I ever compare saved PNG File with Photoshop and Grim (on linux). Their size are quite different, may because of compression level. So these different of file size will effect to game performance at run time? Thank you for your help.
Assuming you don't mind the palette being messed with:
- Load the image
- Convert it to RGB
- Convert back to indexed
Choose 16 for Max Colours (assuming you can afford to lose that many)
Tick "Enable Dithering of Transparency" if the image has any transparency
Then save the Image.
On the Png save options screen,
Untick everything (they reprosent the optional chunks), make sure compression is set to 9.
Note: you may get some odd results, for example, despite asking for only 16 colours, gimp will sometimes put the output file in 8bit rather than 4bit colour. Testing shows this is because oddly, in these few examples, a 8bit palette results in a smaller file than a 4bit palette.