glitch.js

var glitch = module.exports = function(origData, params){

	if(
		checkType(params, 'params', 'object')
	){

		var params = getNormalizedParameters(params), 
			jpg_header_length = getJpegHeaderSize(origData);

		for(i = 0, len = params.iterations; i < len; i++){
			glitchJpegBytes(origData, jpg_header_length, params.seed, params.amount, i, params.iterations);
        }

        return origData;

	}

};

var glitchJpegBytes = function(byte_array, jpg_header_length, seed, amount, i, len){
	var max_index = byte_array.length - jpg_header_length - 4;
	var px_min = parseInt(max_index / len * i, 10);
	var px_max = parseInt(max_index / len * (i + 1), 10);
	var delta = px_max - px_min;
	var px_i = parseInt(px_min + delta * seed, 10);
	if (px_i > max_index) {
		px_i = max_index;
	}
	var index = Math.floor(jpg_header_length + px_i);
	byte_array[index] = Math.floor(amount * 256);
}

var getJpegHeaderSize = function(data){
	var result = 417;
	for (i = 0, len = data.length; i < len; i++){
		if (data[i] === 255 && data[i + 1] === 218){
			result = i + 2;
			break;
		}
	}
	return result;
}

var getNormalizedParameters = function(parameters){
	return {
		'seed': (parameters.seed || 0) / 100,
		'quality': (parameters.quality || 0) / 100,
		'amount': (parameters.amount || 0) / 100,
		'iterations': parameters.iterations || 0
	};
}

var checkType = function(it, name, expected_type){
	if(typeof it === expected_type){
		return true;
	}else{
		error(it, 'typeof ' + name, '\'' + expected_type + '\'', '\'' + typeof it + '\'');
		return false;
	}
};

var error = function(it, name, expected, result) {
	throw new Error('glitch(): Expected ' + name + ' to be ' + expected + ', but it was ' + result + '.');
};