paper.js sketch that generates trees with branches of varying length.

paper_js_trees1.js

var width = view.size.width;
var height = view.size.height;
var averageLengthFac = 0.7;
var angleInc = Math.PI/4;
var iterations = 12;


function drawLine(x1, y1, x2, y2, color) {
    var line = new Path(new Point(x1, y1));
    line.strokeColor = color;
    line.add(new Point(x2, y2));
}

function getNextBranchX(branch) {
    var x = branch[0];
    var length = branch[2];
    var angle = branch[3];
    return x + Math.cos(angle)*length;
}


function getNextBranchY(branch) {
    var y = branch[1];
    var length = branch[2];
    var angle = branch[3];
    return y + Math.sin(angle)*length;
}


function getBranchColor(branch) {
    var length = branch[2];
    var angle = branch[3];
    return new Color(angle/(2*Math.PI), 1 - angle/(2*Math.PI), 1 - length/startLength);
}


function drawBranch(branch) {
    var x = branch[0];
    var y = branch[1];
    drawLine(
        x, y, 
        getNextBranchX(branch), 
        getNextBranchY(branch), 
        getBranchColor(branch)
    );
}


function drawBranches(branches) {
    for (var i = 0; i < branches.length; i++) {
        drawBranch(branches[i]);
    }
}


function getRandomStandardNormalDistribution() {
    var x = 2 * Math.random() - 1;
    if (x < 0) {
        return -1/(Math.sqrt(2)*Math.PI)*Math.pow(
            Math.E, -0.5*Math.pow(x, 2)
        );
    }
    return 1/(Math.sqrt(2)*Math.PI)*Math.pow(
        Math.E, -0.5*Math.pow(x, 2)
    );
}


function getNextBranch(branch, nextAngle) {
    var nextX = getNextBranchX(branch);
    var nextY = getNextBranchY(branch);
    var nextLength = branch[2]*(1 + getRandomStandardNormalDistribution())*averageLengthFac;
    return [nextX, nextY, nextLength, nextAngle];
}


var startLength = height/8;
var groundY = 3*height/4;

var branches = [[width/2, groundY, startLength, -Math.PI/2]];

var background = new Path.Rectangle(view.bounds);
background.fillColor = 'black';
var ground = new Path.Rectangle(
        new Point(0, groundY), 
        new Size(width, height - groundY)
    );
ground.fillColor = new Color(0, 0.5, 0.5);

for (var i = 0; i < iterations; i++) {
    drawBranches(branches);
    if (i != iterations - 1) {
        var nextBranches = [];
        for (var j = 0; j < branches.length; j++) {
            var branch = branches[j];
            var angle = branch[3];
            nextBranches.push(getNextBranch(branch, angle + angleInc));
            nextBranches.push(getNextBranch(branch, angle - angleInc));
        }
        branches = nextBranches;
    }
}