Image Filters with Canvas

HTML5 Rocks

Introduction

The HTML5 canvas element can be used to write image filters. What you need to do is draw an image onto a canvas, read back the canvas pixels, and run your filter on them. You can then write the result onto a new canvas (or heck, just reuse the old one.)

Sounds simple? Good. Let's get cracking!

The original test image

Processing pixels

First, retrieve the image pixels:

Filters = {};
Filters.getPixels = function(img) {
  var c = this.getCanvas(img.width, img.height);
  var ctx = c.getContext('2d');
  ctx.drawImage(img);
  return ctx.getImageData(0,0,c.width,c.height);
};

Filters.getCanvas = function(w,h) {
  var c = document.createElement('canvas');
  c.width = w;
  c.height = h;
  return c;
};

Next, we need a way to filter images. How about a filterImage method that takes a filter and an image and returns the filtered pixels?

Filters.filterImage = function(filter, image, var_args) {
  var args = [this.getPixels(image)];
  for (var i=2; i<arguments.length; i++) {
    args.push(arguments[i]);
  }
  return filter.apply(null, args);
};

Running simple filters

Now that we have the pixel processing pipeline put together, it's time to write some simple filters. To start off, let's convert the image to grayscale.

Filters.grayscale = function(pixels, args) {
  var d = pixels.data;
  for (var i=0; i<d.length; i+=4) {
    var r = d[i];
    var g = d[i+1];
    var b = d[i+2];
    // CIE luminance for the RGB
    // The human eye is bad at seeing red and blue, so we de-emphasize them.
    var v = 0.2126*r + 0.7152*g + 0.0722*b;
    d[i] = d[i+1] = d[i+2] = v
  }
  return pixels;
};

Adjusting brightness can be done by adding a fixed value to the pixels:

Filters.brightness = function(pixels, adjustment) {
  var d = pixels.data;
  for (var i=0; i<d.length; i+=4) {
    d[i] += adjustment;
    d[i+1] += adjustment;
    d[i+2] += adjustment;
  }
  return pixels;
};

Thresholding an image is also quite simple. You just compare the grayscale value of a pixel to the threshold value and set the color based on that:

Filters.threshold = function(pixels, threshold) {
  var d = pixels.data;
  for (var i=0; i<d.length; i+=4) {
    var r = d[i];
    var g = d[i+1];
    var b = d[i+2];
    var v = (0.2126*r + 0.7152*g + 0.0722*b >= threshold) ? 255 : 0;
    d[i] = d[i+1] = d[i+2] = v
  }
  return pixels;
};

Convolving images

Convolution filters are very useful generic filters for image processing. The basic idea is that you take the weighed sum of a rectangle of pixels from the source image and use that as the output value. Convolution filters can be used for blurring, sharpening, embossing, edge detection and a whole bunch of other things.

Filters.tmpCanvas = document.createElement('canvas');
Filters.tmpCtx = Filters.tmpCanvas.getContext('2d');

Filters.createImageData = function(w,h) {
  return this.tmpCtx.createImageData(w,h);
};

Filters.convolute = function(pixels, weights, opaque) {
  var side = Math.round(Math.sqrt(weights.length));
  var halfSide = Math.floor(side/2);
  var src = pixels.data;
  var sw = pixels.width;
  var sh = pixels.height;
  // pad output by the convolution matrix
  var w = sw;
  var h = sh;
  var output = Filters.createImageData(w, h);
  var dst = output.data;
  // go through the destination image pixels
  var alphaFac = opaque ? 1 : 0;
  for (var y=0; y<h; y++) {
    for (var x=0; x<w; x++) {
      var sy = y;
      var sx = x;
      var dstOff = (y*w+x)*4;
      // calculate the weighed sum of the source image pixels that
      // fall under the convolution matrix
      var r=0, g=0, b=0, a=0;
      for (var cy=0; cy<side; cy++) {
        for (var cx=0; cx<side; cx++) {
          var scy = sy + cy - halfSide;
          var scx = sx + cx - halfSide;
          if (scy >= 0 && scy < sh && scx >= 0 && scx < sw) {
            var srcOff = (scy*sw+scx)*4;
            var wt = weights[cy*side+cx];
            r += src[srcOff] * wt;
            g += src[srcOff+1] * wt;
            b += src[srcOff+2] * wt;
            a += src[srcOff+3] * wt;
          }
        }
      }
      dst[dstOff] = r;
      dst[dstOff+1] = g;
      dst[dstOff+2] = b;
      dst[dstOff+3] = a + alphaFac*(255-a);
    }
  }
  return output;
};

Here's a 3x3 sharpen filter. See how it focuses the weight on the center pixel. To maintain the brightness of the image, the sum of the matrix values should be one.

Filters.filterImage(Filters.convolute, image,
  [  0, -1,  0,
    -1,  5, -1,
     0, -1,  0 ]
);

Here's an another example of a convolution filter, the box blur. The box blur outputs the average of the pixel values inside the convolution matrix. The way to do that is to create a convolution matrix of size NxN where each of the weights is 1 / (NxN). That way each of the pixels inside the matrix contributes an equal amount to the output image and the sum of the weights is one.

Filters.filterImage(Filters.convolute, image,
  [ 1/9, 1/9, 1/9,
    1/9, 1/9, 1/9,
    1/9, 1/9, 1/9 ]
);

We can make more complex image filters by combining existing filters. For example, let's write a Sobel filter. A Sobel filter computes the vertical and horizontal gradients of the image and combines the computed images to find edges in the image. The way we implement the Sobel filter here is by first grayscaling the image, then taking the horizontal and vertical gradients and finally combining the gradient images to make up the final image.

Regarding terminology, "gradient" here means the change in pixel value at an image position. If a pixel has a left neighbour with value 20 and a right neighbour with value 50, the horizontal gradient at the pixel would be 30. The vertical gradient has the same idea but uses the above and below neighbours.

var grayscale = Filters.filterImage(Filter.grayscale, image);
// Note that ImageData values are clamped between 0 and 255, so we need
// to use a Float32Array for the gradient values because they
// range between -255 and 255.
var vertical = Filters.convoluteFloat32(grayscale,
  [ -1, 0, 1,
    -2, 0, 2,
    -1, 0, 1 ]);
var horizontal = Filters.convoluteFloat32(grayscale,
  [ -1, -2, -1,
     0,  0,  0,
     1,  2,  1 ]);
var final_image = Filters.createImageData(vertical.width, vertical.height);
for (var i=0; i<final_image.data.length; i+=4) {
  // make the vertical gradient red
  var v = Math.abs(vertical.data[i]);
  final_image.data[i] = v;
  // make the horizontal gradient green
  var h = Math.abs(horizontal.data[i]);
  final_image.data[i+1] = h;
  // and mix in some blue for aesthetics
  final_image.data[i+2] = (v+h)/4;
  final_image.data[i+3] = 255; // opaque alpha
}

To cap off our journey into convolution, here's a little toy for you to play with: A custom 3x3 convolution filter! Yay!




And there's a whole bunch of other cool convolution filters out there just waiting for you to discover them. For instance, try implementing a Laplace filter in the convolution toy above and see what it does.

Conclusion

I hope this small article was useful in introducing the basic concepts of writing image filters in JavaScript using the HTML canvas tag. I encourage you to go and implement some more image filters, it's quite fun!

If you need better performance from your filters, you can usually port them to use WebGL fragment shaders to do the image processing. With shaders, you can run most simple filters in realtime, which allows you to use them for post-processing video and animations.

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