Allow RGBGFX to take an external palette specification

[ISSOtm]

Rationale: everybody's having headaches regarding the "indexed PNG" thing. It makes sense, it should work properly, but tooling for it is in practice abysmal.

Proposal: support input palettes like SuperFamiconv. Input formats:

• PNG (4 pixels/stripes)
• Bin data (quadruplet of either RGB888 or RGB555 colors)
• Text (CSS specs directly via CLI args)

Problem: how to make this work with CGB images?

[Hawkbat]

Would it also be possible to make an "inferred palette" with the first four RGB values encountered?

[ISSOtm]

That's already the case, with the inferred palette being sorted by brightness. But sometimes you want to have white in your sprites, for which this solution isn't satisfactory.

[Rangi42]

GBA has 16 palettes of 16 colors each, rather than 8 of 4. pokeruby/pokeemerald sometimes have grayscale images, but other images are colored entirely with the first palette in a set. For example, https://github.com/pret/pokeemerald/blob/master/graphics/frontier_pass/tiles.png. So CGB images could do the same: let the input palette file specify 4xN colors for N palettes, and expect the input image to be either grayscale or colored with the first 4. In either case, indexing the input image wouldn't be necessary. (Although if two of the colors in the first palette are identical, the image would have to be grayscale to distinguish them.)

[ISSOtm]

Suggested workflow after a lot of discussion: first, create a "default" palette—either by following the PLTE chunk if any, or by gathering all colors in the PNG otherwise. Then, if a palette is specified on the CLI, override.

Also useful: taking the spec both as a list of colors directly (--palettes 000,555,aaa,fff;012,345,678,9ab), or as a separate file (--palette @file)

Suggested: a --bgp shorthand? Maybe --dmg-pal instead, since it also works for OBJ palettes.

[Rangi42]

--dmg-pal as shorthand for --palettes fff,aaa,555,000 would be convenient.

[ISSOtm]

I meant --dmg-pal 0xE4, --dmg-pal 0x11100100. I considered accepting the RGBASM syntax directly, but $ is problematic on Linux, and % on Windows.

[Rangi42]

Base-4 like rgbasm's "Gameboy graphics" numeric format would also be convenient (it matches with how you'd input graphics directly, and the four values are obvious). Maybe --dmg-pal G3210 since backtick would need escaping.

[ISSOtm]

Or prefix-less?