I have stumbled over a strange thing affecting .p8.png files created by older versions of PICO-8 (file version 8 and lower). It appears that the memory image a modern version of PICO-8 (tested 0.2.4c, 0.2.5c) reads from those files differs from what it should be according to the encoding scheme described in https://pico-8.fandom.com/wiki/P8PNGFileFormat and implemented in picotool.
When I read that PNG and look at the pixel data, the bytes starting at 0xc400 look like this:
0c400 fc f0 e9 fc fc f0 ea fc
Decoding them according to the usual scheme gives
fc f0 e9 fc fc f0 ea fc
extract lowest 2 bits (& 3):
00 00 01 00 00 00 02 00
merge (alpha << 6 + red << 4 + green << 2 + blue) :
01 02
Since 0xc400 / 4 == 0x3100, these are the first bytes of the music section. So the first music pattern should have SFX 1 in the first channel and SFX 2 in the second channel. But when I open the file with PICO-8, I instead get SFX 2 in the first channel and SFX 1 in the second channel:
This is confirmed when I re-export the file using PICO-8, i.e. pico8 -export test.p8.png 13010.png. When I look at the pixels of the re-exported file at the same offset, I see
0c400 fc f0 ea fc fc f0 e9 fc
extract lowest 2 bits (& 3):
00 00 02 00 00 00 01 00
merge (alpha << 6 + red << 4 + green << 2 + blue) :
02 01
Those are not the only differences between the original and the re-exported version. There are also changes in the SFX section:
Hello Dan,
I have stumbled over a strange thing affecting
.p8.pngfiles created by older versions of PICO-8 (file version 8 and lower). It appears that the memory image a modern version of PICO-8 (tested 0.2.4c, 0.2.5c) reads from those files differs from what it should be according to the encoding scheme described in https://pico-8.fandom.com/wiki/P8PNGFileFormat and implemented in picotool.Example: https://www.lexaloffle.com/bbs/get_cart.php?cat=7&play_src=2&lid=13010
When I read that PNG and look at the pixel data, the bytes starting at 0xc400 look like this:
Decoding them according to the usual scheme gives
Since
0xc400 / 4 == 0x3100, these are the first bytes of the music section. So the first music pattern should have SFX 1 in the first channel and SFX 2 in the second channel. But when I open the file with PICO-8, I instead get SFX 2 in the first channel and SFX 1 in the second channel:This is confirmed when I re-export the file using PICO-8, i.e.
pico8 -export test.p8.png 13010.png. When I look at the pixels of the re-exported file at the same offset, I seeThose are not the only differences between the original and the re-exported version. There are also changes in the SFX section:
In all the carts I tested, this seems to affect only the SFX and music sections.
Do you have any idea what PICO-8 is doing here?
Have a nice day!