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Canvas image masking / overlapping
(2 answers)
Closed 8 years ago.
I've two images, first is large image which is placed first and then another image is placed on it. Now I want to cut out area of second image over the first.
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var first_img = new Image(),
second_img = new Image();
first_img.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
second_img.src = 'http://tux.crystalxp.net/png/mimipunk-tux-cartoon-1820.png';
var draw = function(){
second_img.onload = function() {
context.drawImage(second_img, 50, 50);
};
first_img.onload = function() {
context.drawImage(first_img, 50, 50);
};
};
draw();
Here is the fiddle.
edit
second image is png and have irregular shape, I want the first image having transparency of area of the first image
The drawImage function provides parameter for that
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D#drawImage()
Related
I have a whiteboard option during a video call in my web application built in angularjs (1.x). Users can draw above the video in a canvas element. I need to take a screenshot of the current video position and drawing on the canvas. I am able to get the video frame as base64 URL(second image) and canvas drawing as base64 URL(third image) separately. But I need to get it as a combined single image in base64 URL like the first image.
HTML
<img id="img1">
<img id="img2">
<canvas id="mergedImage" width="382" height="510"></canvas>
JS
var drawingImg1 = document.getElementById('img1');
drawingImg1.setAttribute('src', img1);
var drawingImg2 = document.getElementById('img2');
drawingImg2.setAttribute('src', img2);
var c = document.getElementById('mergedImage');
var width = 382;
var height = 510;
var ctx = c.getContext("2d");
var imageObj2 = new Image();
imageObj2.onload = function (){
ctx.drawImage(imageObj2, 0, 0, width, height);
var imageObj1 = new Image();
imageObj1.onload = function (){
imageObj1.style.objectFit = "cover";
ctx.drawImage(imageObj1, 0, 0,width, height);
};
imageObj1.src = img2;
};
imageObj2.src = img1;
I tried the above code and it gives the output as video frame image only not the drawing included.
I want the output as the first image. Please, someone, guide me to do this.
I have created a jsfiddle here
The following solution worked for me to overlap 2 images and got a single image in a canvas.
Link
I have 3 canvases. I crop a region from canvas1 and display it on canvas 2 . Then I want to convert canvas 2 image to a URL and to see if can convert that URL back to a image. I want it to be displayed in canvas c4.Any help is appreciated.
// image is drawn here , I want this image to be converted to a dataURL
//then back to image and display it in canvas c4
var c2 = document.getElementById("area_c2");
var ctx = c.getContext("2d");
var ctx2 = c2.getContext("2d");
image_src.width = c2.width;
image_src.height = c2.height;
ctx2.drawImage(image_src, 0, 0,image_src.width, image_src.height);
var c4 = document.getElementById("area_c4");
var ctx4 = c4.getContext("2d");
var dataURL = c2.toDataURL();
var myImg = new Image;
myImg.src = dataURL;
myImg.width = c4.width;
myImg.height = c4.height;
ctx4.drawImage(myImg, 0, 0, image_src.width, image_src.height); //Image //is not displayed here , I want the image to take the size of the canvas
<canvas id ="area_c2" style="width:300px;height:300px;border:3px solid
black;z-index:1" >
</canvas>
<canvas id ="area_c4" style="width:300px;height:300px;border:3px solid
black;z-index:1;background:red">
</canvas>
The reason your solution did not work is because you did not wait for the onload event. Before the onload event the image will not render.
To convert a canvas to a data URL is simple, use the .toDataURL(); method of the canvas element.
Then to convert the data URL back to an canvas image, you would first have to create an Image element and set its source as the dataURL. After getting the image onload event you can draw the image onto the canvas. This could be accomplished as shown:
Assuming the data URL is in a variable called dataURL, the canvas context is in a variable called ctx.
var img = new Image;
img.onload = () => { ctx.drawImage(img, 0, 0); };
img.src = dataURL;
The final solution would go like this:
var cdata2 = c2.toDataURL();
var cimg2 = new Image;
cimg2.onload = () => { ctx4.drawImage(cimg2, 0, 0, c4.width, c4.height); };
PS: You don't have to scale the image object by specifying the width and height, the canvas will do that when you specify the destination width and height.
I'm trying to get a base64 version of a canvas in HTML5.
Currently, the base64 image that I get from the canvas is blank.
I have found similar questions for example this one:
HTML Canvas image to Base64 problem
However, the issue that I have is that because I am adding 2 images in the canvas, I cannot use the example provided in those answers as most of them are using single image.
I understand that I have to wait until the canvas image is loaded properly before trying to get the base64 image. But I don't know how in my case.
This is my code:
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
context.canvas.width = 1000;
context.canvas.height = 1000;
var imageObj1 = new Image();
imageObj1.src = "http://www.offthegridnews.com/wp-content/uploads/2017/01/selfie-psuDOTedu.jpg";
imageObj1.onload = function() {
context.drawImage(imageObj1, 0, 180, canvas.width, canvas.height);
};
var imageObj2 = new Image();
imageObj2.src = "http://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg"
imageObj2.onload = function() {
context.drawImage(imageObj2, 0, 0, canvas.width, 180);
};
// get png data url
//var pngUrl = canvas.toDataURL();
var pngUrl = canvas.toDataURL('image/png');
// get jpeg data url
var jpegUrl = canvas.toDataURL('image/jpeg');
$('#base').val(pngUrl);
<div class="contents" style="overflow-x:hidden; overflow-y:scroll;">
<div style="width:100%; height:90%;">
<canvas id="myCanvas" class="snap" style="width:100%; height:100%;" onclick="takephoto()"></canvas>
</div>
</div>
<p>
This is the base64 image
</p>
<textarea id="base">
</textarea>
and this is a working FIDDLE:
https://jsfiddle.net/3p3e6Ldu/1/
Can someone please advice on this issue?
Thanks in advance.
EDIT:
As suggested in the comments bellow, i tried to use a counter and when the counter reaches a specific number, I convert the canvas to base64.
Like so:https://jsfiddle.net/3p3e6Ldu/4/
In both your examples (the one from the question and the one from the comments), the order of commands does not really respect the async nature of the task at hand.
In your later example, you should put the if( count == 2 ) block inside the onload callbacks to make it work.
However, even then you will run into the next problem: You are loading the images from different domains. You can still draw them (either into the canvas or using an <img> tag), but you are not able to access their contents directly. Not even with the detour of using the <canvas> element.
I changed to code so it would work, if the images are hosted on the same domain. I also used a function to load the image and promises to handle the callbacks. The direct way of using callbacks and a counting variable, seem error-prone to me. If you check out the respective fiddle, you will notice the SecurityError shown. This is the result of the aforementioned problem with the Same-Origin-Policy I mentioned.
A previous question of mine in a similar direction was about how to detect, if I can still read the contents of a <canvas> after adding some images.
const canvas = document.getElementById('myCanvas');
const context = canvas.getContext('2d');
context.canvas.width = 1000;
context.canvas.height = 1000;
// function to retrieve an image
function loadImage(url) {
return new Promise((fulfill, reject) => {
let imageObj = new Image();
imageObj.onload = () => fulfill(imageObj);
imageObj.src = url;
});
}
// get images
Promise.all([
loadImage("http://www.offthegridnews.com/wp-content/uploads/2017/01/selfie-psuDOTedu.jpg"),
loadImage("http://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg"),
])
.then((images) => {
// draw images to canvas
context.drawImage(images[0], 0, 180, canvas.width, canvas.height);
context.drawImage(images[1], 0, 0, canvas.width, 180);
// export to png/jpg
const pngUrl = canvas.toDataURL('image/png');
const jpegUrl = canvas.toDataURL('image/jpeg');
// show in textarea
$('#base').val(pngUrl);
})
.catch( (e) => alert(e) );
Allright, this is going to be quite hard to explain but I will try to explain my problem as clear as possible.
I currently have a HTML5 Canvas with a couple of preloaded shapes which I changed to images. I am using a modified version of this example. On the canvas I have now I can drag, drop and select the shapes which are preloaded.
I have implemented an option to upload a new image to the canvas using this:
var imageLoader = document.getElementById('uploader');
imageLoader.addEventListener('change', handleImage, false);
var canvas = document.getElementById('canvas1');
var ctx = canvas.getContext('2d');
function handleImage(e){
var reader = new FileReader();
reader.onload = function(event){
var imgNew = new Image();
imgNew.onload = function(){
ctx.drawImage(imgNew, 0, 0);
}
imgNew.src = event.target.result;
}
reader.readAsDataURL(e.target.files[0]);
}
This will now just put an image in the top-left corner of the canvas and when I select another shape this image will disappear. All shapes that are already in the canvas are in an array called 'shapes'. In the example there is an option to add a new shape when you dubbelclick on the canvas, this is the code:
// double click for making new shapes
canvas.addEventListener('dblclick', function(e) {
var mouse = myState.getMouse(e);
myState.addShape(new Shape(mouse.x - 10, mouse.y - 10, 20, 20, 'rgba(0,255,0,.6)'));
}, true);
This is the addShape part which is called when double clicking:
CanvasState.prototype.addShape = function(shape) {
this.shapes.push(shape);
this.valid = false;
}
What I am trying to do is when I upload an image using the file upload the image should be added to the 'shapes' array so I can also drag,drop and select the uploaded image. (basically I need to do the same with the uploaded image as I can do with the preloaded ones) My guess is that it should be done something like the doubleclick method and addShape but I can't get this to work.
A working version of my current version can be found here:
http://codepen.io/anon/pen/qLuCy/
If anyone knows how I can get this to work it would be great!
first the result, here : http://codepen.io/anon/pen/qBmnC/
Your shape object can only draw one single hard-coded image, so change this and have shape be either a color or an image. In fact for this you just have to change shape.draw since the 'type' of the fill does not matter in the constructor.
So just test the fill and draw accordingly :
// Tekent de Shape
Shape.prototype.draw = function(ctx) {
var locx = this.x;
var locy = this.y;
var fill = this.fill ;
if (typeof fill == 'string' )
{
ctx.fillStyle = fill;
ctx.fillRect(locx, locy, this.w, this.h);
}
else
{
ctx.drawImage(fill, locx, locy, this.w, this.h);
}
}
then on image load i just add a shape that has this image as fill :
function handleImage(e){
var reader = new FileReader();
reader.onload = function(event){
var imgNew = new Image();
imgNew.onload = function(){
s.addShape(new Shape(60,60,imgNew.width,imgNew.height,imgNew));
}
imgNew.src = event.target.result;
}
reader.readAsDataURL(e.target.files[0]);
}
I had to put the shape collection, s, as a global var.
You can also see that i used the same scheme in init() to
add an image once it's loaded.
After being a long time lurker, this is my first post here! I've been RTFMing and searching everywhere for an answer to this question to no avail. I will try to be as informative as I can, hope you could help me.
This code is for my personal webpage.
I am trying to implement some sort of a modern click-map using HTML5 and jQuery.
In the website you would see the main image and a hidden canvas with the same size at the same coordinates with this picture drawn into it.
When the mouse hovers the main picture, it read the mouse pixel data (array of r,g,b,alpha) from the image drawn onto the canvas. When it sees the pixel color is black (in my case I only check the RED value, which in a black pixel would be 0) it knows the activate the relevant button.
(Originally, I got the idea from this article)
The reason I chose this method, is for the page to be responsive and dynamically change to fit different monitors and mobile devices. To achieve this, I call the DrawCanvas function every time the screen is re-sized, to redraw the canvas with the new dimensions.
Generally, this works OK. The thing is ,there seems to be an inconsistent behavior in Chrome and IE(9). When I initially open the page, I sometimes get no pixel data (0,0,0,0), until i re-size the browser. At first I figured there's some loading issues that are making this happen so I tried to hack it with setTimeout, it still doesn't work. I also tried to trigger the re-size event and call the drawCanvas function at document.ready, still didn't work.
What's bothering me is most, are the inconsistencies. Sometimes it works, sometimes is doesn't. Generally, it is more stable in chrome than in IE(9).
Here is the deprecated code:
<script type="text/javascript">
$(document).ready(function(){setTimeout(function() {
// Get main image object
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
// Create a hidden canvas the same size as the main image and append it to main div
var canvas = document.createElement('canvas');
canvas.height = mapWrapper.clientHeight;
canvas.width = mapWrapper.clientWidth;
canvas.fillStyle = 'rgb(255,255,255)';
canvas.style.display = 'none';
canvas.id = 'hiddencvs';
$('#map_wrapper').append(canvas);
// Draw the buttons image into the canvas
drawCanvas(null);
$("#map_wrapper").mousemove(function(e){
var canvas = document.getElementById('hiddencvs');
var context = canvas.getContext('2d');
var pos = findPos(this);
var x = e.pageX - pos.x;
var y = e.pageY - pos.y;
// Get pixel information array (red, green, blue, alpha)
var pixel = context.getImageData(x,y,1,1).data;
var red = pixel[0];
var main_img = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
if (red == 0)
{
...
}
else {
...
}
});
},3000);}); // End DOM Ready
function drawCanvas(e)
{
// Get context of hidden convas and set size according to main image
var cvs = document.getElementById('hiddencvs');
var ctx = cvs.getContext('2d');
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
cvs.width = mapWrapper.clientWidth;
cvs.height = mapWrapper.clientHeight;
// Create img element for buttons image
var img = document.createElement("img");
img.src = "img/main-page-buttons.png";
// Draw buttons image inside hidden canvas, strech it to canvas size
ctx.drawImage(img, 0, 0,cvs.width,cvs.height);
}
$(window).resize(function(e){
drawCanvas(e);
}
);
function findPos(obj)
{
...
}
</script>
I'd appreciate any help!
Thanks!
Ron.
You don't wait for the image to be loaded so, depending on the cache, you may draw an image or not in the canvas.
You should do this :
$(function(){
var img = document.createElement("img");
img.onload = function() {
var mapWrapper = document.getElementById('map_wrapper').getElementsByTagName('img').item(0);
...
// your whole code here !
...
}
img.src = "img/main-page-buttons.png";
});