Javascript - найти самую темную область изображения

"use strict";
function newEl(tag){return document.createElement(tag)}
function newTxt(txt){return document.createTextNode(txt)}
function byId(id){return document.getElementById(id)}
function allByClass(clss,parent){return (parent==undefined?document:parent).getElementsByClassName(clss)}
function allByTag(tag,parent){return (parent==undefined?document:parent).getElementsByTagName(tag)}
function toggleClass(elem,clss){elem.classList.toggle(clss)}
function addClass(elem,clss){elem.classList.add(clss)}
function removeClass(elem,clss){elem.classList.remove(clss)}
function hasClass(elem,clss){elem.classList.contains(clss)}

// useful for HtmlCollection, NodeList, String types
function forEach(array, callback, scope){for (var i=0,n=array.length; i<n; i++)callback.call(scope, array[i], i, array);} // passes back stuff we need

// callback gets data via the .target.result field of the param passed to it.
function loadFileObject(fileObj, loadedCallback){var a = new FileReader();a.onload = loadedCallback;a.readAsDataURL( fileObj );}

function ajaxGet(url, onLoad, onError)
{
	var ajax = new XMLHttpRequest();
	ajax.onload = function(){onLoad(this);}
	ajax.onerror = function(){console.log("ajax request failed to: "+url);onError(this);}
	ajax.open("GET",url,true);
	ajax.send();
}

function ajaxPost(url, phpPostVarName, data, onSucess, onError)
{
	var ajax = new XMLHttpRequest();
	ajax.onload = function(){ onSucess(this);}
	ajax.onerror = function() {console.log("ajax request failed to: "+url);onError(this);}
	ajax.open("POST", url, true);
	ajax.setRequestHeader("Content-type","application/x-www-form-urlencoded");
	ajax.send(phpPostVarName+"=" + encodeURI(data) );
}

function ajaxPostForm(url, formElem, onSuccess, onError)
{
	var formData = new FormData(formElem);
	ajaxPostFormData(url, formData, onSuccess, onError)
}

function ajaxPostFormData(url, formData, onSuccess, onError)
{
	var ajax = new XMLHttpRequest();
	ajax.onload = function(){onSuccess(this);}
	ajax.onerror = function(){onError(this);}
	ajax.open("POST",url,true);
	ajax.send(formData);
}

function getTheStyle(tgtElement)
{
	var result = {}, properties = window.getComputedStyle(tgtElement, null);
	forEach(properties, function(prop){result[prop] = properties.getPropertyValue(prop);});
	return result;
}

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window.addEventListener('load', onDocLoaded, false);

function onDocLoaded(evt)
{
//	var image_url = 'http://i.imgur.com/j6oJO8s.png'; 
//	var image_url = 'onePixel.png';
	var image_url = 'j6oJO8s.png';
	
	byId('theImage').src = image_url;
	solve_darkest(image_url, function(x,y){alert('x: '+x+' y: '+y);} );
	solve_darkest_2(image_url, function(x,y){alert('x: '+x+' y: '+y);} );
}

function rgbToHsl(r, g, b) 
{
r /= 255, g /= 255, b /= 255;
var max = Math.max(r, g, b),
    min = Math.min(r, g, b);
var h, s, l = (max + min) / 2;

if (max == min) {
    h = s = 0; // achromatic
} else {
    var d = max - min;
    s = l > 0.5 ? d / (2 - max - min) : d / (max + min);
    switch (max) {
        case r:
            h = (g - b) / d + (g < b ? 6 : 0);
            break;
        case g:
            h = (b - r) / d + 2;
            break;
        case b:
            h = (r - g) / d + 4;
            break;
    }
    h /= 6;
}

return ({
    h: h,
    s: s,
    l: l,
})
}

function solve_darkest(url, callback) 
{
	var image = new Image();
	image.src = url;

	image.onload = function()
	{
		var canvas = document.createElement('canvas');
		canvas.width  = 300;
		canvas.height = 300;

		var context = canvas.getContext("2d");
		context.drawImage(image, 0, 0);

		var imgData = context.getImageData(0, 0, 300, 300);

		var pixel = 0;
		var darkest_pixel_lightness = 100;
		var darkest_pixel_location = 0;
		for (var i = 0; i < imgData.data.length; i += 4) 
		{
			var red = imgData.data[i + 0];
			var green = imgData.data[i + 1];
			var blue = imgData.data[i + 2];
			var alpha = imgData.data[i + 3];

			var hsl = rgbToHsl(red, green, blue);
			var lightness = hsl.l;

			if (lightness < darkest_pixel_lightness) 
			{
				darkest_pixel_lightness = lightness;
				darkest_pixel_location = pixel;
				console.log("Darkest found at index: " + pixel);
			}

			pixel++;
		}

	//    var y = Math.floor(darkest_pixel_location/200);
	//    var x = darkest_pixel_location-(y*200);
		var y = Math.floor(darkest_pixel_location/this.width);
		var x = darkest_pixel_location-(y*this.width);
		callback(x,y);
	};
}



function solve_darkest_2(url, callback) 
{
	var image = new Image();
	image.src = url;

	image.onload = function()
	{
		var canvas = document.createElement('canvas');
		canvas.width  = 300;
		canvas.height = 300;
		var context = canvas.getContext("2d");
		context.drawImage(image, 0, 0);
		var imgData = context.getImageData(0,0, canvas.width, canvas.height);

		var darkest_pixel_luminance = 100;
		var darkest_pixel_xPos = 0;
		var darkest_pixel_yPos = 0;
		for (var y=0; y<canvas.height; y++)
		{
			for (var x=0; x<canvas.width; x++)
			{
				var luminance = averagePixels(imgData, x, y, 10);
				if (luminance < darkest_pixel_luminance)
				{
					darkest_pixel_luminance = luminance;
					darkest_pixel_xPos = x;
					darkest_pixel_yPos = y;
				}
			}
		}
		callback(darkest_pixel_xPos,darkest_pixel_yPos);
	};
}

function averagePixels(imgData, xPos, yPos, averagingBlockSize)
{
//	var ctx = canvas.getContext("2d");
//	var imgData = ctx.getImageData( 0, 0, canvas.width, canvas.height );
//	imgData
	
	// we average pixels found in a square region, we need to know how many pixels
	// are in the region to divide the accumalated totals by the number of samples (pixels) in the
	// averaging square
	var numPixelsMax = averagingBlockSize * averagingBlockSize;
	var numPixelsActual = 0;
	
	var red, green, blue;
	red = green = blue = 0;
	
	var rowStride = imgData.width * 4;		// add this to an index into the canvas's data to get the pixel
										// immediatelly below it.
	var x, y;
	var initialIndex = ((yPos * imgData.width) + xPos) * 4;
	var index = initialIndex;
	
	var pixel = 0;
	var darkest_pixel_lightness = 100;
	var darkest_pixel_location = 0;
	
	for (y=0; y<averagingBlockSize; y++)
	{
		index = initialIndex + y * rowStride;
		
		for (x=0; x<averagingBlockSize; x++)
		{
			if ((x+xPos < imgData.width) && (y+yPos < imgData.height))
			{
				red += imgData.data[index+0];
				green += imgData.data[index+1];
				blue += imgData.data[index+2];
				numPixelsActual++;
			}
			index += 4;
		}
	}
	red /= numPixelsActual;
	green /= numPixelsActual;
	blue /= numPixelsActual;
	
	var hsl = rgbToHsl(red, green, blue);
	var luminance = hsl.l;
	return luminance;
}

img
{
	border: solid 1px red;
}

 <h1>300px</h1>
 <img id='theImage'/>