How to change the opacity (alpha, transparency) of an element in a canvas element after it has been drawn?

Question

Using the HTML5 element, I would like to load an image file (PNG, JPEG, etc.), draw it to the canvas completely transparently, and then fade it i

Answer 1

I think this answers the question best, it actually changes the alpha value of something that has been drawn already. Maybe this wasn't part of the api when this question was asked.

given 2d context c.

function reduceAlpha(x, y, w, h, dA) {
    var screenData = c.getImageData(x, y, w, h);
    for(let i = 3; i < screenData.data.length; i+=4){
    screenData.data[i] -= dA; //delta-Alpha
    }
    c.putImageData(screenData, x, y );
}