xo-chip-colour-proposal.md

This is still very much a work in progress. So far it's just a place to document some ideas, thoughts and findings.

XO-CHIP colour proposal

John Earnest mentions in his proposal for XO-CHIP that a theoretical sixteen colours are possible with the plane opcode:

Encoding is chosen such that it would be possible to provide 4 bitplanes (and thus 16 colors!) in the future should it prove necessary.

However, Octo doesn't implement this theoretical 16-colour feature, and it seems to have been left as an exercise to the reader. This document proposes a concrete specification for the possible colours in XO-CHIP's two additional planes.

Constraints

Memory

Octo has chosen to make the four colours that it does have configurable. Considering the program size contraints of 3.5K / 32K this makes sense; adding the other two bits of colour doubles the size of all image data in your program. But still, programmatically having more colours opens up possibilities that four configurable colours just don't provide, like showing a colourful Mandelbrot set, using colour to indicate different players or enemies, et cetera.

Also, it would be possible to still use two planes, but two different planes for some sprites, effectively giving those sprites access to a different colour palette without sacrificing storage space. This means that the colour palette that we choose should preferably be "useful" when using one, two, three or four bits of colour, in any plane permutation.

Things I consider "useful" would for example be:

• Being able to switch palettes to re-use the same sprite for different purposes (the red player and the blue player, the green goblin and the brown goblin)
• Having more vivid colours in another plane than more dull colours. Combined with screen scolling, this allows for a brighter "foreground" and a more subdued "background" that can scroll independently
• The same goes for having a game world with a palette in "earthy" colours with more vivid sprites for entities (players, enemies, pickups)

Colour compatibility

I thought it valuable for developers implementing this proposal to keep the least significant plane compatible with standard CHIP-8, which means that in plane 1, 0 indicates a non-active pixel (dark) and 1 an active pixel (light).

Compatibility with Octo's implementation of XO-CHIP is kind of impossible, because due to its configurability different values in plane 3 can correspond to any colour. However, we can take a look at the default colours Octo uses:

Bits	Colour
0b00	#996600
0b01	#FFCC00
0b10	#FF6600
0b11	#662200

As you can see these are fairly warm colours on a spectrum from lighter to darker. Essentially giving the user four levels of "grayscale", but in a much less boring palette. There is one issue with this palette though, and that is that 0b11 is a darker colour than 0b00, even though we just concluded that for the CHIP-8 "palette" 0b0 needs to be a non-active pixel. So this colour would have to be darker than a non-active pixel..? 😄

So trying to get Octo-compatibility out of the box, even in the more liberal sense of using four shades of gray to represent the Octo palette, doesn't really seem to work very well. Combined with Octo's configurability it would seem like a waste of time to worry about this too much. Interpreter developers wanting to support a version of XO-Chip that's really compatible with Octo will have to make the two-bit palette configurable anyway.

Candidate palettes

There have been plenty of 16-colour palettes on old machines. So let's take a look at what those who came before us have chosen to do.

(I guess I should have done this before I came up with my own Silicon8 v1.0 palette. Oh well 😉)

Microsoft Windows and IBM OS/2 default 16-color palette

Microsoft seems to have had a very mathematical approach to dividing up the colour space. Every combination of the extremes (0x00 and 0xFF) is represented, with the in-between colours using 0x80 (128), which is pretty much exactly in the middle. It lays the colours out in such a way that bit 3 becomes a "brightness toggle", which is nice I guess.

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#800000	Maroon
2	0b0010	#008000	Green
3	0b0011	#808000	Olive
4	0b0100	#000080	Navy
5	0b0101	#800080	Purple
6	0b0110	#008080	Teal
7	0b0111	#C0C0C0	Silver
8	0b1000	#808080	Gray
9	0b1001	#FF0000	Red
10	0b1010	#00FF00	Lime
11	0b1011	#FFFF00	Yellow
12	0b1100	#0000FF	Blue
13	0b1101	#FF00FF	Fuchsia
14	0b1110	#00FFFF	Aqua
15	0b1111	#FFFFFF	White

Apple Macintosh default 16-color palette

Of course, Apple being Apple, this palette is much more pleasing on the eyes. The colours are laid out in an order that feels like it makes sense, a bit like a rainbow. And they aren't quite so extreme, so the colours seem to "scream" at you a lot less.

#	Binary #	HEX colour	Colour
0	0b0000	#FFFFFF	White
1	0b0001	#fbf305	Yellow
2	0b0010	#ff6403	Orange
3	0b0011	#dd0907	Red
4	0b0100	#f20884	Magenta
5	0b0101	#4700a5	Purple
6	0b0110	#0000d3	Blue
7	0b0111	#02abea	Cyan
8	0b1000	#1fb714	Green
9	0b1001	#006412	Dark Green
10	0b1010	#562c05	Brown
11	0b1011	#90713a	Tan
12	0b1100	#C0C0C0	Light gray
13	0b1101	#808080	Medium gray
14	0b1110	#404040	Dark gray
15	0b1111	#000000	Black

RISC OS default palette

The designers of this palette clearly thought that having lots of different levels of gray was important. Which is very nice for some purposes, like good looking fonts, but probably not so great for others, like flashy games.

#	Binary #	HEX colour	Colour
0	0b0000	#FFFFFF	White
1	0b0001	#dddddd	Gray #1
2	0b0010	#bbbbbb	Gray #2
3	0b0011	#999999	Gray #3
4	0b0100	#777777	Gray #4
5	0b0101	#555555	Gray #5
6	0b0110	#333333	Gray #6
7	0b0111	#000000	Black
8	0b1000	#004499	Dark blue
9	0b1001	#eeee00	Yellow
10	0b1010	#00cc00	Green
11	0b1011	#dd0000	Red
12	0b1100	#eeeebb	Beige
13	0b1101	#558800	Dark green
14	0b1110	#ffbb00	Gold/Orange
15	0b1111	#00bbff	Light blue

CGA 16-color palette

The CGA palette looks a lot like the Microsoft one, except that it uses a different in-between value of 0xAA and reorders the colours a bit. Again, bit 3 is a "brightness toggle".

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#0000aa	Blue
2	0b0010	#00aa00	Green
3	0b0011	#00aaaa	Cyan
4	0b0100	#aa0000	Red
5	0b0101	#aa00aa	Magenta
6	0b0110	#aa5500	Brown
7	0b0111	#aaaaaa	Light gray
8	0b1000	#555555	Dark gray
9	0b1001	#5555ff	Light blue
10	0b1010	#55ff55	Light green
11	0b1011	#55ffff	Light cyan
12	0b1100	#ff5555	Light red
13	0b1101	#ff55ff	Light magenta
14	0b1110	#ffff55	Yellow
15	0b1111	#ffffff	White

Pico-8 16-colour palette

If I understand correctly the 16 colours in Pico-8 can be mapped to other colours, but these are the 16 system colors the screen palette is mapped to when PICO-8 starts up.

I like how this palette is much more stylized, like the Apple one, but still manages to look very 8-bit-like when used.

#	Binary #	HEX colour	Colour
0	0b0000	#000000	black
1	0b0001	#1D2B53	dark-blue
2	0b0010	#7E2553	dark-purple
3	0b0011	#008751	dark-green
4	0b0100	#AB5236	brown
5	0b0101	#5F574F	dark-grey
6	0b0110	#C2C3C7	light-grey
7	0b0111	#FFF1E8	white
8	0b1000	#FF004D	red
9	0b1001	#FFA300	orange
10	0b1010	#FFEC27	yellow
11	0b1011	#00E436	green
12	0b1100	#29ADFF	blue
13	0b1101	#83769C	lavender
14	0b1110	#FF77A8	pink
15	0b1111	#FFCCAA	light-peach

Silicon8 palette

v1.0

The current palette that Silicon8 uses is the following:

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
2	0b0010	#AAAAAA	Light gray
3	0b0011	#555555	Dark gray
4	0b0100	#FF0000	Red
5	0b0101	#00FF00	Green
6	0b0110	#0000FF	Blue
7	0b0111	#FFFF00	Yellow
8	0b1000	#880000	Bordeaux
9	0b1001	#008800	Olive
10	0b1010	#000088	Navy
11	0b1011	#888800	Orange
12	0b1100	#FF00FF	Pink
13	0b1101	#00FFFF	Cyan
14	0b1110	#880088	Purple
15	0b1111	#008888	Ocean

This palette looks a lot like the Microsoft and CGA ones, except that my in-between value is 0x88 (which may or may not be a good idea) and that I chose a different order for the colours.

Advantages of this palette:

• The grays are in the first four spots, so that XO-Chip programs get four levels of grayscale by default, without any additional effort on the part of the developer of the interpreter (albeit in the "wrong" order)
• The primary colours red, green, blue and yellow can be used by adding just one extra plane to an image
• In use, it very much reminds me of the screamy bright "good old days" of CGA

Disadvantages of this palette:

• Bit 3 doesn't function like a "brightness toggle" like the MS and CGA palettes do (is that a big disadvantage? I don't know)
• The colours aren't as pretty as the Apple or Pico-8 ones
• Individual planes don't represent very useful colours (white, light gray, red, bordeax)

Let's break this palette up in it's "sub-palettes".

Individual planes

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
2	0b0010	#AAAAAA	Light gray
4	0b0100	#FF0000	Red
8	0b1000	#880000	Bordeaux

Planes 0 and 1

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
2	0b0010	#AAAAAA	Light gray
3	0b0011	#555555	Dark gray

Planes 0 and 2

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
4	0b0100	#FF0000	Red
5	0b0101	#00FF00	Green

Planes 0 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
8	0b1000	#880000	Bordeaux
9	0b1001	#008800	Olive

Planes 1 and 2

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
2	0b0010	#AAAAAA	Light gray
4	0b0100	#FF0000	Red
6	0b0110	#0000FF	Blue

Planes 1 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
2	0b0010	#AAAAAA	Light gray
8	0b1000	#880000	Bordeaux
10	0b1010	#000088	Navy

Planes 2 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
4	0b0100	#FF0000	Red
8	0b1000	#880000	Bordeaux
12	0b1100	#FF00FF	Pink

Planes 0, 1 and 2

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
2	0b0010	#AAAAAA	Light gray
3	0b0011	#555555	Dark gray
4	0b0100	#FF0000	Red
5	0b0101	#00FF00	Green
6	0b0110	#0000FF	Blue
7	0b0111	#FFFF00	Yellow

Planes 0, 1 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
2	0b0010	#AAAAAA	Light gray
3	0b0011	#555555	Dark gray
8	0b1000	#880000	Bordeaux
9	0b1001	#008800	Olive
10	0b1010	#000088	Navy
11	0b1011	#888800	Orange

Planes 0, 2 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
1	0b0001	#FFFFFF	White
4	0b0100	#FF0000	Red
5	0b0101	#00FF00	Green
8	0b1000	#880000	Bordeaux
9	0b1001	#008800	Olive
12	0b1100	#FF00FF	Pink
13	0b1101	#00FFFF	Cyan

Planes 1, 2 and 3

#	Binary #	HEX colour	Colour
0	0b0000	#000000	Black
2	0b0010	#AAAAAA	Light gray
4	0b0100	#FF0000	Red
6	0b0110	#0000FF	Blue
8	0b1000	#880000	Bordeaux
10	0b1010	#000088	Navy
12	0b1100	#FF00FF	Pink
14	0b1110	#880088	Purple

Conclusion so far

I'm not overly happy with the current "sub-palettes", so I think a redo may be in order. But probably not any time soon 😄