Generating a canvas color picker with full rgb spa

Question

Since RGB needs a cube for displaying all colors there are more ways to display all the colors. I would like to have a circle displaying all the colors in a rainbow at full color - and when clicking showing all the different brightnesses for that selected color in it's own little 2D space.

I want to generate something that looks like this image using canvas:

My attempt for this:

Javascript:

function ColorPicker(element) {
    this.element = element;

    this.init = function() {
        var diameter = this.element.offsetWidth;

        var canvas = document.createElement('canvas');
        canvas.height = diameter;
        canvas.width = diameter,
        this.canvas = canvas;

        this.renderColorMap();

        element.appendChild(canvas);

        this.setupBindings();
    };

    this.renderColorMap = function() {
        var canvas = this.canvas;
        var ctx = canvas.getContext('2d');

        var radius = canvas.width / 2;
        var toRad = (2 * Math.PI) / 360;
        var step = 1 / radius;

        ctx.clearRect(0, 0, canvas.width, canvas.height);

        for(var i = 0; i < 360; i += step) {
            var rad = i * toRad;
            ctx.strokeStyle = 'hsl(' + i + ', 100%, 50%)';
            ctx.beginPath();
            ctx.moveTo(radius, radius);
            ctx.lineTo(radius + radius * Math.cos(rad), radius + radius * Math.sin(rad));
            ctx.stroke();
        }

        ctx.fillStyle = 'rgb(255, 255, 255)';
        ctx.beginPath();
        ctx.arc(radius, radius, radius * 0.8, 0, Math.PI * 2, true);
        ctx.closePath();
        ctx.fill();

        // render the rainbow box here ----------
    };

    this.renderMouseCircle = function(x, y) {
        var canvas = this.canvas;
        var ctx = canvas.getContext('2d');

        ctx.strokeStyle = 'rgb(255, 255, 255)';
        ctx.lineWidth = '2';
        ctx.beginPath();
        ctx.arc(x, y, 10, 0, Math.PI * 2, true);
        ctx.closePath();
        ctx.stroke();
    };

    this.setupBindings = function() {
        var canvas = this.canvas;
        var ctx = canvas.getContext('2d');
        var self = this;

        canvas.addEventListener('click', function(e) {
            var x = e.offsetX || e.clientX - this.offsetLeft;
            var y = e.offsetY || e.clientY - this.offsetTop;

            var imgData = ctx.getImageData(x, y, 1, 1).data;
            var selectedColor = new Color(imgData[0], imgData[1], imgData[2]);
            // generate this square here.

            self.renderMouseCircle(x, y);
        }, false);
    };

    this.init();
}

new ColorPicker(document.querySelector('.color-space'));

JSFiddle: http://jsfiddle.net/yH6JE/

I have a hard time figuring out how to generate this square in the middle. Basically I want the color in the far left middle to be the same as the one on the ring - that I've selected by clicking.

How can I generate this type of gradient square?

Answer 1

If you like, you can also use a circle. I think it is easier to choose a color that way.

I made this: https://github.com/JeroenvO/html5-colorpicker

Maybe you can use it as a guide. I used only images with some opacity and no gradiënt calculations in javascript.

Answer 2

It looks like you want an HSL colour rect, with the hue being selected from the colour wheel.

Here's a function i put together to draw the rect:

function draw(canvas, hue){
    var ctx = canvas.getContext('2d');
    for(row=0; row<100; row++){
        var grad = ctx.createLinearGradient(0, 0, 100,0);
        grad.addColorStop(0, 'hsl('+hue+', 100%, '+(100-row)+'%)');
        grad.addColorStop(1, 'hsl('+hue+', 0%, '+(100-row)+'%)');
        ctx.fillStyle=grad;
        ctx.fillRect(0, row, 100, 1);
    }   
}

Hopefully it does what you want. Example here: colour rect js fiddle