SIXEL support?

[jerch]

For higher quality images you might want to add optional SIXEL output (supported by several terminals).

[sindresorhus]

Are there actually any popular terminals that support SIXEL?

[jerch]

@sindresorhus I think so:

• xterm (behind compile switch)
• vte (currently still WIP, but covers 60% of linux desktop terminals once released)
• iTerm2
• mintty
• and several more

The support got way better in the last years. The hard part might be to decide, whether to use SIXEL on a certain terminal. Maybe a switch would do in the beginning, so users could test themselves, if their terminal can handle it.

[jerch]

The patch below adds basic SIXEL support. Flaws to be addressed, if someone wants to integrate that further:

• way to get the real viewport size is just a bad hack based on your "cell pixels"
• quantizer lib is not that good, quite slow and even buggy (find a better one?)
• needs better integration (own export?), prolly with some of the settings being exposed

diff --git a/index.js b/index.js
index 896bb8e..78c468d 100644
--- a/index.js
+++ b/index.js
@@ -6,6 +6,8 @@ const Jimp = require('jimp');
 const termImg = require('term-img');
 const renderGif = require('render-gif');
 const logUpdate = require('log-update');
+const sixel = require('sixel');
+const RgbQuant = require('rgbquant');
 
 // `log-update` adds an extra newline so the generated frames need to be 2 pixels shorter.
 const ROW_OFFSET = 2;
@@ -97,11 +99,44 @@ async function render(buffer, {width: inputWidth, height: inputHeight, preserveA
 	return result;
 }
 
-exports.buffer = async (buffer, {width = '100%', height = '100%', preserveAspectRatio = true} = {}) => {
+async function renderSixel(buffer, {width: inputWidth, height: inputHeight, preserveAspectRatio}) {
+	// some conversion settings
+	const BACKGROUND_SELECT = 0;
+	const MAX_COLORS = 64;	// higher gives better quality
+
+	const image = await Jimp.read(buffer);
+	const {bitmap} = image;
+
+	// CELL_TO_PIXEL_FACTOR: hack to get close to terminal coverage (scales roughly with selected font size)
+	// ideally the real terminal viewport size would be queried from ioctl(TIOCGWINSZ) or by CSI 14 t
+	const CELL_TO_PIXEL_FACTOR = 7;
+	let {width, height} = calculateWidthHeight(bitmap.width, bitmap.height, inputWidth, inputHeight, preserveAspectRatio);
+	width *= CELL_TO_PIXEL_FACTOR;
+	height *= CELL_TO_PIXEL_FACTOR;
+
+	image.resize(width, height);
+
+	// quantisation
+	// Note: quite slow for bigger images and more colors, qualifies for worker or better quant lib
+	const q = new RgbQuant({colors: MAX_COLORS, dithKern: 'FloydSteinberg', dithSerp: true});
+	q.sample(image.bitmap.data);
+	const palette = q.palette(true);
+	const quantizedData = q.reduce(image.bitmap.data);
+
+	// sixel encoding
+	const result = sixel.sixelEncode(quantizedData, width, height, palette);
+
+	// return sixel data embedded in escape sequence
+	return sixel.introducer(BACKGROUND_SELECT) + result + sixel.FINALIZER;
+}
+
+exports.buffer = async (buffer, {width = '100%', height = '100%', preserveAspectRatio = true, sixel = false} = {}) => {
 	return termImg(buffer, {
 		width,
 		height,
-		fallback: () => render(buffer, {height, width, preserveAspectRatio})
+		fallback: () => sixel
+			? renderSixel(buffer, {height, width, preserveAspectRatio})
+			: render(buffer, {height, width, preserveAspectRatio})
 	});
 };
 
diff --git a/package.json b/package.json
index 26dda9e..f12eea5 100644
--- a/package.json
+++ b/package.json
@@ -48,6 +48,8 @@
 		"jimp": "^0.14.0",
 		"log-update": "^4.0.0",
 		"render-gif": "^2.0.4",
+		"rgbquant": "^1.1.2",
+		"sixel": "^0.12.0",
 		"term-img": "^5.0.0"
 	},
 	"devDependencies": {

[Richienb]

We can use the Primary Device Attributes ANSI escape code to detect support. The only challenge is writing the escape code to stdout and then capturing the stdin straight after. See: https://stackoverflow.com/questions/63051961/querying-primary-device-attributes

[Richienb]

Alright, we can now automatically detect sixel support so that it is possible to autonomously switch between regular and sixel rendering:

const supportsSixel = require("supports-sixel");

(async () => {
	const isSupported = await supportsSixel();

	console.log(isSupported ? "Sixels are supported!" : "Sixels aren't supported!");
})();

[jerch]

Also have an optimized quantizer in the making on my playground branch, with this dealing with the image processing semantics will be a no-brainer (well, hopefully :smile_cat:).

[jerch]

@Richienb Created a PR with an included quantizer: https://github.com/jerch/node-sixel/pull/16

This turns out to be much faster (at least 5 times), and comes with an easy to use interface as:

const { image2sixel } = require('sixel');  // not working from npm as not published yet

console.log(image2sixel(pixelData, width, height));

It supports more optional arguments, that might be needed sometimes (like the terminal might restrict the usable colors).

[jerch]

@Richienb Published an updated version of the lib, the snippet in my last comment should now work.