HTML5 canvas - rotate object without moving coordinates

Question

What I have is this:

What I want is to rotate the red rectangle e.g. 20 degrees, but

Answer 1

You can use a simple function to do this as an alternative to translate and rotate:

This function will let you draw a line starting at x and y, and end at length at angle:

function lineToAngle(ctx, x1, y1, length, angle) {

    angle *= Math.PI / 180;

    var x2 = x1 + length * Math.cos(angle),
        y2 = y1 + length * Math.sin(angle);

    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);

    return {x: x2, y: y2};
}

Usage:

lineToAngle(ctx, x, y, length, angle);

Demo

var canvas = document.querySelector('canvas'),
    ctx = canvas.getContext('2d'),
    x = 100, y =50, length = 40, angle = 0, dlt = -2;

(function animate() {
    
    //clear
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    
    ctx.beginPath();
    lineToAngle(ctx, x, y, length, angle);
    ctx.lineWidth = 10;
    ctx.stroke();

    angle += dlt;
    if (angle < -90 || angle > 0) dlt = -dlt;
    
    requestAnimationFrame(animate);
})();

function lineToAngle(ctx, x1, y1, length, angle) {

    angle *= Math.PI / 180;
    
    var x2 = x1 + length * Math.cos(angle),
        y2 = y1 + length * Math.sin(angle);
    
    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);

    return {x: x2, y: y2};
}

body {background-color:#555557}
canvas {background:#ffffd;border:1px solid #000}