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How to see if PNG has a transparent background with Javascript [duplicate]

Is there a way to read transparent pixels from a picture using javascript?
I think, that it could be something similar to what PNG fixes does for IE (reading transparent pixels and applying some stuff, lol). But yes, for every browser..
Ah, would be awesome if it could be achieved without HTML5.

Well this question is actually answered by the dude from GoogleTechTalks in this video on javascript-based game engines.
http://www.youtube.com/watch?feature=player_detailpage&v=_RRnyChxijA#t=1610s
It should start at the point where it is explained.
Edit:
So I will summarize what is told in the video and provide a code-example.
It was a lot tougher than I had expected. The trick is to load your image onto a canvas and then check each pixel if it is transparent. The data is put into a two dimension array. Like alphaData[pixelRow][pixelCol]. A 0 is representing transparency while a 1 is not. When the alphaData array is completed it is put in global var a.
var a;
function alphaDataPNG(url, width, height) {
var start = false;
var context = null;
var c = document.createElement("canvas");
if(c.getContext) {
context = c.getContext("2d");
if(context.getImageData) {
start = true;
}
}
if(start) {
var alphaData = [];
var loadImage = new Image();
loadImage.style.position = "absolute";
loadImage.style.left = "-10000px";
document.body.appendChild(loadImage);
loadImage.onload = function() {
c.width = width;
c.height = height;
c.style.width = width + "px";
c.style.height = height + "px";
context.drawImage(this, 0, 0, width, height);
try {
try {
var imgDat = context.getImageData(0, 0, width, height);
} catch (e) {
netscape.security.PrivilegeManager.enablePrivilege("UniversalBrowserRead");
var imgDat = context.getImageData(0, 0, width, height);
}
} catch (e) {
throw new Error("unable to access image data: " + e);
}
var imgData = imgDat.data;
for(var i = 0, n = imgData.length; i < n; i += 4) {
var row = Math.floor((i / 4) / width);
var col = (i/4) - (row * width);
if(!alphaData[row]) alphaData[row] = [];
alphaData[row][col] = imgData[i+3] == 0 ? 0 : 1;
}
a=alphaData;
};
loadImage.src = url;
} else {
return false;
}
}
I got errors when running local in Firefox and the try catch statement solved it. Oh I gotta eat...
Edit 2:
So I finished my dinner, I'd like to add some sources I used and wich can be helpful.
https://developer.mozilla.org/En/HTML/Canvas/Pixel_manipulation_with_canvas
Info about the imageData object.
http://blog.nihilogic.dk/2008/05/compression-using-canvas-and-png.html
Even more info about the imageData object and it's use.
http://www.nihilogic.dk/labs/canvascompress/pngdata.js
A really helpful example of the use of imageData, the code I provided resembles this one for a big part.
http://www.youtube.com/watch?v=_RRnyChxijA
Infos on scripting game-engines in javascript, really really interesting.
http://blog.project-sierra.de/archives/1577
Infos about the use of enablePrivilege in firefox.

This is a bit tricky problem, since the only way to access files directly from Javascript is by using FileReader, which is a relatively new feature and not yet supported in most browsers.
However, you could get the desired result by using a canvas. If you have a canvas, you could assign it some distinctive color (such as neon green used in green screens). Then you could insert the image onto canvas and use the method mentioned here to get each individual pixel. Then you could check each pixel's color and see whether that point corresponds to your background color (ergo it's transparent) or does it have some other color (not transparent).
Kind of hackish, but don't think there's anything else you can do with pure JS.

It appears that GameJS can do this and much, much more. I am referencing this SO question for any/all of my knowledge, as I don't claim to actually have any about this topic.
Of course, this is HTML5, and uses the canvas element.

Fastest way to change image pixels before rendering on an HTML5 canvas

I have a (largish) HTML5 canvas. Its rendering a pictures from a file, using context.drawImage() and this is quite fast. (Note that there are more than one picture on the same canvas).
Now I need to perform some manipulations to the pixels on the canvas, basically I need to perform Alpha Blending which darkens certain areas of the picture. So instead I used this approach.
//create an invisible canvas so that we don't do the actual rendering of the image
var invisibleCanvas = document.createElement('canvas');
invisibleCanvas.width = myWidth;
invisibleCanvas.height = myHeight;
var invContext = invisibleCanvas.getContext('2d');
invContext.drawImage(imageObj, 0, 0, invisibleCanvas.width, invisibleCanvas.height);
var imageData = invContext.getImageData(0, 0, invisibleCanvas.width, invisibleCanvas.height)
var pixelComponents = imageData.data;
var dkBlendingAmount = 0.5;
for (var i = 0; i < pixelComponents.length; i += 4)
{
//there are a few extra checks here to see if we should do the blending or not
pixelComponents[i] = pixelComponents[i] * dkBlendingAmount;
pixelComponents[i+1] = pixelComponents[i+1] * dkBlendingAmount;
pixelComponents[i+2] = pixelComponents[i+2] * dkBlendingAmount;
}
//this is the real place where I want it
context.putImageData(imageData, xOffset, yOffset);
Is there a way to make this faster? Is there a way to get the image data directly from my imageObj rather than having to put it on a canvas, get the data, convert it and put it on another canvas?

resetting canvas strokes each re-iteration through the function(animation). how?

At the moment the lines stick permanently creating a drawing. I want the line to be drawm and reset everytime the function is called via setInterval() therefore creating a brain-wave ripple like animation.
Here's my current source code:
var rippleEffect = function(){
var co = [];
for(var i=0; i<=5; i++){
var r = Math.floor(Math.random()*250);
co.push(r);
//each time through the loop throws/pushes 6 random numbers to array 'co' for bezier curve.
}
var canvas = document.getElementById('brainWave');
var context = canvas.getContext('2d');
context.beginPath();
context.moveTo(500, 75);
context.bezierCurveTo(co[0], co[1], co[2], co[3], co[4], co[5]);
context.lineWidth = 2;
// line color
context.strokeStyle = '#444';
context.stroke(); //exicute the stroke based on the structions we've provided.
};
setInterval(ripple,100); //this re-calls the functions every few miller-seconds.
is .clearRect() some part of the solution?

I'm not quite sure what you're trying to do, but here are some observations on your code:
clearRect is essential if you don't want the drawings to accumulate on top of each other.
setInterval is calling "ripple", but it should be calling "rippleEffect".
var canvas and var context should be pulled outside-before rippleEffect (they just need to be created once, not with every loop.
Totally random control points for a Bezier curve yields spaghetti, not brain-waves.
Good luck with your project :)

Collision Detection with Javascript, Canvas, and Alpha Detection

I'm currently working on a basic javascript game that has two sprites that are not to be collided together. However, basic bounding box collision won't suffice as there are portions of the sprites that are transparent and wouldn't 'count' as colliding. I found a solution to the problem that I am having, but I can't get it to work. What I would like to do is calculate the transparent portions of the sprites and make sure that if the transparent portions overlap, that there is no collision detected. Here is what I found that solves the problem.
http://blog.weeblog.net/?p=40#comments
/**
* Requires the size of the collision rectangle [width, height]
* and the position within the respective source images [srcx, srcy]
*
* Returns true if two overlapping pixels have non-zero alpha channel
* values (i.e. there are two vissible overlapping pixels)
*/
function pixelCheck(spriteA, spriteB, srcxA, srcyA, srcxB, srcyB, width, height){
var dataA = spriteA.getImageData();
var dataB = spriteB.getImageData();
for(var x=0; x<width; x++){
for(var y=0; y<height; y++){
if( (dataA[srcxA+x][srcyA+y] > 0) && (dataB[srcxB+x][srcyB+y] > 0) ){
return true;
}
}
}
return false;
}
And for calculating the image data:
/**
* creating a temporary canvas to retrieve the alpha channel pixel
* information of the provided image
*/
function createImageData(image){
$('binaryCanvas').appendTo('body');
var canvas = document.getElementById('binaryCanvas');
var ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0);
var canvasData = ctx.getImageData(0, 0, canvas.width, canvas.height);
var imageData = [image.width];
for(var x=0; x<image.width; x++){
imageData[x] = [image.height];
for(var y=0; y<image.height; y++){
var idx = (x + y * image.width) * 4;
imageData[x][y] = canvasData.data[idx+3];
}
}
$("#binaryCanvas").remove();
return imageData;
}
The problem is that I don't know how to implement this solution, or if this is the best solution to my problem. Is this what I'm looking for? And if so, where do I put these methods? The thing that I'm most confused about is what I should be passing to spriteA and spriteB. I've tried passing Images and I've tried passing the imageData returned from the pixelCheck method, but receiving the same error: that the object or image has no method 'getImageData'. What am I doing wrong?

Two things are wrong with this.
You made a function:
function createImageData(image){...}
But what you are calling is:
spriteA.getImageData();
spriteB.getImageData();
The dot notates a property of an object. You were trying to call a function that was never part to the objects. There are some simple fixes.
add the createImageData() function to your constructor :
function Sprite(){
...
this.createImageData = function(image){...};
...
}
or :
Sprite.createImageData = function(image{...};
or just call it correctly:
createImageData(spriteA.image); // or whatever the image is
Second, your function calls for an image parameter, but you aren't supplying one. Simply remember to supply the image when you call it. You could also remove the parameter and get the image from within the function.
Sort of like this:
function Sprite(){
...
this.createImageData = function(){
var image = this.image;
// or just use this.image, or whatever it is
...
}
...
}
Hope this helped.

Image appearing multiple times on canvas?

I'm drawing a simple dynamic canvas and I'm wondering how I can make the .png file in my drawImage method appear like 40 times at different places on my canvas at the same time?
Thanks beforehand! :)
Thank you all very much for your reply! This is as far as I've gotten now:
<script type="text/javascript">
var ctx;
var imgBg;
var imgDrops;
var x = 40;
var y = 0;
function setup() {
var canvas = document.getElementById('canvasRegn');
if (canvas.getContext) {
ctx = canvas.getContext('2d');
setInterval('draw();', 36);
imgBg = new Image();
imgBg.src = 'dimma.jpg';
imgDrops = new Image();
imgDrops.src = 'drop.png';
}
}
function draw() {
drawBackground();
for(var i=0; i <= 40; i++) {
ctx.drawImage (imgDrops, x, y);
y += 3;
if(y > 450)
y = -20;
x=Math.random()*600;
}
}
function drawBackground(){
ctx.drawImage(imgBg, 0, 0);
}
</script>
My problem is now that the images are jumping all over the place... I want them "falling" down slowly from above and coming back around:(

Have a look at this fiddle http://jsfiddle.net/joevallender/D83uC/10/
I made it to explain some basics of canvas to a friend recently. Although I'm using shapes instead of .png files I think the loop you are looking for is the same.
The key bit of code being this loop below
setInterval(function(){
// clear stage
context.clearRect(0, 0, width, height);
for(var i = 0; i < balls.length; i++) {
balls[i].move(balls);
}
}, 1000/FPS)
FPS is a variable, and .move() is a function that calculated new co-ordinates for and then re-draws the ball object.
I think it might simply not clearing the 'stage' context.clearRect(0, 0, width, height);
EDIT Perhaps that example had too much going on in it to be useful.
Please see a much earlier version http://jsfiddle.net/joevallender/D83uC/2 that simple animates the ball. The main point remains though, you need to clear the canvas if you don't want the 'trails'
Think of canvas like windows paint, not flash. The things you have drawn aren't editable objects. You need to redraw the whole thing each time. (Unless you use a JS library that makes things seem more like flash - but I'm guessing you want to learn without helper libraries at first)
As I said, I was explaining canvas to someone recently and you can see the various stages between the two links I've sent you by changing the number on the end of the URL http://jsfiddle.net/joevallender/D83uC/3
http://jsfiddle.net/joevallender/D83uC/4
etc.
EDIT2
Or if I've misunderstood, post a jsfiddle and let us know what is wrong with it

This is what you need: http://edumax.org.ro/extra/new/imagetut/
You can get the code here: http://edumax.org.ro/extra/new/imagetut/script.js
The relevant part is this:
function draw_image(){
//draw one image code
}
window.onload = function () {
for (var i=0;i<10;i++){
for (var j=0;j<10;j++){
draw_image(10+i*40, 10+j*40, 40, 40);
}
}
}
This code only explains the concept, it will not work by itself, for a full version check the link above.