Are there any open source sprite format?

PNG just plainly sucks. I has no support for animations, timing or delta compression and sequence names. Also, PNG is slow at random access (you have to decompress image before use). Proprietary formats (like Diablo 2 DCC) are much better, but highly specialized for their games: some have no true color, other have no directions.

>>71
Only when the back buffer itself was 8-bit paletted, and you had to use a single palette for the entire scene.

Someone else mentioned using a 32-bit RGBA back buffer with 8-bit paletted sprites being faster earlier in the thread, perhaps it was you.

You don't have hardware support for that, you have to manually lookup each pixels color in the palette when rendering and that incurs an extra indirection and instruction dependencies. Even if it's all in the L1 cache, it can still be 4-5 extra cycles of latency for the indirection per pixel, and even slower than that if you can't adequately pad the instruction pipeline to spread out instruction dependencies.

What used to be the case is no longer true. You're living in the past.