Develop Reference

Preloading JS calculation prior to images loading

This is essentially a follow up question to this question:
Positioning a div relative to a fixed div with responsive content
So we are on the same page, it might help if you read through that article! :)
Thank you in advance.
I am using the Javascript from the linked article's answer for a separate project where the "color" div is now a "content" div and inside of my content div are roughly 30 images. I'm having no issue getting my "content" div to position correctly with the fixed div now, but the issue I'm having is when I incorporate the Javascript that positions the "content" div, it is calculating and positioning the "content" div AFTER everything in that div is loaded. So essentially the first image is getting hidden under the fixed header until all the images are loaded and then it will position it where it needs to be. I need it to position the "content" div prior to any of the images inside of the "content" div loading.
I have done research trying to figure out how to pick and choose which code gets read first, but all of the things I have tried haven't worked yet. My most recent attempt was putting the JS that positions the "content" div right below the "head" and before the "body" so it is read first, but it still doesn't position the "content" div until everything in it fully loads.
Where am I going wrong?
Here is a JSFiddle with the code: https://jsfiddle.net/8wkotamf/
Here is the Javascript (also in the JSFiddle):
window.addEventListener('load', function() {
var headerHeight = document.getElementById('header').clientHeight;
document.getElementById("content").style.paddingTop = headerHeight + "px";
var footerHeight = document.getElementById('footer').clientHeight;
document.getElementById("content").style.paddingBottom = footerHeight + "px";
}, true);
window.addEventListener('resize', function() {
var headerHeight = document.getElementById('header').clientHeight;
document.getElementById("content").style.paddingTop = headerHeight + "px";
var footerHeight = document.getElementById('footer').clientHeight;
document.getElementById("content").style.paddingBottom = footerHeight + "px";
}, true);
It will be nearly impossible to recreate the issue on the JSFiddle because since the images aren't actually embedded, it loads instantly and you can't tell that the positioning happens after they are all fully loaded. Hopefully all the code will make it an easy fix though! Fingers are crossed!
Thank you guys so much. I appreciate it immensely.

window.addEventListener('load' ... fires when everything in the window has loaded - the HTML document and all content including images. You want your function to run when the document is loaded, ie the DOM is ready, but not necessarily waiting till all other resources such as images are fully loaded.
If you don't need to support IE 8 or lower try document.addEventListener("DOMContentLoaded", function() {... instead. This will fire when the DOM is parsed and ready, not waiting till images etc are loaded. If you do need to support old IE versions use Jquery's $(document).ready().
See window.onload vs $(document).ready() for more information.

Wait for image added to CSS to load - JQuery

I have an image and I want to draw on it. To do that, I use jQuery to hide the image:
$("img").hide();
And then I create a canvas and put it in the same div with id drawing in the html. I then set the background of the canvas to be the same as the img src for the image I hid. This makes it look like an image but now it is actually a canvas with the image as it's background. I do this by:
$('#drawing > canvas').css('background-image','url('+$(".image img").attr('src')+')');
context.canvas.width = $("img").width();
context.canvas.height = $("img").height();
The issue I am having is that sometimes, the image isn't displayed in the canvas and the canvas is not the size of the image. I think it's probably because of some loading issue. How can I wait for the canvas to have the image displayed in the background for sure? Thank you
Edit: Note that in the DOM, the canvas always has the right src. It just doesn't display it
Edit 2: Here's the JSfiddle. Here, everything seems fine but I have a lot more going on in my code including fetching stuff from the server so it's slower there. Hope this helps you guys to understand the problem: http://jsfiddle.net/wL3ezLke/2/
Thanks again

You need to use:
$(function(){
// Code executed once the DOM is loaded.
});
Official documentation:
https://api.jquery.com/ready/

If I understand correctly your problem is knowing when the image loaded (from what you describe it could be a lot of other problems though).
To test if an image has loaded it's pretty simple.
var $img = $('#hiddenImg');
if ($img[0].complete) doCanvasStuff();
else {
$img.on('load', function(e) {
var $canvas = $('#drawCanvas');
$canvas.css({width: $img.width(), height: $img.height()});
//you can go ahead with the background image, but this is preferable
var ctx = $canvas[0].getContext("2d");
ctx.drawImage(img,0,0);
}
}
This should make sure you canvas loads an image only after it was loaded, or do canvas stuff right away if image was loaded, fiddle

Change
$('#drawing > canvas').css('background-image','url('+$(".image img").attr('src')+')');
context.canvas.width = $("img").width();
context.canvas.height = $("img").height();
to
context.canvas.width = $("img").width();
context.canvas.height = $("img").height();
$('#drawing > canvas').css('background-image','url('+$(".image img").attr('src')+')');
so the height and width are set before the image goes into the background.

Add Transparent Image on top of other Images with JavaScript

I need to add a Transparent image on top of all images on a page. The goal is if a user were to do a simple right click and save of an image, they would save the transparent image.
I do realize this is not a guaranteed method and that none exist to prevent image theft but simply a measure that a client wants added to prevent your average non tech person from saving images.
Using JavaScript I would like to find all images or all images within a certain Div.
Apply a new image overlay on top of these images that will have the same width and height of the image they are covering
I am not sure how to do this with JavaScript and was hoping someone would have a quick fix or example. I was unable to find anything so far on Google or SO. Appreciate any help
I have this JS which gets all images on a page so far...
// Get all images on a Page
function checkimages() {
var images = document.images;
for (var i=0; i<images.length; i++){
var img =images[i].src;
// Add new transparent image on top of this image
alert(img);
}
}

I would advise you to work with jQuery (or similar library) to keep things easier. I would even write a small jquery extension to make it easy to recycle the code, and apply it on any div (or other wrapper), wich child images you want to be overlayed.
My code would look something like this:
// jquery plugin to create overlays
// src is the url of the overlay image
// apply to any container that contains images to be overlayed
$.fn.overlayImages = function(src) {
// loop trough the images
$(this).find('img').each(function() {
// cache some variables
var $img = $(this);
var $parent = $img.parent();
// make the parent relative, if not yet absolute or fixed, for easy positioning
if ($parent.css('position') !== 'fixed' && $parent.css('position') !== 'absolute') {
$parent.css('position', 'relative');
}
// get the position of the image
var position = $img.position();
// clone the image
var $overlay = $img.clone();
// set the styling, based on the img, for exact positioning
$overlay.css({
top: position.top,
left: position.left,
position: 'absolute',
width: $img.width(),
height: $img.height()
});
// change the src attribute for the overlay
$overlay.attr('src', src);
// insert the overlay to the DOM
$overlay.insertAfter($img);
});
}
// when the DOM is loaded (not just ready, the images need to be there to copy their position and size)
$(window).load(function() {
// apply the overlay plugin to the wrapper of the images
$('#replace-images').overlayImages("http://www.riptideinnovations.com/images/watermark.png");
});
I added the step by step explanation inside the code as comments, but do feel free to ask if you want any further explanation.
I set up a small fiddle to demonstrate: http://jsfiddle.net/pP96f/6/

I don't know if this would help, but you could make your images all div's with backgrounds like this:
<div style="background-image: url('<your_image>');"></div>

Javascript - Image change an resize to full background

As a quick explanation, I created an image that resizes to fill the background using this function which works great:
function resize_bg(element_id){
$("#"+element_id).css("left","0");
var doc_width = $(window).width();
var doc_height = $(window).height();
var image_width = $("#"+element_id).width();
var image_height = $("#"+element_id).height();
var image_ratio = image_width/image_height;
var new_width = doc_width;
var new_height = Math.round(new_width/image_ratio);
if(new_height<doc_height){
new_height = doc_height;
new_width = Math.round(new_height*image_ratio);
var width_offset = Math.round((new_width-doc_width)/2);
$("#"+element_id).css("left","-"+width_offset+"px");
}
$("#"+element_id).width(new_width);
$("#"+element_id).height(new_height);
return true;
}
So no problem for the full background image. The problem appears, when I change the image source using Javascript. In other words, I have 1 image set as background but on hover of certain elements on the page, the image changes but it doesn't change the resize right. So the first image on load is resized and positioned correctly, but when I switch the image using .attr('src',newimg) the image is not resized correctly even though I call the function again.
Here is the code I use to change the image and resize it:
$('#menu_work li a').hover(function(){
$('#content').hide();
var img_src = $(this).next('img').attr('src');
$('#full_screen_img').attr('src', img_src );
resize_bg();
$('#full_screen_img').show();
},function(){
$('#full_screen_img').hide();
$('#content').show();
});
Thanks for any help.

It appears that you have left out the element_id argument when calling resize_bg() in the hover event handler. As a result, resize_bg() can't find the element you want to resize.

#maxedison is right, you forgot to pass the element id.
Another problem is that when you change the src, the new image might not be loaded yet, so you won't get the right dimensions in resize_bg until it is.
In that case you'll need to resize the image once it's loaded:
$('#full_screen_img').attr('src', img_src ).load(function() {
resize_bg('<ELEMENT_ID>');
});
resize_bg('<ELEMENT_ID>');
On another note, I'd recommend you change resize_bg to get a jQuery object instead of an id, or even write a plugin ($.fn.resize_bg) if it's a functionality you want to use often.

Programmatically Clip/Cut image using Javascript

Are there any documents/tutorials on how to clip or cut a large image so that the user only sees a small portion of this image? Let's say the source image is 10 frames of animation, stacked end-on-end so that it's really wide. What could I do with Javascript to only display 1 arbitrary frame of animation at a time?
I've looked into this "CSS Spriting" technique but I don't think I can use that here. The source image is produced dynamically from the server; I won't know the total length, or the size of each frame, until it comes back from the server. I'm hoping that I can do something like:
var image = getElementByID('some-id');
image.src = pathToReallyLongImage;
// Any way to do this?!
image.width = cellWidth;
image.offset = cellWidth * imageNumber;

This can be done by enclosing your image in a "viewport" div. Set a width and height on the div (according to your needs), then set position: relative and overflow: hidden on it. Absolutely position your image inside of it and change the position to change which portions are displayed.
To display a 30x40 section of an image starting at (10,20):
<style type="text/css">
div.viewport {
overflow: hidden;
position: relative;
}
img.clipped {
display: block;
position: absolute;
}
</style>
<script type="text/javascript">
function setViewport(img, x, y, width, height) {
img.style.left = "-" + x + "px";
img.style.top = "-" + y + "px";
if (width !== undefined) {
img.parentNode.style.width = width + "px";
img.parentNode.style.height = height + "px";
}
}
setViewport(document.getElementsByTagName("img")[0], 10, 20, 30, 40);
</script>
<div class="viewport">
<img class="clipped" src="/images/clipped.png" alt="Clipped image"/>
</div>
The common CSS properties are associated with classes so that you can have multiple viewports / clipped images on your page. The setViewport(…) function can be called at any time to change what part of the image is displayed.

In answer to :
Alas, JavaScript simply isn't capable of extracting the properties of the image you'd require to do something like this. However, there may be salvation in the form of the HTML element combined with a bit of server-side scripting.
...
< ? (open php)
$large_image = 'path/to/large_image';
$full_w = imagesx($large_image);
$full_h = imagesy($large_image);
(close php) ? >
This can be done in Javascript, just google a bit :
var newimage = new Image();
newimage.src = document.getElementById('background').src;
var height = newimage.height;
var width = newimage.width;
This generates a new image from an existing one and captures this way in java script the original height and width properties of the original image (not the one id'ed as background.
In answer to :
The width/height properties of the document's image object are read only. If you could change them, however, you would only squish the frames, not cut the frames up like you desire. The kind of image manipulation you want can not be done with client-side javascript. I suggest cutting the images up on the server, or overlay a div on the image to hide the parts you do not wish to display.
...
var newimage = new Image();
newimage.src = document.getElementById('background').src;
var height = newimage.height;
var width = newimage.width;
newimage.style.height = '200px';
newimage.style.width = '200px';
newimage.height = '200px';
newimage.width = '200px';
and if wanted :
newimage.setAttribute('height','200px');
The doubled newimage.style.height and newimage.height is needed in certain circumstances in order to make sure that a IE will understand in time that the image is resized (you are going to render the thing immediately after, and the internal IE processing is too slow for that.)
Thanks for the above script I altered and implemented on http://morethanvoice.net/m1/reader13.php (right click menu... mouseover zoom lent) correct even in IE , but as you will notice the on mousemove image processing is too fast for the old styled IE, renders the position but only once the image. In any case any good idea is welcome.
Thanks to all for your attention, hope that the above codes can help someone...
Claudio Klemp
http://morethanvoice.net/m1/reader13.php

CSS also defines a style for clipping. See the clip property in the CSS specs.

The width/height properties of the document's image object are read only. If you could change them, however, you would only squish the frames, not cut the frames up like you desire. The kind of image manipulation you want can not be done with client-side javascript. I suggest cutting the images up on the server, or overlay a div on the image to hide the parts you do not wish to display.

What spriting does is essentially position a absolutely-positioned DIV inside another DIV that has overflow:hidden. You can do the same, all you need to do is resize the outer DIV depending on the size of each frame of the larger image. You can do that in code easily.
You can just set the inner DIV's style:
left: (your x-position = 0 or a negative integer * frame width)px
Most JavaScript Frameworks make this quite easy.

Alas, JavaScript simply isn't capable of extracting the properties of the image you'd require to do something like this. However, there may be salvation in the form of the HTML <canvas> element combined with a bit of server-side scripting.
PHP code to go about extracting the width and height of the really large image:
<?php
$large_image = 'path/to/large_image';
$full_w = imagesx($large_image);
$full_h = imagesy($large_image);
?>
From here, you'd then load the image into a <canvas> element, an example of which is documented here. Now, my theory was that you may be able to extract pixel data from a <canvas> element; assuming that you can, you would simply make sure to have some form of definite divider between the frames of the large image and then search for it within the canvas. Let's say you found the divider 110 pixels from the left of the image; you would then know that each "frame" was 110 pixels wide, and you've already got the full width stored in a PHP variable, so deciphering how much image you're working with would be a breeze.
The only speculative aspect to this method is whether or not JavaScript is capable of extracting color data from a specified location within an image loaded into a <canvas> element; if this is possible, then what you're trying to accomplish is entirely feasible.

I suppose you want to take a thumbnail for your image. You can use ImageThumbnail.js that created from prototype library in this way:
<script type="text/javascript" src="prototype.js"></script>
<script type="text/javascript" src="ImageThumbnail.js"></script>
<input type="file" id="photo">
<img src="empty.gif" id="thumbnail" width="80" height="0">
<script type="text/javascript">
<!--
new Image.Thumbnail('thumbnail', 'photo');
//-->
</script>
for more information

try use haxcv library haxcv js by simple functions
go to https://docs.haxcv.org/Methods/cutImage to read more about his library
var Pixels = _("img").cutImage (x , y , width , height );
_("img").src (Pixels.src);
// return cut image
but try to include library first