gistfile1.js

function createOrtho2D(left,right,top,bottom) {
	var near = -1, far = 1, rl = right-left, tb = top-bottom, fn = far-near;
	return [2/rl,0,0,0,
		0,2/tb,0,0,
		0,0,-2/fn,0,
		-(right+left)/rl, -(top+bottom)/tb, -(far+near)/fn,1];
}

createOrtho2D(0,canvas.offsetWidth,0,canvas.offsetHeight) * [10,10,0] = [-0.9804878048598766,0.9450549436733127,0]

I was expecting [10/canvas.offsetWidth,1-(10/canvas.offsetHeight),0] ??

If I do createOrtho2D(0,canvas.offsetWidth,canvas.offsetHeight,0) then the point 10,10 is 10 pixels in from the bottom left, and everything draws upside down