Javascript module for color quantization, based on Leptonica

quantize.js

/*! 
 * quantize.js Copyright 2008 Nick Rabinowitz.
 * Licensed under the MIT license: http://www.opensource.org/licenses/mit-license.php
 */

// fill out a couple protovis dependencies
/*!
 * Block below copied from Protovis: http://mbostock.github.com/protovis/
 * Copyright 2010 Stanford Visualization Group
 * Licensed under the BSD License: http://www.opensource.org/licenses/bsd-license.php
 */
if (!pv) {
    var pv = {
        map: function(array, f) {
          var o = {};
          return f
              ? array.map(function(d, i) { o.index = i; return f.call(o, d); })
              : array.slice();
        },
        naturalOrder: function(a, b) {
            return (a < b) ? -1 : ((a > b) ? 1 : 0);
        },
        sum: function(array, f) {
          var o = {};
          return array.reduce(f
              ? function(p, d, i) { o.index = i; return p + f.call(o, d); }
              : function(p, d) { return p + d; }, 0);
        },
        max: function(array, f) {
          return Math.max.apply(null, f ? pv.map(array, f) : array);
        }
    }
}
 
/**
 * Basic Javascript port of the MMCQ (modified median cut quantization)
 * algorithm from the Leptonica library (http://www.leptonica.com/).
 * Returns a color map you can use to map original pixels to the reduced
 * palette. Still a work in progress.
 * 
 * @author Nick Rabinowitz
 * @example

// array of pixels as [R,G,B] arrays
var myPixels = [[190,197,190], [202,204,200], [207,214,210], [211,214,211], [205,207,207]
                // etc
                ];
var maxColors = 4;

var cmap = MMCQ.quantize(myPixels, maxColors);
var newPalette = cmap.palette();
var newPixels = myPixels.map(function(p) { 
    return cmap.map(p); 
});
 
 */
var MMCQ = (function() {
    // private constants
    var sigbits = 5,
        rshift = 8 - sigbits,
        maxIterations = 1000,
        fractByPopulations = 0.75;
    
    // get reduced-space color index for a pixel
    function getColorIndex(r, g, b) {
        return (r << (2 * sigbits)) + (g << sigbits) + b;
    }
    
    // Simple priority queue
    function PQueue(comparator) {
        var contents = [],
            sorted = false;
        
        function sort() {
            contents.sort(comparator);
            sorted = true;
        }
        
        return {
            push: function(o) {
                contents.push(o);
                sorted = false;
            },
            peek: function(index) {
                if (!sorted) sort();
                if (index===undefined) index = contents.length - 1;
                return contents[index];
            },
            pop: function() {
                if (!sorted) sort();
                return contents.pop();
            },
            size: function() {
                return contents.length;
            },
            map: function(f) {
                return contents.map(f);
            },
            debug: function() {
                if (!sorted) sort();
                return contents;
            }
        };
    }
    
    // 3d color space box
    function VBox(r1, r2, g1, g2, b1, b2, histo) {
        var vbox = this;
        vbox.r1 = r1;
        vbox.r2 = r2;
        vbox.g1 = g1;
        vbox.g2 = g2;
        vbox.b1 = b1;
        vbox.b2 = b2;
        vbox.histo = histo;
    }
    VBox.prototype = {
        volume: function(force) {
            var vbox = this;
            if (!vbox._volume || force) {
                vbox._volume = ((vbox.r2 - vbox.r1 + 1) * (vbox.g2 - vbox.g1 + 1) * (vbox.b2 - vbox.b1 + 1));
            }
            return vbox._volume;
        },
        count: function(force) {
            var vbox = this,
                histo = vbox.histo;
            if (!vbox._count_set || force) {
                var npix = 0,
                    i, j, k;
                for (i = vbox.r1; i <= vbox.r2; i++) {
                    for (j = vbox.g1; j <= vbox.g2; j++) {
                        for (k = vbox.b1; k <= vbox.b2; k++) {
                             index = getColorIndex(i,j,k);
                             npix += (histo[index] || 0);
                        }
                    }
                }
                vbox._count = npix;
                vbox._count_set = true;
            }
            return vbox._count;
        },
        copy: function() {
            var vbox = this;
            return new VBox(vbox.r1, vbox.r2, vbox.g1, vbox.g2, vbox.b1, vbox.b2, vbox.histo);
        },
        avg: function(force) {
            var vbox = this,
                histo = vbox.histo;
            if (!vbox._avg || force) {
                var ntot = 0,
                    mult = 1 << (8 - sigbits),
                    rsum = 0,
                    gsum = 0,
                    bsum = 0,
                    hval,
                    i, j, k, histoindex;
                for (i = vbox.r1; i <= vbox.r2; i++) {
                    for (j = vbox.g1; j <= vbox.g2; j++) {
                        for (k = vbox.b1; k <= vbox.b2; k++) {
                             histoindex = getColorIndex(i,j,k);
                             hval = histo[histoindex] || 0;
                             ntot += hval;
                             rsum += (hval * (i + 0.5) * mult);
                             gsum += (hval * (j + 0.5) * mult);
                             bsum += (hval * (k + 0.5) * mult);
                        }
                    }
                }
                if (ntot) {
                    vbox._avg = [~~(rsum/ntot), ~~(gsum/ntot), ~~(bsum/ntot)];
                } else {
                    console.log('empty box');
                    vbox._avg = [
                        ~~(mult * (vbox.r1 + vbox.r2 + 1) / 2),
                        ~~(mult * (vbox.g1 + vbox.g2 + 1) / 2),
                        ~~(mult * (vbox.b1 + vbox.b2 + 1) / 2)
                    ];
                }
            }
            return vbox._avg;
        },
        contains: function(pixel) {
            var vbox = this,
                rval = pixel[0] >> rshift;
                gval = pixel[1] >> rshift;
                bval = pixel[2] >> rshift;
            return (rval >= vbox.r1 && rval <= vbox.r2 &&
                    gval >= vbox.g1 && rval <= vbox.g2 &&
                    bval >= vbox.b1 && rval <= vbox.b2);
        }
    };
    
    // Color map
    function CMap() {
        this.vboxes = new PQueue(function(a,b) { 
            return pv.naturalOrder(
                a.vbox.count()*a.vbox.volume(), 
                b.vbox.count()*b.vbox.volume()
            ) 
        });;
    }
    CMap.prototype = {
        push: function(vbox) {
            this.vboxes.push({
                vbox: vbox,
                color: vbox.avg()
            });
        },
        palette: function() {
            return this.vboxes.map(function(vb) { return vb.color });
        },
        size: function() {
            return this.vboxes.size();
        },
        map: function(color) {
            var vboxes = this.vboxes;
            for (var i=0; i<vboxes.size(); i++) {
                if (vboxes.peek(i).vbox.contains(color)) {
                    return vboxes.peek(i).color;
                }
            }
            return this.nearest(color);
        },
        nearest: function(color) {
            var vboxes = this.vboxes,
                d1, d2, pColor;
            for (var i=0; i<vboxes.size(); i++) {
                d2 = Math.sqrt(
                    Math.pow(color[0] - vboxes.peek(i).color[0], 2) +
                    Math.pow(color[1] - vboxes.peek(i).color[1], 2) +
                    Math.pow(color[1] - vboxes.peek(i).color[1], 2)
                );
                if (d2 < d1 || d1 === undefined) {
                    d1 = d2;
                    pColor = vboxes.peek(i).color;
                }
            }
            return pColor;
        },
        forcebw: function() {
            // XXX: won't  work yet
            var vboxes = this.vboxes;
            vboxes.sort(function(a,b) { return pv.naturalOrder(pv.sum(a.color), pv.sum(b.color) )});
            
            // force darkest color to black if everything < 5
            var lowest = vboxes[0].color;
            if (lowest[0] < 5 && lowest[1] < 5 && lowest[2] < 5)
                vboxes[0].color = [0,0,0];
            
            // force lightest color to white if everything > 251
            var idx = vboxes.length-1,
                highest = vboxes[idx].color;
            if (highest[0] > 251 && highest[1] > 251 && highest[2] > 251)
                vboxes[idx].color = [255,255,255];
        }
    };
    
    // histo (1-d array, giving the number of pixels in
    // each quantized region of color space), or null on error
    function getHisto(pixels) {
        var histosize = 1 << (3 * sigbits),
            histo = new Array(histosize),
            index, rval, gval, bval;
        pixels.forEach(function(pixel) {
            rval = pixel[0] >> rshift;
            gval = pixel[1] >> rshift;
            bval = pixel[2] >> rshift;
            index = getColorIndex(rval, gval, bval);
            histo[index] = (histo[index] || 0) + 1;
        });
        return histo;
    }
    
    function vboxFromPixels(pixels, histo) {
        var rmin=1000000, rmax=0, 
            gmin=1000000, gmax=0, 
            bmin=1000000, bmax=0, 
            rval, gval, bval;
        // find min/max
        pixels.forEach(function(pixel) {
            rval = pixel[0] >> rshift;
            gval = pixel[1] >> rshift;
            bval = pixel[2] >> rshift;
            if (rval < rmin) rmin = rval;
            else if (rval > rmax) rmax = rval;
            if (gval < gmin) gmin = gval;
            else if (gval > gmax) gmax = gval;
            if (bval < bmin) bmin = bval;
            else if (bval > bmax)  bmax = bval;
        });
        return new VBox(rmin, rmax, gmin, gmax, bmin, bmax, histo);
    }
    
    function medianCutApply(histo, vbox) {
        if (!vbox.count()) return;
        
        var rw = vbox.r2 - vbox.r1 + 1,
            gw = vbox.g2 - vbox.g1 + 1,
            bw = vbox.b2 - vbox.b1 + 1,
            maxw = pv.max([rw, gw, bw]);
        // only one pixel, no split
        if (vbox.count() == 1) {
            return [vbox.copy()]
        }
        /* Find the partial sum arrays along the selected axis. */
        var total = 0,
            partialsum = [],
            lookaheadsum = [],
            i, j, k, sum, index;
        if (maxw == rw) {
            for (i = vbox.r1; i <= vbox.r2; i++) {
                sum = 0;
                for (j = vbox.g1; j <= vbox.g2; j++) {
                    for (k = vbox.b1; k <= vbox.b2; k++) {
                        index = getColorIndex(i,j,k);
                        sum += (histo[index] || 0);
                    }
                }
                total += sum;
                partialsum[i] = total;
            }
        }
        else if (maxw == gw) {
            for (i = vbox.g1; i <= vbox.g2; i++) {
                sum = 0;
                for (j = vbox.r1; j <= vbox.r2; j++) {
                    for (k = vbox.b1; k <= vbox.b2; k++) {
                        index = getColorIndex(j,i,k);
                        sum += (histo[index] || 0);
                    }
                }
                total += sum;
                partialsum[i] = total;
            }
        }
        else {  /* maxw == bw */
            for (i = vbox.b1; i <= vbox.b2; i++) {
                sum = 0;
                for (j = vbox.r1; j <= vbox.r2; j++) {
                    for (k = vbox.g1; k <= vbox.g2; k++) {
                        index = getColorIndex(j,k,i);
                        sum += (histo[index] || 0);
                    }
                }
                total += sum;
                partialsum[i] = total;
            }
        }
        partialsum.forEach(function(d,i) { 
            lookaheadsum[i] = total-d 
        });
        function doCut(color) {
            var dim1 = color + '1',
                dim2 = color + '2', 
                left, right, vbox1, vbox2, d2, count2=0;
            for (i = vbox[dim1]; i <= vbox[dim2]; i++) {
                if (partialsum[i] > total / 2) {
                    vbox1 = vbox.copy();
                    vbox2 = vbox.copy();
                    left = i - vbox[dim1];
                    right = vbox[dim2] - i;
                    if (left <= right)
                        d2 = Math.min(vbox[dim2] - 1, ~~(i + right / 2));
                    else d2 = Math.max(vbox[dim1], ~~(i - 1 - left / 2));
                    // avoid 0-count boxes
                    while (!partialsum[d2]) d2++;
                    count2 = lookaheadsum[d2];
                    while (!count2 && partialsum[d2-1]) count2 = lookaheadsum[--d2];
                    // set dimensions
                    vbox1[dim2] = d2;
                    vbox2[dim1] = vbox1[dim2] + 1;
                    console.log('vbox counts:', vbox.count(), vbox1.count(), vbox2.count());
                    return [vbox1, vbox2];
                }
            }
        
        }
        // determine the cut planes
        return maxw == rw ? doCut('r') :
            maxw == gw ? doCut('g') :
            doCut('b');
    }

    function quantize(pixels, maxcolors) {
        // short-circuit
        if (!pixels.length || maxcolors < 2 || maxcolors > 256) {
            console.log('wrong number of maxcolors');
            return false;
        }
        
        // XXX: check color content and convert to grayscale if insufficient
        
        var histo = getHisto(pixels),
            histosize = 1 << (3 * sigbits);
        
        // check that we aren't below maxcolors already
        var nColors = 0;
        histo.forEach(function() { nColors++ });
        if (nColors <= maxcolors) {
            // XXX: generate the new colors from the histo and return
        }
        
        // get the beginning vbox from the colors
        var vbox = vboxFromPixels(pixels, histo),
            pq = new PQueue(function(a,b) { return pv.naturalOrder(a.count(), b.count()) });
        pq.push(vbox);
        
        // inner function to do the iteration
        function iter(lh, target) {
            var ncolors = 1,
                niters = 0,
                vbox;
            while (niters < maxIterations) {
                vbox = lh.pop();
                if (!vbox.count())  { /* just put it back */
                    lh.push(vbox);
                    niters++;
                    continue;
                }
                // do the cut
                var vboxes = medianCutApply(histo, vbox),
                    vbox1 = vboxes[0],
                    vbox2 = vboxes[1];
                    
                if (!vbox1) {
                    console.log("vbox1 not defined; shouldn't happen!");
                    return;
                }
                lh.push(vbox1);
                if (vbox2) {  /* vbox2 can be null */
                    lh.push(vbox2);
                    ncolors++;
                }
                if (ncolors >= target) return;
                if (niters++ > maxIterations) {
                    console.log("infinite loop; perhaps too few pixels!");
                    return;
                }
            }
        }
        
        // first set of colors, sorted by population
        iter(pq, fractByPopulations * maxcolors);
        // console.log(pq.size(), pq.debug().length, pq.debug().slice());
        
        // Re-sort by the product of pixel occupancy times the size in color space.
        var pq2 = new PQueue(function(a,b) { 
            return pv.naturalOrder(a.count()*a.volume(), b.count()*b.volume()) 
        });
        while (pq.size()) {
            pq2.push(pq.pop());
        }
        
        // next set - generate the median cuts using the (npix * vol) sorting.
        iter(pq2, maxcolors - pq2.size());
        
        // calculate the actual colors
        var cmap = new CMap();
        while (pq2.size()) {
            cmap.push(pq2.pop());
        }
        
        return cmap;
    }
    
    return {
        quantize: quantize
    }
})();

I have tried an implementation that does not make use of canvas but XMLHttpRequest object and a byte array. The image encoding/decoding part is done by jpg.js for JPEGS and could be done by png.js for PNGs. Currently, there are some issues that prevents to generate the same color palette that RGB byte array from color-theft described above:
The live demo is here: http://www.parisilabs.com/colors2/

There's one more thing... see https://github.com/olivierlesnicki/quantize/pull/6/files
The palette doesn't always return the correct number of colors...