How can I combine alpha with compositing in Images?

Question

I have an HTML5 Canvas. I am using the KineticJS(KonvaJS) canvas library. The canvas contains an Image as shown below. Now I want to erase pixels but being combined with a certain transparency level. The red dot erases the pixels with transparency 0. This was solved in this question. The green dot erases the pixel with transparency level of 0.5. The higher the transparency the smaller is the effect of the eraser.

How can I define the strength of the eraser?

Answer 1

You can use 'destination-out` compositing to "erase" pixels on a canvas. By default, the erasure will be completely through to the background.

ctx.drawImage(img,0,0);
ctx.globalCompositeOperation='destination-out';
ctx.fillRect(100,50,50,50);

But you can also control how much alpha is erased by setting the globalAlpha to less than 1.00. This causes the erasure to reduce the alpha of the existing pixels--similar to color blending.

ctx.drawImage(img,0,0);
ctx.globalCompositeOperation='destination-out';
ctx.globalAlpha=0.50;
ctx.fillRect(200,50,50,50);

Answer 2

// Get the canvas pixel array.
var img = context.getImageData(x, y, w, h);

// then loop through the array 
// modifying each pixel one by one as you need
for (var i = 0, l = img.data.length; i < l; i += 4) {
    img.data[i  ] = ...; // red
    img.data[i+1] = ...; // green
    img.data[i+2] = ...; // blue
    img.data[i+3] = ...; // alpha
}

// put back the data in canvas
context.putImageData(img, x, y);

The strength of the erasor would probably be set via the alpha channel. Hope that gets you started.