index.js

/*
 * Ben Postlethwaite
 * January 2013
 * License MIT
 */
'use strict';

var colorScale = require('./colorScale');
var lerp = require('lerp')

module.exports = createColormap;

function createColormap (spec) {
    /*
     * Default Options
     */
    var indicies, fromrgba, torgba,
        nsteps, cmap, colormap, format,
        nshades, colors, alpha, i;

    if ( !spec ) spec = {};

    nshades = (spec.nshades || 72) - 1;
    format = spec.format || 'hex';

    colormap = spec.colormap;
    if (!colormap) colormap = 'jet';

    if (typeof colormap === 'string') {
        colormap = colormap.toLowerCase();

        if (!colorScale[colormap]) {
            throw Error(colormap + ' not a supported colorscale');
        }

        cmap = colorScale[colormap];

    } else if (Array.isArray(colormap)) {
        cmap = colormap.slice();

    } else {
        throw Error('unsupported colormap option', colormap);
    }

    if (cmap.length > nshades + 1) {
        throw new Error(
            colormap+' map requires nshades to be at least size '+cmap.length
        );
    }

    if (!Array.isArray(spec.alpha)) {

        if (typeof spec.alpha === 'number') {
            alpha = [spec.alpha, spec.alpha];

        } else {
            alpha = [1, 1];
        }

    } else if (spec.alpha.length !== 2) {
        alpha = [1, 1];

    } else {
        alpha = spec.alpha.slice();
    }

    // map index points from 0..1 to 0..n-1
    indicies = cmap.map(function(c) {
        return Math.round(c.index * nshades);
    });

    // Add alpha channel to the map
    alpha[0] = Math.min(Math.max(alpha[0], 0), 1);
    alpha[1] = Math.min(Math.max(alpha[1], 0), 1);

    var steps = cmap.map(function(c, i) {
        var index = cmap[i].index

        var rgba = cmap[i].rgb.slice();

        // if user supplies their own map use it
        if (rgba.length === 4 && rgba[3] >= 0 && rgba[3] <= 1) {
            return rgba
        }
        rgba[3] = alpha[0] + (alpha[1] - alpha[0])*index;

        return rgba
    })


    /*
     * map increasing linear values between indicies to
     * linear steps in colorvalues
     */
    var colors = []
    for (i = 0; i < indicies.length-1; ++i) {
        nsteps = indicies[i+1] - indicies[i];
        fromrgba = steps[i];
        torgba = steps[i+1];

        for (var j = 0; j < nsteps; j++) {
            var amt = j / nsteps
            colors.push([
                Math.round(lerp(fromrgba[0], torgba[0], amt)),
                Math.round(lerp(fromrgba[1], torgba[1], amt)),
                Math.round(lerp(fromrgba[2], torgba[2], amt)),
                lerp(fromrgba[3], torgba[3], amt)
            ])
        }
    }

    //add 1 step as last value
    colors.push(cmap[cmap.length - 1].rgb.concat(alpha[1]))

    if (format === 'hex') colors = colors.map( rgb2hex );
    else if (format === 'rgbaString') colors = colors.map( rgbaStr );
    else if (format === 'float') colors = colors.map( rgb2float );

    return colors;
};

function rgb2float (rgba) {
    return [
        rgba[0] / 255,
        rgba[1] / 255,
        rgba[2] / 255,
        rgba[3]
    ]
}

function rgb2hex (rgba) {
    var dig, hex = '#';
    for (var i = 0; i < 3; ++i) {
        dig = rgba[i];
        dig = dig.toString(16);
        hex += ('00' + dig).substr( dig.length );
    }
    return hex;
}

function rgbaStr (rgba) {
    return 'rgba(' + rgba.join(',') + ')';
}