPOT (non-power-of-two) textures



On Sat, Jan 16, 2010 at 4:15 PM, Chris Marrin <cmarrin@apple.com> wrote:
>
> Ok, I have some new data. Seems that the demos that were falling because I had turned on mipmapping. I used generateMipmap and was using LINEAR_MIPMAP_LINEAR for the minification filter. That's what was causing his hardware to fall back to software. It is the fact that the X1600 hardware can do NPOT textures only when non-mipmapped is what makes it non-compliant with the RB_texture_non_power_of_two extension.
>
> So  that brings up a similar, but somewhat less serious issue. What should be done in cases like this, where the hardware is "almost" compliant, but not quite. Is the best approach:
>
> 1) Disallow this hardware from using WebGL (have it return a null pointer from getContext())?
>
> 2) Allow use of this hardware for WebGL, but disallow the use of mipmaps with NPOT textures. Have glError return an error when either calling texImage2D with a NPOT texture and LINEAR_MIPMAP_LINEAR is set, or when setting LINEAR_MIPMAP_LINEAR when a NPOT texture is set.

These semantics aren't workable. It's a basic expectation that the
developer is allowed to call texImage2D and texParameteri in any order
against a given texture object; they shouldn't be required to change
the default minification filter from LINEAR_MIPMAP_LINEAR before
uploading NPOT texture data.

> 3) Allow use of this hardware for WebGL, but replace LINEAR_MIPMAP_LINEAR with LINEAR when an NPOT texture is in use.

This seems too magical. See below.

> 4) Do nothing other that add some words in a non-normative spec section warning the author that some older hardware will fall back to software (or fail to work?) when using NPOT with mipmaps.
>
> (1) seems pretty harsh for such a minor issue. I would be fine with any of the other 3 choices.

The OpenGL ES spec is clear about what the behavior should be in this
case. Section 3.8.2 says that if a two-dimensional sampler is called
in a fragment shader for a non-power-of-two texture and either the
wrap mode is not CLAMP_TO_EDGE, or the minification filter is not
NEAREST or LINEAR, then the sampler returns the color (0, 0, 0, 1).

We can implement this behavior in WebGL running on top of desktop GL
by swapping out any NPOT textures that are not set up correctly with a
1x1 black texture when a draw call is made, and restoring them after
the draw call. Going forward, there seems to be some discussion of
convergence between desktop GL and GL ES, so I think we would be wise
to implement the GL ES semantics rather than invent new ones.

-Ken

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