I am using canvas drawImage() twice. One is a background image, and the other is some kind of 'mask' used to get the wanted image from the background.
The resulting image is the part of the background that's behind the overlay. I want to save that part of the image. Here's an example where I get the Hulk's face -- I want to create an image out of that face and save it on my server. Saving this image as is (right-click save image as) takes the full size of the canvas, and not just the wanted face.
http://jsfiddle.net/s248a76w/
HTML looks like:
<canvas id="canvas" width="900" height="500"></canvas>
JS:
var canvas = document.getElementById("canvas");
if (canvas.getContext) {
var ctx = canvas.getContext("2d");
//Load the images to be blended
var userPic = new Image;
userPic.src = "http://img2.wikia.nocookie.net/__cb20090320033607/marvelmovies/images/5/5e/The-hulk-2003.jpg";
var maskTemplate = new Image;
maskTemplate.src = "http://www.botaiusti.com/img/1.svg";
//Set the globalCompositeOperation to lighter
// ctx.globalCompositeOperation = "lighter";
ctx.globalCompositeOperation = "destination-atop";
//Draw the source image
userPic.onload = function() {
ctx.drawImage(userPic, 100, 00);
}
//Draw the destination image
maskTemplate.onload = function() {
ctx.drawImage(maskTemplate, 340, 70);
}
}
Related
I have an image which is loaded from server and contains 4 bands in RGB format. Because of the alpha band which causes the image to look transparently, then I need to crop this alpha band and save it as JPEG format. One way I can think is using canvas, however, I'm not sure it possible or not as the test I did show that the image is the same.
<p>Image to use:</p>
<img id="scream" src="data/with_alpha.png" alt="The Scream">
<p>Canvas:</p>
<canvas id="myCanvas" width="2400" height="2097" style="border:1px solid #d3d3d3;">
</canvas>
window.onload = function() {
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.globalAlpha = 1;
var img = document.getElementById("scream");
ctx.drawImage(img, 0, 0);
// This ones does not do anything to remove the alpha band
c.toDataURL("image/jpeg", 0.0);
}
I found the answer from http://sunbox.github.io/image-to-canvas-to-image/ it can be like this to convert canvas to image:
// Get the image
var sampleImage = document.getElementById("ringoImage"),
canvas = convertImageToCanvas(sampleImage),
image = convertCanvasToImage(canvas);
// Actions
document.getElementById("pngHolder").appendChild(image);
// Converts image to canvas; returns new canvas element
function convertImageToCanvas(image) {
var canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;
canvas.getContext("2d").drawImage(image, 0, 0);
return canvas;
}
// Converts canvas to an image
function convertCanvasToImage(canvas) {
var image = new Image();
image.src = canvas.toDataURL("image/jpeg");
return image;
}
I am working on a dynamic web map with HTML5 and canvas.
I loaded the map in PNG format, but whenever I try to draw on it, it draws behind the map like this:
I need it to draw over the white squares.
This is my code:
window.onload = function() {
var canvas = document.getElementById('canvas1');
var ctx = canvas.getContext('2d');
var img = new Image();
img.onload = function() {
ctx.drawImage(img, 0, 0);
}
img.src = "img/plano.png"; //transparent png
ctx.fillRect(1039, 150, 50, 50);
}
Thanks in advance!
The fillRect needs to happen after the image has loaded. The image loads asynchronously so the only safe place for this to happen is after the drawImage...
img.onload = function() {
ctx.drawImage(img, 0, 0);
ctx.fillRect(200,200,50,50); // Draw rect after image
}
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<canvas id="spriteCanvas" width="500" height="500">
<img id="coin" width="440" height="40" src="coin.png">
</canvas>
</body>
</html>
I tried placing an image inside a canvas element, but it won't display on the browser. I know the image tag works because it's displayed if I place it outside of the canvas element. Also, if I inspect the canvas element, I can see that the image is inside, but its dimensions are 0 by 0. Can somebody explain why this isn't working?
EDIT: My original code added the image through javascript, but it wouldn't show on the canvas. It was giving me the same problems as above. But I just realized I was missing "onload".
original code:
var coinImage = new Image();
coinImage.src = "coin.png";
var sCanvas = document.getElementById('spriteCanvas');
function Sprite(canvas, width, height, image){
this.context = canvas.getContext("2d");
this.width = width;
this.height = height;
this.image = image;
}
Sprite.prototype.render = function() {
var that = this;
this.context.drawImage(that.image, 0, 0);
}
function init() {
var coin = new Sprite(sCanvas, 100, 100, coinImage);
coin.render();
}
init();
editted code:
var coinImage = new Image();
coinImage.src = "coin.png";
var sCanvas = document.getElementById('spriteCanvas');
function Sprite(canvas, width, height, image){
this.context = canvas.getContext("2d");
this.width = width;
this.height = height;
this.image = image;
}
Sprite.prototype.render = function() {
var that = this;
this.context.drawImage(that.image, 0, 0);
}
coinImage.onload = function () {
var coin = new Sprite(sCanvas, 100, 100, coinImage);
coin.render();
}
This isn't how a <canvas> tag works. If you want your image to appear in your canvas, you will have to use JavaScript to place the pixels of the image into your canvas.
<canvas>s are exactly what they state: canvases. They are an element for you to draw on programmatically. If you just want to display an image on a page, you don't need a canvas, you just need the <img> tag. In fact, elements should not be placed in <canvas>.
Take a look at CanvasRenderingContext2D.drawImage() and this tutorial: HTML5 Canvas Image Tutorial.
And this snippet:
var canvas = document.getElementById("painting"),
ctx = canvas.getContext("2d"),
image = new Image();
image.onload = function() {
ctx.drawImage(image, 30, 50);
};
image.src = "http://cdn.sstatic.net/Sites/stackoverflow/img/sprites.png?v=10a9e8743fb0";
<canvas id="painting" width="300" height="300"></canvas>
To draw an image on a canvas, use the following method:
drawImage(image,x,y)
If the image you want to draw is not in the DOM already you can load an image directly from a URL with a few lines of javascript.
function loadAndDrawImage(url)
{
// Create an image object. This is not attached to the DOM and is not part of the page.
var image = new Image();
// When the image has loaded, draw it to the canvas
image.onload = function()
{
// draw image...
}
// Now set the source of the image that we want to load
image.src = url;
}
loadAndDrawImage("http://www---.png");
This question already has an answer here:
HTML Canvas: Drawing grid below a plot
(1 answer)
Closed 6 years ago.
I have a canvas, and I want to use drawImage to draw an image behind the current content on the canvas.
Due to the fact that there is content already on the canvas (I'm using Literally Canvas to create a canvas containing an image, so I can't really draw the image first), I cannot use drawImage before I render the rest of my content.
Is it possible to drawImage behind all other content on a canvas?
Yes you can just use globalCompositeOperation destination-over, but note that your first image needs some transparency, otherwise, you will obviously not see anything :
var img1 = new Image();
var img2 = new Image();
var loaded = 0;
var imageLoad = function(){
if(++loaded == 2){
draw();
}
};
img1.onload = img2.onload = imageLoad;
var draw = function(){
var ctx = c.getContext('2d');
ctx.drawImage(img1, 100,100);
// wait a little bit before drawing the background image
setTimeout(function(){
ctx.globalCompositeOperation = 'destination-over';
ctx.drawImage(img2, 0,0);
}, 500);
}
img1.src = "https://dl.dropboxusercontent.com/s/4e90e48s5vtmfbd/aaa.png";
img2.src = "https://picsum.photos/200/200";
<canvas id="c" width="200" height="200"></canvas>
Sorry about the previous post, I didn't properly read your post
Perhaps you could save the canvas, draw your image, and then reload the old content on top of your drawn image? Here's some JS psuedocode:
var imgData=ctx.getImageData(0,0,canvas.width,canvas.height);
ctx.drawImage('Your Image Watermark Stuff');
ctx.putImageData(imgData,0,0);
You can use KonvaJS. And then use layers for it.
<!DOCTYPE html>
<html>
<head>
<script src="https://cdn.rawgit.com/konvajs/konva/0.13.0/konva.min.js"></script>
<meta charset="utf-8">
<title>Konva Rect Demo</title>
<style>
body {
margin: 0;
padding: 0;
overflow: hidden;
background-color: #F0F0F0;
}
</style>
</head>
<body>
<div id="container"></div>
<script>
var width = window.innerWidth;
var height = window.innerHeight;
var stage = new Konva.Stage({
container: 'container',
width: width,
height: height
});
var layer = new Konva.Layer();
var imageObj = new Image();
imageObj.onload = function() {
var baseImage = new Konva.Image({
x: 50,
y: 50,
width: width,
height: height,
image: image
});
// add the shape to the layer
layer.add(rect);
// add the layer to the stage
stage.add(layer);
};
imageObj.src = 'url to your image'
</script>
</body>
</html>
A simple solution would be to use another canvas behind the first one.
Normally canvas pixels are initialized to transparent black and therefore are perfectly see-through.
If your first canvas is created opaque instead the only other option I can think to is
create a temporary canvas of the same size
draw your image in this temporary canvas
get the ImageData object of both the temporary canvas and of the original canvas
copy from the temporary canvas to the original canvas only where the original canvas is not set at the background color
In code:
var tmpcanvas = document.createElement("canvas");
tmpcanvas.width = canvas.width;
tmpcanvas.height = canvas.height;
var temp_ctx = tmpcanvas.getContext("2d");
// ... draw your image into temporary context ...
var temp_idata = temp_ctx.getImageData(0, 0, canvas.width, canvas.height);
var temp_data = temp_idata.data;
// Access the original canvas pixels
var ctx = canvas.getContext("2d");
var idata = ctx.getImageData(0, 0, canvas.width, canvas.height);
var data = idata.data;
// Find the background color (here I'll use first top-left pixel)
var br_r = data[0], bg_g = data[1], bg_b = data[2];
// Replace all background pixels with pixels from temp image
for (var i=0,n=canvas.width*canvas.height*4; i<n; i+=4) {
if (data[i] == bg_r && data[i+1] == bg_g && data[i+2] == bg_b) {
data[i] = tmp_data[i];
data[i+1] = tmp_data[i+1];
data[i+2] = tmp_data[i+2];
data[i+3] = tmp_data[i+3];
}
}
// Update the canvas
ctx.putImageData(idata, 0, 0);
this approach however will have a lower quality if the original canvas graphics has been drawn with antialiasing or if pixels of the background color are also used in the image (e.g. an object on #FFF white background where object highlights are also #FFF). Another problem is if the background color is not a perfectly uniform RGB value (this will happen if the image has been compressed with a lossy algorithm like jpeg).
All these problems could be mitigated with more sophisticated algorithms like range matching, morphological adjustments and color-to-alpha conversions (basically the same machinery used for chroma-keying).
I'm looking for a way to overlay the visible areas of a transparent black silhouette PNG file with a certain pattern.
Is there a CSS, JavaScript, mixed solution for this?
Example: I have one image of a weapon silhoutte and a set of camo patterns which I want to lay over the weapon.
Demo: In Photoshop the same result is givin when selecting layer mask > overlay.
Ps: my question is similar to this thread: Silhouette a PNG image using CSS except I am looking for the exact opposite.
You can do this using canvas with globalCompositeOperation set to destination-in. For example
var canvas = document.createElement('canvas');
canvas.width = 250;
canvas.height = 250;
var canvas_context = canvas.getContext("2d");
var img = new Image();
img.onload = function(){
var msk = new Image();
msk.onload = function(){
canvas_context.drawImage(img, 0, 0);
canvas_context.globalCompositeOperation = "destination-in";
canvas_context.drawImage(msk, 0, 0);
canvas_context.globalCompositeOperation = "source-over";
};
msk.src = 'silhouette.png';
}
img.src = 'pattern.jpg';
document.body.appendChild(canvas);