Click area on sprite in canvas
I have a tutorial that does almost exactly what you need for the hit-testing. See the code here.
When you click, the code draws each shape (I use rectangles but it works beautifully with semi-transparent images) to a canvas in memory (ghostcanvas) and checks to see if the mouse pixel is on a pixel that is not-transparent.
Relevant code pasted below:
function myDown(e){
getMouse(e);
clear(gctx); // clear the ghost canvas from its last use
// run through all the boxes
var l = boxes.length;
for (var i = l-1; i >= 0; i--) {
// draw shape onto ghost context
drawshape(gctx, boxes[i], 'black');
// get image data at the mouse x,y pixel
var imageData = gctx.getImageData(mx, my, 1, 1);
var index = (mx + my * imageData.width) * 4;
// if the mouse pixel exists, select and break
if (imageData.data[3] > 0) {
mySel = boxes[i];
offsetx = mx - mySel.x;
offsety = my - mySel.y;
mySel.x = mx - offsetx;
mySel.y = my - offsety;
isDrag = true;
canvas.onmousemove = myMove;
invalidate();
clear(gctx);
return;
}
}
// havent returned means we have selected nothing
mySel = null;
// clear the ghost canvas for next time
clear(gctx);
// invalidate because we might need the selection border to disappear
invalidate();
}
You could have the background be transparent and check images for transparency at the clicked pixel. Here's some code from one of my game prototypes:
function getAlphaInImage(img, x, y) {
var tmp = document.createElement("CANVAS");
tmp.setAttribute('width', img.width);
tmp.setAttribute('height', img.height);
var tmpCtx = tmp.getContext('2d');
tmpCtx.drawImage(img, 0, 0);
var imageData = tmpCtx.getImageData(x, y, 1, 1);
var alpha = imageData.data[3];
tmp = null;
imageData = null;
return alpha;
}
Before calling this I first check if the mouseclick was within the whole image.