Question

I’m trying to figure out why drawing a shape, then drawing over it in a new color (as though to highlight it), and then re-drawing the original (un-highlighting it) is leaving traces of the highlighted color.

I’ve reproduced the issue in this fiddle. The wedge is drawn in a light-blue color. There’s a red button that’ll draw over it in red, then another button that re-draws the original shape. All parameters are identical (except for the color), but yet after clicking the button to reset the color, there’s a faint trace of red over the wedge.

Before:

After:

Here’s the relevant code:

drawWedge(250, 250, 200, 0, 18, "rgb(150, 254, 223)");

$("#red").click(function () {
    drawWedge(250, 250, 200, 0, 18, "rgb(255, 0, 0)");
});

$("#back").click(function () {
    drawWedge(250, 250, 200, 0, 18, "rgb(150, 254, 223)");
});

function d2r(degrees) {
    return degrees * (Math.PI / 180.0);
}

function drawWedge(centerX, centerY, r, start, end, color) {
    context.beginPath();
    context.moveTo(centerX, centerY);
    context.arc(centerX, centerY, r, d2r(start), d2r(end), false);
    context.closePath();
    context.fillStyle = color;
    context.fill();
}

Answer 1

When drawing on canvas, it just keeps stacking things on top of each other until you clear it. The easiest way to clear it is ctx.clearRect(0,0,width,height)

I put that in your drawWedge function here:

http://jsfiddle.net/X7deh/1