Develop Reference

How to quickly find all colors in an image

I need to analyze an image to find all colors in a PNG or GIF image. I currently load the image to a canvas, then get the image data, then loop through every pixel and check it against each color in the palette. It takes forever, the browser thinks the script has stopped and it sometimes just crashes. Hoping there is a better way.
//load file
var fileReader = new FileReader();
fileReader.onload = function(e) {
var img = new Image();
img.onload = function() {
//create a new pixel with the images dimentions
newPixel(this.width, this.height, []);
//draw the image onto the canvas
context.drawImage(img, 0, 0);
var colorPalette = [];
var imagePixelData = context.getImageData(0,0,this.width, this.height).data;
console.log(imagePixelData)
for (var i = 0; i < imagePixelData.length; i += 4) {
var color = rgbToHex(imagePixelData[i],imagePixelData[i + 1],imagePixelData[i + 2]);
if (colorPalette.indexOf(color) == -1) {
colorPalette.push(color);
//don't allow more than 256 colors to be added
if (colorPalette.length >= settings.maxColorsOnImportedImage) {
alert('The image loaded seems to have more than '+settings.maxColorsOnImportedImage+' colors.')
break;
}
}
}
createColorPalette(colorPalette, false);
//track google event
ga('send', 'event', 'Pixel Editor Load', colorPalette.length, this.width+'/'+this.height); /*global ga*/
};
img.src = e.target.result;
};
fileReader.readAsDataURL(this.files[0]);

This will speed things up a little.
The function indexof will search all of the array if it can not find an entry. This can be very slow, you can improve the speed of the search by adding to the top of the array moving down then searching for the index from the top most entry. This ensures that the search is only over added entries not the entire array.
Also use 32Bit words rather than 8bit bytes, use typed arrays as they are significantly quicker that pushing onto a array and don't convert to hex inside the loop it is redundant and can be done after on the smaller data set.
See comments for logic.
//draw the image onto the canvas
ctx.drawImage(image, 0, 0);
// get size
const size = image.width * image.height;
// create pallete buffer
const colors32 = new Uint32Array(256);
// current pos of palette entry
var palettePos = colors32.length - 1; // Start from top as this will speed up indexOf function
// pixel data as 32 bit words so you can handle a pixel at a time rather than bytes.
const imgData = new Uint32Array(ctx.getImageData(0, 0, image.width, image.height).data.buffer);;
// hold the color. If the images are low colour (less 256) it is highly probable that many pixels will
// be the same as the previous. You can avoid the index search if this is the case.
var color= colors32[palettePos --] = imgData[0]; // assign first pixels to ease logic in loop
for (var i = 1; i < size && palettePos >= 0; i += 1) { // loop till al pixels read if palette full
if(color !== imgData[i]){ // is different than previouse
if (colors32.indexOf(imgData[i], palettePos) === -1) { // is in the pallet
color = colors32[palettePos --] = imgData[i]; // add it
}
}
}
// all the performance issues are over so now convert to the palette format you wanted.
const colorPalette = [];
colors32.reverse();
const paletteSize = (255 - palettePos) * 4;
const colors8 = new Uint8Array(colors32.buffer);
for(i = 0; i < paletteSize; i += 4){
colorPalette.push(rgbToHex(colors8[i],colors8[i + 1],colors8[i + 2]));
}

How do I determine the efficiency of mouse movement from Point A to Point B in Javascript?

I am trying to create an analytical program which keeps track of user mouse movement on a website and stores the data in a DB. Here is where I am stuck:
Assuming the mouse is always starting at the middle of the screen, and the user is instructed to move it to a particular element, how do I determine the efficiency and accuracy of that movement. I need to keep in mind the duration from start of hovering till the click, but I want to also include the hovering path of the mouse.
A perfect score would be a perfect line from Point A to Point B in x seconds, how do I determine the score of a curved path in 2x seconds, or an instance where the path goes in the wrong direction before proceeding to Point B? Are there any algorithms in existence?
Thanks for your help!

Here is a JSFiddle that I created. Click on the START box and then click on the FINISH box. Hopefully this will help you get started.
var start = false;
var start_time,end_time;
var points = [];
$("#start").click(function() {
start = true;
points = [];
start_time = Date.now();
});
$("#finish").click(function() {
start = false;
distance = travelledDistance();
time = (Date.now() - start_time)/1000;
var center_x_start = $("#start").offset().left + $("#start").width() / 2;
var center_y_start = $("#start").offset().top + $("#start").height() / 2;
var center_x_finish = $("#finish").offset().left + $("#finish").width() / 2;
var center_y_finish = $("#finish").offset().top + $("#finish").height() / 2;
var straight_distance = Math.round(Math.sqrt(Math.pow(center_x_finish - center_x_start, 2) + Math.pow(center_y_finish - center_y_start, 2)));
$("#time").text(+time+"s");
$("#distance").text(distance+"px");
$("#straight_distance").text(straight_distance+"px");
});
$(document).mousemove(function( event ) {
if(!start)
return;
points.push(event.pageX + "," + event.pageY);
});
function travelledDistance(){
var distance = 0;
for (i = 0; i < points.length - 1; i++) {
start_point = points[i].split(",");
end_point = points[i+1].split(",");
distance += Math.round(Math.sqrt(Math.pow(end_point[0] - start_point[0], 2) + Math.pow(end_point[1] - start_point[1], 2)));
}
return distance;
}
UPDATE
I made a new version here. Now you can drag the targets to check the different results.

move div smoothly with javascript/jquery and an array of pos

I'm looking for a way to move a div from an array of position with javascript/jquery.
I have trying to do it with jquery.animate but he moved the div with a pause at each iteration of my array.
That could be something like move the div from 0,0 to 120px,230px passing by the 23px,35px;45px,50px etc...
That is for moving an game character on a Tile map
So as requested, some bit of code
First you have a global timer that call a function at short interval to see if it have any action to execute.
In this loop a routine look if some mobile tiles are waiting of any mouvement.
Mobiles are declared as Object class and have a sub function that do the deplacement like that
setPos:function(coord){
var pos = jQuery("#"+this.id).position();
var x = (coord[0] - 32 + this.screenOffX + this.xOffset) - pos.left;
var y =(coord[1] + this.yOffset) - pos.top;
//this.stopAnimation();
//this.startAnimation(this.walkingAnimation);
jQuery("#"+this.id).animate({
left: '+='+ x,
top: '+='+ y
}, 33, function() {
// Animation complete.
});
},
That is a bit messy cause i trying a lot of thing to do the smooth movement that i'm looking for.
so setPos is calling in another place like that
stepMobile:function(mobile){
var wp;/*TEST*/
mobile.changeState("idle");
var ind = mobile.getWayPointIndex();
while(ind < (mobile.getWayPoints()).length - 1){
if (ind < (mobile.getWayPoints()).length - 1) {
wp = (mobile.getWayPoints())[ind + 1];
if (getTime() > wp.time) {
mobile.setWayPointIndex(ind + 1);
ind = ind +1;
}
}
wp = (mobile.getWayPoints())[ind];
var x;
var y = 0;
var z;
x = this.tileWidth * (wp.getTile()).getCol();
z = this.tileHeight * (wp.getTile()).getRow();
var elapsed = getTime() - wp.getTime();
console.log(elapsed);
if (ind == (mobile.getWayPoints()).length - 1) {
console.log('checkForOnStopEvent()');
} else {
//x += 1 * mobile.getWalkSpeed() * mobile.getCosAngle();
//z += 1 * mobile.getWalkSpeed() * mobile.getSinAngle();
}
var coord = this.mapToScreen(x, y, -z);
mobile.setPos(coord);
ind = mobile.getWayPointIndex();
}
},
Again lot of junk code here cause i literally burned my brain but i didn't get any good result.
And you have that global function that run this function over all mobiles waiting for deplacement.

Assign index # according to current section

Say I have a total width of 585px. And I wanted to divide the space into equal sections and assign each an index value within position. I could do something like this if I had lets say 6 sections: (assigned by total width / number of sections)
//Set up elements with variables
this.sliderContent = config.sliderContent;
this.sectionsWrap = config.sectionsWrap;
//Selects <a>
this.sectionsLinks = this.sectionsWrap.children().children();
//Create drag handle
this.sectionsWrap.parent().append($(document.createElement("div")).addClass("handle-containment")
.append($(document.createElement("a")).addClass("handle ui-corner-all").text("DRAG")));
//Select handle
this.sliderHandle = $(".handle");
var left = ui.position.left,
position = [];
var position = ((left >= 0 && left <= 80) ? [0, 1] :
((left >= 81 && left <= 198) ? [117, 2] :
((left >= 199 && left <= 315) ? [234, 3] :
((left >= 316 && left <= 430) ? [351, 4] :
((left >= 431 && left <= 548) ? [468, 5] :
((left >= 549) ? [585, 6] : [] ) ) ) ) ) );
if (position.length) {
$(".handle").animate({
left : position[0]
}, 400);
Slider.contentTransitions(position);
}
But what if I had an x number of sections. These sections are just elements like
<li><a></a></li>
<li><a></a></li>
<li><a></a></li>
Or
<div><a></a></div>
<div><a></a></div>
<div><a></a></div>
<div><a></a></div>
How would I divide the total of 585px and classify the index in position according to the current left value of the .handle element? I can know where the drag handle is by using ui.position.left, what I want is to be able to set an index for each element and be able to animate handle depending on where the handle is within the indexed elements. Since each element is indexed I later call a transition method and pass in the current index # to be displayed. The code I show above works, but isn't really efficient. I also need to account for the width of the handle to fit the section width. http://jsfiddle.net/yfqhV/1/

Ok, there is a slight inconsistency in the difference between the range figures in the question, which makes it hard to algorithmise [ my made-up-word de jour =) ] this exactly:
81 to 199 = 118
199 to 316 = 117
316 to 431 = 115
431 to 518 = 118
If you can adjust for that, I have a solution - it's not especially clever JavaScript, so there may well be better ways to do this (SO JS people, feel free to educate me!) but it works.
First we need a function to find the index of an array range, a given value falls within (this replaces your nested if-else shorthands), then we have a function to set up the positional arrays, and finally we can do a range search and return the corresponding array of values.
This solution should dynamically deal with a varying number of sections, as long as this line:
var len = $("#sectionContainer").children().length;
is adjusted accordingly. The only other values that may need adjusting are:
var totalWidth = 585;
var xPos = 81;
although you could set them if you have elements you can draw the values from, making it even more of a dynamic solution.
/**
* function to find the index of an array element where a given value falls
* between the range of values defined by array[index] and array[index+1]
*/
function findInRangeArray(arr, val){
for (var n = 0; n < arr.length-1; n++){
if ((val >= arr[n]) && (val < (arr[n+1]))) {
break;
}
}
return n;
}
/**
* function to set up arrays containing positional values
*/
function initPositionArrays() {
posArray = [];
leftPosArray = [];
var totalWidth = 585;
var xPos = 81;
var len = $("#sectionContainer").children().length;
var unit = totalWidth/(len - 1);
for (var i=1; i<=len; i++) {
pos = unit*(i-1);
posArray.push([Math.round(pos), i]);
xMin = (i >= 2 ? (i==2 ? xPos : leftPosArray[i-2] + posArray[1][0]) : 0);
leftPosArray.push(Math.round(xMin));
}
}
var left = ui.position.left;
initPositionArrays();
// find which index of "leftPosArray" range that "left" falls within
foundPos = findInRangeArray(leftPosArray, left);
var position = posArray[foundPos];
if (position.length) {
$(".handle").animate({
left : position[0]
}, 400);
Slider.contentTransitions(position);
}
I've set up a jsFiddle to illustrate.
Enjoy!
Edit
I've looked at #JonnySooter s own answer, and whilst it calculates the positioning correctly, it won't deal with a variable number of sections.
To get it to work with any number of sections, the handleContainment div (that is created on-the-fly) needs to have it's width set dynamically (via inline styling).
This is calculated by multiplying the number of sections by the width of each section (which is actually the same as the width of the slider).
This is all done after creating the handle so that the width can be extracted from the "handle" css class, meaning a change to the width of the handle will cascade into the routine when applied at the css level.
See this jsFiddle where the number of sections can be altered and the slider behaves properly.

var numSections = // ...;
var totalWidth = // ...;
var sectionWidth = totalWidth / numSections;
var index = Math.floor($(".handle").position().left / sectionWidth);
var leftPosition = index * sectionWidth;
var rightPosition = leftPosition + sectionWidth - 1;

UPDATE:
I worked on trying to find a solution myself and this is what I came up with:
function( event, ui ) {
var left = ui.position.left, //Get the current position of the handle
self = Slider, //Set to the Slider object cus func is a callback
position = 1;
sections_count = self.sectionsLinks.length, //Count the sections
section_position = Math.floor(self.sectionsWrap.width() / sections_count); //Set width of each section according to total width and section count
left = Math.round(left / section_position); //Set the index
position = (left * section_position); //Set the left ammount
if(position < section_position){ //If handle is dropped in the first section
position = 0.1; //Set the distance to animate
left = 0; //Set index to first section
}
if (position.length) {
$(this).animate({
left : position //Animate according to distance
}, 200);
left = left += 1; //Add one to the index so that I can use the nth() child selector later.
self.contentTransitions(left);
}
}

How to track divs scrolling over point on page with JavaScript/jQuery

I'm looking for a very fast solution to a div scrolling problem.
I have a set of divs, like forum posts, that are laid out one on top of the other. As the page scrolls down or up, I'd like to know when one of those divs hit's an arbitrary point on the page.
One way I tried was adding an onScroll event to each item, but as the number of items grow the page really starts to lag.
Anyone know a more efficient way to do this? Thanks /w

Well, I'm new to all this, so may be someone should correct me :)
I propose to
cache posts position
caсhe current
use binary search
Demo: http://jsfiddle.net/zYe8M/
<div class="post"></div>
<div class="post"></div>
<div class="post"></div>
...
var posts = $(".post"), // our elements
postsPos = [], // caсhe for positions
postsCur = -1, // cache for current
targetOffset = 50; // position from top of window where you want to make post current
// filling postsPos with positions
posts.each(function(){
postsPos.push($(this).offset().top);
});
// on window scroll
$(window).bind("scroll", function(){
// get target post number
var targ = postsPos.binarySearch($(window).scrollTop() + targetOffset);
// only if we scrolled to another post
if (targ != postsCur) {
// set new cur
postsCur = targ;
// moving cur class
posts.removeClass("cur").eq(targ).addClass("cur");
}
});
// binary search with little tuning on return to get nearest from bottom
Array.prototype.binarySearch = function(find) {
var low = 0, high = this.length - 1,
i, comparison;
while (low <= high) {
i = Math.floor((low + high) / 2);
if (this[i] < find) { low = i + 1; continue; };
if (this[i] > find) { high = i - 1; continue; };
return i;
}
return this[i] > find ? i-1 : i;
};

You shouldn't bind scroll event to all the divs but only to window instead. Then, you should check whether one of the divs overlap with the target point by making a simple calculation of the element offset values.
$(window).scroll(function(event)
{
var isCaptured = capture();
console.log(isCaptured);
});
function capture()
{
var c = $('.box'); //this is the divs
var t = $('#target'); //this is the target element
var cPos = c.offset(); var tPos = t.offset();
var overlapY = (cPos.top <= tPos.top + t.height() && cPos.top + c.height() >= tPos.top);
var overlapX = (cPos.left <= tPos.left + t.width() && cPos.left + c.width() >= tPos.left);
return overlapY && overlapX;
}
Instead of the $('#target') element, you can pass top and left (X, Y) offset values directly to the function.
Well, here is a dirty demonstration.