XXIIVV — sixels

Sixel is a graphics format made of 64 patterns six pixels high and one wide.

An image is encoded by breaking up the bitmap into a series of 6-pixel high strips that is then converted into a single ASCII character, offset by 0x3f so that the first sixel is encoded as ?. This ensures that the sixels remain within the printable character range of the ASCII character set.

Enter Sixels Mode	DCS	0x90	Start sequence
Enter Sixels Mode	q	0x71	End optional parameters
Sixels Body	!	0x21	RLE Encoding
	$	0x24	Beginning of current line
	-	0x2d	Beginning of next line
	?~	0x3x-0x7f	Sixels Tiles
Leave Sixels Mode	ST	0x9c	Terminate sequence

RLE Encoding

The ! character, followed by a string of decimal digit characters, preceding any valid sixel-data character, causes that sixel to be repeated the number of times represented by the decimal string. RLE Encoding shouldn't be used for less than 4 repetitions. For example, seven repetitions of the sixel represented by the letter "A" could be transmitted either as AAAAAAA or !7A.

Tiles

The pixels can be read as a binary number, with the top pixel being the least significant bit. Add the value of the pixels together, which gives a number between 0 and 63 inclusive. This is converted to a character code by adding 63, which is the code of the question mark character, ?. The correspondence between each possible combination of six pixels and its sixel character is illustrated below.

Uxntal Implementation

@draw-sixels ( str* -- )
	[ LIT2 02 -Screen/auto ] DEO
	.Screen/x DEI2 ,&anchor STR2
	&w ( -- )
		LDAk [ LIT "- ] NEQ ?{
			[ LIT2 &anchor $2 ] .Screen/x DEO2
			.Screen/y DEI2k #0006 ADD2 ROT DEO2
			!& }
		LDAk [ LIT "? ] SUB ,&t STR
		#0600
	&l ( -- )
		[ LIT &t $1 ] OVR SFT #01 AND .Screen/pixel DEO
		INC GTHk ?&l
	POP2
	( | advance )
	.Screen/x DEI2k INC2 ROT DEO2
	.Screen/y DEI2k #0006 SUB2 ROT DEO2
	& INC2 LDAk ?&w
	POP2 JMP2r

@sample [ "???owYn||~ywo??-?IRJaVNn^NVbJRI $1 ]

incoming gly format