Canvas Prototype: Contrast

HTMLCanvasElement.prototype.contrast.js

HTMLCanvasElement.prototype.contrast = function (value) {
	var
	canvas = this,
	context = canvas.getContext('2d'),
	imageData = context.getImageData(0, 0, canvas.width, canvas.height),
	canvasPixelArray = imageData.data,
	canvasPixelArrayLength = canvasPixelArray.length,
	i = 0;

	value = (parseFloat(value) || 0) + 1;

	for (; i < canvasPixelArrayLength; i += 4) {
		canvasPixelArray[i] = ((((canvasPixelArray[i] / 255) - 0.5) * value) + 0.5) * 255;
		canvasPixelArray[i + 1] = ((((canvasPixelArray[i + 1] / 255) - 0.5) * value) + 0.5) * 255;
		canvasPixelArray[i + 2] = ((((canvasPixelArray[i + 2] / 255) - 0.5) * value) + 0.5) * 255;
	}

	context.putImageData(imageData, 0, 0);

	return canvas;
};