Question

There are hundreds of tutorials, how one can crop an image by drawImage() on a canvas.

context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight);

However, I have a canvas that fills the user’s browser. By exporting the canvas as an image I would like to export only an area of 640px*480px from (0|0).

Problem: How can I tell javascript to use only 640*480 of the canvas for the toDataURL()?

Here is what I have so far:

$("#submitGraphic").click( function(){
    var canvas = document.getElementsByTagName("canvas");
    // canvas context
    var context = canvas[0].getContext("2d");
    // get the current ImageData for the canvas
    var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
    // store the current globalCompositeOperation
    var compositeOperation = context.globalCompositeOperation;
    // set to draw behind current content
    context.globalCompositeOperation = "destination-over";
    //set background color
    context.fillStyle = "#FFFFFF";
    // draw background/rectangle on entire canvas
    context.fillRect(0,0,canvas[0].width,canvas[0].height);

    // not working, seems to clear the canvas? browser hangs?
    // seems that I can click a white image in the background
    /*canvas[0].width = 640;
    canvas[0].height = 480;*/

    // not working either
    /*canvas[0].style.width  = '640px';
    canvas[0].style.height = '480px';*/

    // not working at all
    /*context.canvas.width = 640;
    context.canvas.height = 480;*/

    // write on screen
    var img = canvas[0].toDataURL("image/png");
    document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
})

PS: I do not want to resize or scale, just clipping/cropping to the fixed window. Here I read that you only specifiy canvas.width and canvas.height – but this clears the canvas.

Answer 1

The best way is to just create a temporary canvas to draw onto from the current canvas. The user will never see this temp canvas. Then you just need use toDataUrl() on the temp canvas.

Live Demo

$("#submitGraphic").click( function(){
    var canvas = document.getElementsByTagName("canvas");
    // canvas context
    var context = canvas[0].getContext("2d");
    // get the current ImageData for the canvas
    var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
    // store the current globalCompositeOperation
    var compositeOperation = context.globalCompositeOperation;
    // set to draw behind current content
    context.globalCompositeOperation = "destination-over";
    //set background color
    context.fillStyle = "#FFFFFF";
    // draw background/rectangle on entire canvas
    context.fillRect(0,0,canvas[0].width,canvas[0].height);

    var tempCanvas = document.createElement("canvas"),
        tCtx = tempCanvas.getContext("2d");

    tempCanvas.width = 640;
    tempCanvas.height = 480;

    tCtx.drawImage(canvas[0],0,0);

    // write on screen
    var img = tempCanvas.toDataURL("image/png");
    document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
})​