Higher DPI graphics with HTML5 canvas

Question

Is there a way to set a custom DPI/PPI when creating an image using the HTML5 canvas? I know how can I draw on the canvas and export it as an image, but how can I make sure the output image is of certain DPI/PPI. I guess using SVG elemnts to draw on the canvas is a way, but wouldn't that be flattened out when I export the whole canvas as an image? Or calculating the device DPI and then scaling the image to meet my DPI requirement, but that doesn't seem like the correct solution.

Answer 1

Canvases have two different 'sizes': their DOM width/height and their CSS width/height. You can increase a canvas' resolution by increasing the DOM size while keeping the CSS size fixed, and then using the .scale() method to scale all of your future draws to the new bigger size. Here's an example:

function changeResolution(canvas, scaleFactor) {
    // Set up CSS size.
    canvas.style.width = canvas.style.width || canvas.width + 'px';
    canvas.style.height = canvas.style.height || canvas.height + 'px';

    // Resize canvas and scale future draws.
    canvas.width = Math.ceil(canvas.width * scaleFactor);
    canvas.height = Math.ceil(canvas.height * scaleFactor);
    var ctx = canvas.getContext('2d');
    ctx.scale(scaleFactor, scaleFactor);
}

The canvas default resolution is 96dpi (CSS inches, not based on the actual screen). So a scaleFactor of 2 gives 192dpi, 3 is 288dpi, etc. In fact, here's a version that should give your desired DPI:

function setDPI(canvas, dpi) {
    // Set up CSS size.
    canvas.style.width = canvas.style.width || canvas.width + 'px';
    canvas.style.height = canvas.style.height || canvas.height + 'px';

    // Resize canvas and scale future draws.
    var scaleFactor = dpi / 96;
    canvas.width = Math.ceil(canvas.width * scaleFactor);
    canvas.height = Math.ceil(canvas.height * scaleFactor);
    var ctx = canvas.getContext('2d');
    ctx.scale(scaleFactor, scaleFactor);
}

Have fun! Note that both these code samples can only be used once per canvas, they assume the current DOM size is the original (they could be tweaked to change that). Also the rescaling needs to happen before you do any drawing on the canvas. Thanks to this post for the method and information!

Edit: Here is a more robust function that will scale future draws and maintain existing canvas contents. This can be called to rescale multiple times.

function setDPI(canvas, dpi) {
    // Set up CSS size.
    canvas.style.width = canvas.style.width || canvas.width + 'px';
    canvas.style.height = canvas.style.height || canvas.height + 'px';

    // Get size information.
    var scaleFactor = dpi / 96;
    var width = parseFloat(canvas.style.width);
    var height = parseFloat(canvas.style.height);

    // Backup the canvas contents.
    var oldScale = canvas.width / width;
    var backupScale = scaleFactor / oldScale;
    var backup = canvas.cloneNode(false);
    backup.getContext('2d').drawImage(canvas, 0, 0);

    // Resize the canvas.
    var ctx = canvas.getContext('2d');
    canvas.width = Math.ceil(width * scaleFactor);
    canvas.height = Math.ceil(height * scaleFactor);

    // Redraw the canvas image and scale future draws.
    ctx.setTransform(backupScale, 0, 0, backupScale, 0, 0);
    ctx.drawImage(backup, 0, 0);
    ctx.setTransform(scaleFactor, 0, 0, scaleFactor, 0, 0);
}

Answer 2

You cannot (ugh) access the DPI of a display of the current web page in any browser:

Detecting the system DPI/PPI from JS/CSS?

For printing: You most likely cannot set the DPI of exported <canvas> image (PNG, JPEG) using browser standard functions. However, if you use a pure Javascript encoder image encoder you are free to create any sort of binary file you wish and manually adjust the DPI value embedded int he binary.

https://gist.github.com/1245476

Answer 3

If you just want to set the dpi of the PNG (ie not increase the number of pixels) then this library lets you set the pHYs chunk (amongst other things):

https://github.com/imaya/CanvasTool.PngEncoder

Minimal example to export an HTML5 canvas to base64-encoded PNG:

        // convert dots per inch into dots per metre
        var pixelsPerM = dpi * 100 / 2.54;

        var param = {
            bitDepth : 8,
            colourType : 2,
            filterType : 0,
            height : canvas.height,
            interlaceMethod : 0,
            phys : {
                unit : 1,
                x : pixelsPerM,
                y : pixelsPerM
            },
            width : canvas.width
        };

        var array = canvas.getContext('2d').getImageData(0, 0, canvas.width,
                canvas.height).data;

        var png = new window.CanvasTool.PngEncoder(array, param).convert();

        var base64 = 'data:image/png;base64,' + btoa(png);