PVR Texture Compression Tiling (exposing edge context)

Question

I've got PVR texture compression working all happy and good in my iPhone game, but I've got issues when tiling multiple textures together. Basically, I've got a very large background which is split into multiple 512x512 tiles, all PVR compressed. Then they're drawn together to look like one big background image. The way PVR works, because it doesn't know that it's supposed to be compressing the texture as if it were a really big texture - i.e. use a neighbor's tiled information to determine how to perform the PVR compression.

I can think of maybe a couple ways to do this.

1) Somehow tell the texturetool command line program to accommodate for other images that will be adjacent.
2) Use the command line program to generate a huge PVR texture that represents the whole image, then somehow split up the bytes into multiple images - probably impossible.
3) Do some kind of OpenGL ES trickiness that blends the edges nicely.
4) Do some trickiness where I have redundant information in each tile and then clip those areas when the texture is drawn (please no).

Hopefully I can do 1, 2 or 3, or there is some other well known solution.

Answer 1

My best advice, but not what you asked, is to know when PVRTC isn't appropriate. By far the simplest solution is to just not use PVRTC for those tiles. I've spent a lot of time trying to bend PVRTC to work in situations it just isn't suited for.

That said,

When using PVRTC, the texture is always assumed to tile (with itself), thus pixels on the right edge affect the pixels on the left edge (same with top & bottom). So choices 1 or 2 likely won't work.

One possibility is to add an alpha channel to the tiles and allow them to fade out around the edges so that when you overlap them, they fade into each other. Keep in mind PVRTC tends to work better with gradual alpha fades. Hard alpha edges often have artifacts in PVRTC.

Answer 2

I ended up going with option 4. I don't think this was a situation where PVRTC isn't appropriate - in fact it's almost a necessity. When I've got a total of 24 512x512 textures in memory at once (representing a very large background and foreground), putting those in uncompressed is suicide. So I simply used PVR compression as normal, then my edited a few lines of code in my tiling algorithm so that they overlap and trim on 15 pixels on each end. Voila, looks great. Took a couple days and was pretty annoying, but I think this is a good option for people who need very large tiled backgrounds on the iPhone.