HTML 5 canvas drawing image not showing css

Question

When I try to draw an image on a canvas with a pre-loaded image with css like:

img.style.backgroundColor="red";
ctx.drawImage(img,0,0,100,100);

I find that the image is being drawn as it would appear without the css. Do HTML canvases support css? If not is there a way to overlay a png with transparency with a color only on the pixels that are not transparent?

Answer 1

It is possible to overlay the image drawn on a canvas by analyzing each pixel of the drawn image and then overlaying a 1x1 rectangle with the desired color. Here is an example of how to do so:

function overlayColor(img,x,y,a,r,g,b,drawWidth,drawHeight){
    //create a canvas in memory and draw the image there to analyze it
    var tmpCanvas = document.createElement('canvas');
    var context = tmpCanvas.getContext('2d');
    tmpCanvas.width = drawWidth;
    tmpCanvas.height = drawHeight;
    context.drawImage(img,0,0,drawWidth,drawHeight);
    let pData = new Array();
    for (let i = 0; i < tmpCanvas.width; i++){
        for (let j = 0; j < tmpCanvas.height; j++){
            if (context.getImageData(i, j, 1, 1).data[3] != 0){//if the pixel is not transparent then add the coordinates to the array
                pData.push([i,j]);
            }
        }
    }
    //then every pixel that wasn't transparent will be overlayed with a 1x1 rectangle of the inputted color 
    //drawn at the (x,y) origin which was also given by the user
    //this uses ctx, the context of the canvas visible to the user
    ctx.fillStyle="rgba(" + r + "," + g + "," + b + "," + a + ")";
    for (let i = 0; i < pData.length; i++){
        ctx.fillRect(x + pData[i][0],y + pData[i][1],1,1);
    }

}

Since the function takes an x and y value the image given by the user will be analyzed and overlayed only on pixels that are not transparent at the coordinate given by the user with the rgba value also given. I have found that this process can result in some lag but it could be overcome by saving the pData array and using the second half of the function to draw the array on screen again.