A rather unoptimised colour averaging algorithm from an image trianglifier for a blog post over at https://starbeamrainbowlabs.com

getaveragepixelcolour.js

function getaveragepixelcolour(pixeldata, width, height, x, y, radius)
{
	var colours = [];
	for(var cx = x - radius; cx < x + radius; cx++)
	{
		for(var cy = y - radius; cy < y + radius; cy++)
		{
			if(cx < 0 || cy < 0)
				continue;
			if(cx >= width || cy >= height)
				continue;
			
			colours.push(getpixelcolour(pixeldata, width, cx, cy));
		}
	}
	var colour = { r: 0, g: 0, b: 0, a: 255};
	for(var i = 0; i < colours.length; i++)
	{
		colour.r += colours[i].r;
		colour.g += colours[i].g;
		colour.b += colours[i].b;
		colour.a += colours[i].a;
	}
	colour.r /= colours.length + 1;
	colour.g /= colours.length + 1;
	colour.b /= colours.length + 1;
	colour.a /= colours.length + 1;
	colour.r = Math.floor(colour.r);
	colour.g = Math.floor(colour.g);
	colour.b = Math.floor(colour.b);
	colour.a = Math.floor(colour.a);
	return colour;
}