Unfortunately the exact problem is still not known, and the guidelines seem very vague to me.
Do the color indices of the palette need to match the exact RGB of the game's palette ? AFAIK, no, because sprites are loaded before the game even knows which palette will be used to display them.
The index of pixels expected to be transparent has to be zero, OK, but does this color also HAVE to be transparent as per the PNG specification ? It's the opposite of what's required for GIFs, which must
not have transparent color zero
- Using a "tRNS" chunk ? Does it HAVE to be a 256-byte chunk with 1 time "0" and 255 times "255" ? Or can it be shortened to 1 x "0", which has the same meaning ?
- What if there's an alpha channel which gives an alpha of zero to all 0-index pixels ? Does it more harm than good ? It's not supposed to be PNG standard (color type 5 in Image Header,
forbidden), but some painting program make them, and they actually render OK in web browsers.