corner.js

function curve(x) {
	return Math.sqrt(1 - x * x);
}
function curveInt(x) {
	return 0.5*(Math.sqrt(1-x*x)*x+Math.asin(x));
}
function corner(x, y, w, h) {
	if ((x+w)*(x+w)+(y+h)*(y+h) <= 1) return 1;
	if (x*x+y*y >= 1) return 0;
	//if (x > 1 || y > 1) return 0;
	
	var start = x;
	var end = x + w;
	var result = 0;
	if (curve(x) > y + h) {
		start = curve(y+h);
		result += (start-x) / w;
	}
	if (curve(y) < end) {
		end = curve(y);
	}
	result += (curveInt(end)-curveInt(start)-(end-start)*y)/(w*h);
	if (x <= 50/512) console.log(start*512 + ", " + end*512 + ", result " + result);
	if (result < 0) console.log("error");
	return result;
}
function easyCorner(x, y, w, h) {
	return corner(x/w, y/h, 1/w, 1/h) * 256;
}