OpenGL ES Tiled Texture Mipmap problem - iPad/iPhone

I'm running into the traditional tile/mipmap problem on the iPad using OpenGL ES. Basically, if you have a large texture (larger than 1k X 1k), you can break it up into pieces and map those pieces onto individual polygons. You can clamp the texture coordinates to the edges and it mostly works, but you get artifacts along the boundaries.

Now I know why you get this and know what the traditional solution is. To wit, you make a border around the outside of each littler texture (say 6 pixels). You sample from the little textures into the big one so you're just using those inside pixels (say 256-2*6). Then you smear the valid pixels out into the border area. Lastly, you map your texture coordinates to just use those valid inside pixels. Works okay.

If you're not nodding along at this point, don't try to answer. :-)

Anyway, OpenGL introduced clamping modes way back in the da开发者_如何转开发y to solve this. I don't see those modes in OpenGL ES (at least on this hardware) and I see other references to this problem. What I'm wonder is if I'm missing something. Is there a newer way to solve the tile/edge problem that I'm not aware of?

[Update] A screen shot of the result is attached here. The visible line is at the end of one texture and the start of another. This is using CLAMP_TO_EDGE.

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GLES supplies GL_CLAMP_TO_EDGE but not GL_CLAMP, which clamps to the centres of the outermost pixels in a texture rather than to the extreme edges. So out-of-bounds (border or wraparound) accesses are completely prevented with CLAMP_TO_EDGE but not with CLAMP.

CLAMP_TO_EDGE is a part of the GL ES specification (as per here for 1.1 and here for 2.0), so if your hardware doesn't support it then it's not technically GL ES compliant. It's also available in full Open GL, but I think only as of version 1.2. It's implied that CLAMP_TO_EDGE made the leap to ES but CLAMP didn't because the former is considered to be a fixed version of the latter.

It sounds to me like CLAMP_TO_EDGE should be suitable for what you're doing — have I misunderstood?

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In the end the problem was related to texture compression. The lines were due to the compression method assuming the texture wrapped around.

I solved the problem by building slightly larger textures than needed, compressing and then using only an area within each texture, thus leaving a border.