HTML5 canvas text resizes - html

This must be obvious to most of you, so sorry for asking, but how come when I change the width of my canvas my text resizes as well? And how can I prevent this from happening?
code used:
canvas = document.getElementById('canvas');
context = canvas.getContext('2d');
context.fillText("text",10,10);
btw: before I get voted down / bashed at, I tried searching about this for like twenty minutes.. no results.

If you aren't re-sizing it using CSS You need to set the context font like so.
Live Demo
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
function drawText() {
ctx.font = 'italic 20px Calibri';
ctx.fillText("HELLO!", 100, 100);
}
window.addEventListener("resize", function() {
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
drawText()
});
drawText()​
If you are resizing the canvas element using css like the following example
canvas{
width:200px;
}
Then theres not much you can do to fix it. When you resize the canvas using CSS you just stretch it, but the pixel density stays the same. You should never use css to resize the canvas element, rather you should be using the width and height properties via JS like I do in my example above.

This problem might occur due to changing the attributes by using CSS. It seems that canvas gets re-scaled when you access its properties through css. I think your problem might be solved if you use this in your javascript code instead of changing it through css :
document.getElementById("yourCanvasId").height = 150; // height you want to change to
document.getElementById("yourCanvasId").width = 150; // width you want to change to
Same problem occurred with me. I did this and the problem was solved. Hope it helps !

Related

HTML Canvas imageSmoothingEnabled kills Draw Image in Chrome on Windows

We are using a canvas to draw an image, and require that the image is smoothed.
so we use context.imageSmoothingEnabled in the 2d Canvas context.
We notice that if we use a jpg and smoothing is true, then the image fails to scale correctly on some machines (mainly windows 64bit).
In addition, if smoothingQuality is set to 'low' (rather than medium or high) then it works, but of course, who wants low.
If you modify the code to point to a PNG file, it seems to work ok.
Here is the JS Code
// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
// Create an image element
var img = document.createElement('IMG');
// When the image is loaded, draw it
img.onload = function () {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.imageSmoothingQuality = "medium";
// set to true and the image fails to scale
ctx.imageSmoothingEnabled = true;
ctx.drawImage(img, 0, 0, 50, 50);
}
// Specify the src to load the image
img.src = "http://i.imgur.com/gwlPu.jpg";
body {
background: #CEF;
}
<canvas id="c" width="800" height="600"></canvas>
Thanks in advance.
I have tried it on a Win10 64bit, Win10 32Bit (64bitCPU) and Win10 32bit on Chrome Beta, & Canary
Can not repeat problems you mention.
The image format (jpeg,png) will not effect the result of any of the smoothing or quality settings (apart from source image quality).
What can make a difference, especially at small scales is the ratio of the scaled down image to the original. Ratios like 1/2 1/4 1/8th will produce far better results than 1/3 2/9 3/19th. When comparing images of different sizes you should compare at same scale ratio rather than same size (if original have different sizes)
Below I used your code and the image you linked to to draw the image at the same size you had. Along the top from left to right is smoothing true from quality high on left to low on right.
The second row is same but smoothing turned off.
Below that is the same but zoomed via copy of canvas and no smoothing to show more details of the differences the setting make (if any).
If as your question outlines this was reproducible it would be a major issue for chrome and would be quickly fixed. I would say it is something very specific, maybe all the graphics cards are the same type and there is a driver issue, or images are cached and some old bad version is appearing.
If you want more help the best you could do is provide some screen shots of the problems, showing the problem and the desired result. That way we can at least narrow it down somewhat.
var ctx = c.getContext('2d');
var img = new Image;
img.src = "http://i.imgur.com/gwlPu.jpg";
img.onload = function () {
ctx.imageSmoothingQuality = "high";
ctx.imageSmoothingEnabled = true;
ctx.drawImage(img, 0, 0,50,50);
ctx.imageSmoothingQuality = "medium";
ctx.drawImage(img, 60, 0,50,50);
ctx.imageSmoothingQuality = "low";
ctx.drawImage(img, 120, 0,50,50);
ctx.imageSmoothingQuality = "high";
ctx.imageSmoothingEnabled = false;
ctx.drawImage(img, 0, 60,50,50);
ctx.imageSmoothingQuality = "medium";
ctx.drawImage(img, 60, 60,50,50);
ctx.imageSmoothingQuality = "low";
ctx.drawImage(img, 120, 60,50,50);
ctx.imageSmoothingQuality = "low"; // turn off all filtering
ctx.imageSmoothingEnabled = false;
ctx.drawImage(c,0,0,170,110,0,120,680,440)
}
body {
background: #CEF;
}
<canvas id="c" width="800" height="600"></canvas>
Following content edited in by OP user2739963
Here is a screen shot of this not working on a machine (We have many of these).

SVG stroke width is messy on a html 5 canvas

I am trying to bind an SVG to a canvas. The SVG has a stroke width setup and it seems to be thicker than what it needs to be when bound to the canvas. What could be causing this and how to fix this?
The library that I use to bind svg to the html5 canvas is canvg
The JS fiddle below shows both the SVG and the bound svg on a canvas.
JS Fiddle: http://jsfiddle.net/fYAAf/111/
var image = new Image();
var canvas = document.createElement('canvas');
canvas.width =1090;
canvas.height = 1875;
var context = canvas.getContext('2d');
context.drawSvg(xml, 0, 0);
image.src = canvas.toDataURL();
According to the link: http://jsfiddle.net/fYAAf/111/
In the javascript code you are embedding the characteristic of SVG in a type tag (image). The result is something like this:

<img src="data:image/png;basQAACAnWQXByGjnghZGYT8......characteristic_of_SVG">
So you should try to put all the XML properties within an SVG tag, as it is written in the HTML document in the top of the example of jsFiddle.
In JavaScript something fast would:
var xml = "<svg> (Write here xml properties) <g><text (Write here text properties)><tspan dy="35" x="0">Test</tspan></text></g></svg>";
document.getElementById('container').innerHTML = xml;
<div id="container"></div>
I don't know what the cause of this behavior in img tags.

Make canvas occupy as much space as possible

I'm trying to make my HTML5 Canvas occupy as much space as possible. I'm making a game fore Firefox OS that uses the canvas and testing with the Firefox OS simulator.
By "as much space as possible", I mean the whole screen, without scrollbars.
Ideally, a cross-platform way (that also works on Android and iOS) is preferred.
You need to do a few things, set the the canvas position to absolute, and make sure there is no padding, or margins set in the containing element.
The following is what I use in all of my canvas demos
CSS
body, html {
width: 100%;
height: 100%;
margin: 0;
padding: 0;
overflow: hidden;
}
canvas {
position:absolute;
}
JavaScript
var canvas = document.getElementById("canvas"),
width = window.innerWidth,
height = document.body.offsetHeight;
window.onresize = function () {
height = canvas.height = document.body.offsetHeight;
width = canvas.width = document.body.offsetWidth;
};
Full Screen Demo
This should do the trick:
$("canvas").width($("body").width());
$("canvas").height($("body").height());
window.onresize = function(){
$("canvas").width($("body").width());
$("canvas").height($("body").height());
};
Replace canvas with canvas selector. This requires jQuery.

How do I generate a thumbnail client-side in a modern browser?

I'm looking for an elegant way to generate a thumbnail for use with the FileAPI. Currently I get a DataURL representing an image. Problem is, if the image is very large, than moving it around and rerendering it becomes CPU intensive. I can see 2 options to get around this.
Generate a thumbnail on the client
Generate a thumbnail on the server, send the thumbnail back to the client (AJAX).
With HTML5 we have a canvas element? Does anyone know how to use it to generate thumbnails from pictures? They don't have to be perfect -- sampling quality is acceptable. Is there a jQuery plugin that will do this for me? Are there any other way to speed up the clientside use of large images?
I'm using HTML5, and Firefox 3.6+: there is no need to support anything other than Firefox 3.6+, please don't provide suggestions for IE 6.0
Here’s what you can do:
function getThumbnail(original, scale) {
var canvas = document.createElement("canvas");
canvas.width = original.width * scale;
canvas.height = original.height * scale;
canvas.getContext("2d").drawImage(original, 0, 0, canvas.width, canvas.height);
return canvas
}
Now, to create thumbnails, you simply do the equivalent of this:
var image = document.getElementsByTagName("img")[0];
var thumbnail = getThumbnail(image, 1/5);
document.body.appendChild(thumbnail);
Note: Remember to make sure that the image is loaded (using onload) before trying to make a thumbnail of it.
Okay, the way I can see this working is drawing the image to the canvas at a smaller size, then exporting the canvas. Say you want a 64px thumbnail:
var thumbSize = 64;
var canvas = document.getElementById("canvas");
canvas.width = thumbSize;
canvas.height = thumbSize;
var c = canvas.getContext("2d");
var img = new Image();
img.onload = function(e) {
c.drawImage(this, 0, 0, thumbSize, thumbSize);
document.getElementById("thumb").src = canvas.toDataURL("image/png");
};
img.src = fileDataURL;
With this code, an image element with the id "thumb" is used as the thumbnail element. fileDataURL is the data URL that you got from the file API.
More information on drawing images to the canvas: http://diveintohtml5.info/canvas.html#images
And on exporting canvas data: http://msdn.microsoft.com/en-us/library/ie/ff975241(v=vs.85).aspx

Flip <canvas> (rotate 180deg) after being published on page

I'm trying to rotate a canvas element AFTER it's been appended to the DOM.
Canvas is 600x50 and this is the code at hand:
var canvas = document.getElementsByTagName('canvas')[2];
var ctx = canvas.getContext('2d');
ctx.translate(300, 25); // rotate # center
ctx.rotate(angle * Math.PI/180);
which isn't accomplishing the task. Am I missing something?
Thanks
Dug around and found this working solution;
context.scale(1,-1); //flip vertically
context.translate(0,-height); //move beneath original position
works wonders!