I'm currently attempting to dynamically retrieve the location of an image and load it into a html5 canvas. The canvas is in a bootstrap responsive environment hence I never know the actual width of the canvas until the document is ready. Using the code below I've managed to resize the image and canvas to what I require but the image quality is very poor. I'm thinking it is because of the vast reduction in size but again unfortunately I have no control over the size of the original image uploaded.
function load_canvas_image(photo_type, vehicle_index) {
image_location = get_image_location(photo_type, vehicle_index);
var img = new Image();
img.src = image_location;
var canvas = $("#myCanvas")[0];
var canvas_context = canvas.getContext("2d");
canvas_width = canvas.width;
canvas_height = canvas.height;
img.onload = function() {
cur_height = img.height;
cur_width = img.width;
aspect_ratio = cur_width/canvas_width;
new_height = cur_height/aspect_ratio;
canvas.height = new_height;
canvas_context.drawImage(img, 0, 0, canvas.width, canvas.height);
}
};
$(document).on("change", "#add_damage_location", function() {
load_canvas_image($(this).val(), $('#vehicle_index').val());
});
I'm hoping someone knows how to use a similar method to reduce the width and height of an image using a similar method to the above whilst keeping the quality of the image as good as possible, I know to expect some reduction in quality but in it's current state it is unusable.
Thanks in Advance
Related
I'm trying to use the Incode functionality to resize an image in LogicApps.
I'm not sure if this is possible with the absence of HTML.
var inputImage = workflowContext.actions.GetFileContent.outputs.body.$content;
function resize_image(imagesrc)
{
let image = new Image();
var base = "data:image/png;base64,";
image.src = base.concat(imagesrc);
image.onload = () => {
let width = image.width;
let height = image.height;
let canvas = document.createElement('canvas');
canvas.width = newWidth ;
canvas.height = newHeight;
let context = canvas.getContext('2d');
context.drawImage(image, 450, 0, newWidth-500, newHeight);
}
return image;
}
return resize_image(inputImage);
The error I receive
The inline code action 'JavaScriptCode' execution failed, with error 'Image is not defined'.
$content is the image in Base64, for example, starts like this:
iVBORw0KGgoAAAANSUhEUgAACFwAAAMMCAYAAABkSiF3...
Inline code can only perform some simple JavaScript operations, it may not be able to install canvas.
You can create an Azure Function App to resize your image.
For more details, you can refer to Call functions from Azure Logic Apps.
I have created a basic shape in HTML canvas element which works fine.
The problem occurs when I resize the canvas, all the drawing in the canvas disappears. Is this the normal behavior? or is there a function that can be used to stop this?
One way to fix this could be to call drawing function again on canvas resize however this may not be very efficient if there is huge content to be drawn.
What's the best way?
Here is the link to sample code https://gist.github.com/2983915
You need to redraw the scene when you resize.
setting the width or height of a canvas, even if you are setting it to the same value as before, not only clears the canvas but resets the entire canvas context. Any set properties (fillStyle, lineWidth, the clipping region, etc) will also be reset.
If you do not have the ability to redraw the scene from whatever data structures you might have representing the canvas, you can always save the entire canvas itself by drawing it to an in-memory canvas, setting the original width, and drawing the in-memory canvas back to the original canvas.
Here's a really quick example of saving the canvas bitmap and putting it back after a resize:
http://jsfiddle.net/simonsarris/weMbr/
Everytime you resize the canvas it will reset itself to transparant black, as defined in the spec.
You will either have to:
redraw when you resize the canvas, or,
don't resize the canvas
One another way is to use the debounce if you are concerned with the performance.
It doesnt resize or redraw every position you are dragging. But it will resize only when the it is resized.
// Assume canvas is in scope
addEventListener.("resize", debouncedResize );
// debounce timeout handle
var debounceTimeoutHandle;
// The debounce time in ms (1/1000th second)
const DEBOUNCE_TIME = 100;
// Resize function
function debouncedResize () {
clearTimeout(debounceTimeoutHandle); // Clears any pending debounce events
// Schedule a canvas resize
debounceTimeoutHandle = setTimeout(resizeCanvas, DEBOUNCE_TIME);
}
// canvas resize function
function resizeCanvas () { ... resize and redraw ... }
I had the same problem. Try following code
var wrapper = document.getElementById("signature-pad");
var canvas = wrapper.querySelector("canvas");
var ratio = Math.max(window.devicePixelRatio || 1, 1);
canvas.width = canvas.offsetWidth * ratio;
canvas.height = canvas.offsetHeight * ratio;
It keeps the drawing as it is
One thing that worked for me was to use requestAnimationFrame().
let height = window.innerHeight;
let width = window.innerWidth;
function handleWindowResize() {
height = window.innerHeight;
width = window.innerWidth;
}
function render() {
// Draw your fun shapes here
// ...
// Keep this on the bottom
requestAnimationFrame(render);
}
// Canvas being defined at the top of the file.
function init() {
ctx = canvas.getContext("2d");
render();
}
I had the same problem when I had to resize the canvas to adjust it to the screen.
But I solved it with this code:
var c = document.getElementById('canvas');
ctx = c.getContext('2d');
ctx.fillRect(0,0,20,20);
// Save canvas settings
ctx.save();
// Save canvas context
var dataURL = c.toDataURL('image/jpeg');
// Resize canvas
c.width = 50;
c.height = 50;
// Restore canvas context
var img = document.createElement('img');
img.src = dataURL;
img.onload=function(){
ctx.drawImage(img,20,20);
}
// Restote canvas settings
ctx.restore();
<canvas id=canvas width=40 height=40></canvas>
I also met this problem.but after a experiment, I found that Resizing the canvas element will automatically clear all drawings off the canvas!
just try the code below
<canvas id = 'canvas'></canvas>
<script>
var canvas1 = document.getElementById('canvas')
console.log('canvas size',canvas1.width, canvas1.height)
var ctx = canvas1.getContext('2d')
ctx.font = 'Bold 48px Arial'
var f = ctx.font
canvas1.width = 480
var f1 = ctx.font
alert(f === f1) //false
</script>
First of all I've looked through all the similar questions and tried multiple times to get this to work but have had no luck. I'm trying to take a signature that I capture via a canvas element on either a desktop or mobile device and resize it. Because the device sizes are different, so is my canvas for each device. Before I save the signature image, I want to resize it so that all my saved images are the same size.
Here is the code I currently have.
resize(){
var image = new Image();
var t = this;
image.onload = function(){
image.width = '100px';
image.height = 'auto';
t.canvas.width = image.width;
t.canvas.height = image.height;
t.ctx.drawImage(image, 0,0);
console.log(t.canvas.toDataURL());
return t.canvas.toDataURL();
}
image.src = this.canvas.toDataURL("image/png");
}
This code doesn't produce any errors, however after I "resize" the image, canvas.toDataURL(); returns "data:," rather than the image. Before I attempt the resize, I am able to grab a valid png image from this.canvas.toDataURL();.
I also tried changing the style width and height but this causes my canvas to reset which causes my image to disappear.
this.canvas.style.width = 100;
this.canvas.style.height = 100;
Any help would be appreciated.
UPDATE
I've put together a Codepen here that shows the original image on the left and on the right you will see that same image that I have attempted to resize with JavaScript. I added an orange background for readability purposes only.
Codepen
The only problem I could spot in your codepen is this line
context.drawImage(img,200,150);
The values 200 and 150 specify the position on the canvas where the image should be drawn. This essentially draws it ouside the visible area.
If you change it to
context.drawImage(img, 0, 0, img.width, img.height);
it works as the second pair of numbers is the clipping area.
Here's an example:
var canvas = document.getElementById("can");
var context = canvas.getContext('2d');
var img = new Image();
img.onload = function() {
canvas.width = 200;
canvas.height = 150;
img.width = 200;
img.height = 150;
context.drawImage(img, 0, 0, img.width, img.height);
}
img.src = document.getElementById("theImg").src;
canvas {
background: orange;
}
<img id="theImg" src="data:image/png;base64,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">
<canvas id="can"></canvas>
I just wrote a little script that resizes uploaded images into canvas thumbnails. While resizing images it slows down the rest of the web page.
I just discovered web workers which I was hoping I could use to alleviate this problem by moving the resizing task into the background.
The MDN article says you can't access the DOM, which is fine, but I was hoping I could still create DOM elements and pass a canvas back.
However, the Image object doesn't even seem to exist from inside my worker thread. Chrome tells me:
Uncaught ReferenceError: Image is not defined resize_image.js:4
So... is there a way to do this without using those elements? I just want to manipulate the image data and then pass it back to the main thread in a usable form. I don't really care about the DOM elements themselves, but I'm not sure if the JS API gives me any other options?
My current resize method, which requires access to DOM elements:
resizeImage: function(file, width, height, callback) {
var reader = new FileReader();
reader.onload = function (fileLoadEvent) {
var img = new Image();
img.onload = function() {
var srcX = 0, srcY = 0, srcWidth = img.width, srcHeight = img.height, ratio=1;
var srcRatio = img.width/img.height;
var dstRatio = width/height;
if(srcRatio < dstRatio) {
ratio = img.width/width;
srcHeight = height*ratio;
srcY = (img.height-srcHeight)/2;
} else {
ratio = img.height/height;
srcWidth = width*ratio;
srcX = (img.width-srcWidth)/2;
}
var canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
var ctx = canvas.getContext('2d');
ctx.drawImage(this, srcX, srcY, srcWidth, srcHeight, 0, 0, width, height);
callback(canvas);
};
img.src = reader.result;
};
reader.readAsDataURL(file);
},
I have a background image for a canvas, and added a few basic elements to the canvas. Now I want to save the canvas (in .png), with the background image of the canvas style.
Tried:
var canvas = document.getElementById("mycanvas");
var img = canvas.toDataURL("image/png");
But this doesn't seem to save the background image of the canvas. Is there a way out?
When you want to save the Canvas + background as an image, you will need to do a sequence of events:
Create an in-memory canvas just as big as your normal canvas. Call it can2
ctx2.drawImage(can1, 0, 0) // paint first canvas onto new canvas
ctx.clearRect(0, 0, width, height) // clear first canvas
ctx.drawImage(background, 0, 0) // draw image on first canvas
ctx.drawImage(can2, 0, 0) // draw the (saved) first canvas back to itself
To save an image location, I believe your looking for:
window.location = canvas.canvas.toDataURL('image/png');
The first canvas call is your variable the second is the canvas object.
You should probably rename your variable to something unique.
To set an image in canvas and make that the background requires some more work:
var myCanvas = document.querySelector('myCanvas'),
img = document.createElement('img'),
ctx = myCanvas.getContext ? myCanvas.getContext('2d') : null;
myCanvas.width = window.innerWidth;
myCanvas.height = window.innerHeight;
img.onload = function () {
ctx.drawImage(img, 0, 0, myCanvas.width, myCanvas.height);
};
img.src = 'image.png';
updated to redraw the image.