After experimenting with composite operations and drawing images on the canvas I'm now trying to remove images and compositing. How do I do this?
I need to clear the canvas for redrawing other images; this can go on for a while so I don't think drawing a new rectangle every time will be the most efficient option.
Given that canvas is a canvas element or an OffscreenCanvas object, use clearRect:
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
Use: context.clearRect(0, 0, canvas.width, canvas.height);
This is the fastest and most descriptive way to clear the entire canvas.
Do not use: canvas.width = canvas.width;
Resetting canvas.width resets all canvas state (e.g. transformations, lineWidth, strokeStyle, etc.), it is very slow (compared to clearRect), it doesn't work in all browsers, and it doesn't describe what you are actually trying to do.
Dealing with transformed coordinates
If you have modified the transformation matrix (e.g. using scale, rotate, or translate) then context.clearRect(0,0,canvas.width,canvas.height) will likely not clear the entire visible portion of the canvas.
The solution? Reset the transformation matrix prior to clearing the canvas:
// Store the current transformation matrix
context.save();
// Use the identity matrix while clearing the canvas
context.setTransform(1, 0, 0, 1, 0, 0);
context.clearRect(0, 0, canvas.width, canvas.height);
// Restore the transform
context.restore();
Edit:
I've just done some profiling and (in Chrome) it is about 10% faster to clear a 300x150 (default size) canvas without resetting the transform. As the size of your canvas increases this difference drops.
That is already relatively insignificant, but in most cases you will be drawing considerably more than you are clearing and I believe this performance difference be irrelevant.
100000 iterations averaged 10 times:
1885ms to clear
2112ms to reset and clear
If you are drawing lines, make sure you don't forget:
context.beginPath();
Otherwise the lines won't get cleared.
Others have already done an excellent job answering the question but if a simple clear() method on the context object would be useful to you (it was to me), this is the implementation I use based on answers here:
CanvasRenderingContext2D.prototype.clear =
CanvasRenderingContext2D.prototype.clear || function (preserveTransform) {
if (preserveTransform) {
this.save();
this.setTransform(1, 0, 0, 1, 0, 0);
}
this.clearRect(0, 0, this.canvas.width, this.canvas.height);
if (preserveTransform) {
this.restore();
}
};
Usage:
window.onload = function () {
var canvas = document.getElementById('canvasId');
var context = canvas.getContext('2d');
// do some drawing
context.clear();
// do some more drawing
context.setTransform(-1, 0, 0, 1, 200, 200);
// do some drawing with the new transform
context.clear(true);
// draw more, still using the preserved transform
};
This is 2018 and still there is no native method to completely clear canvas for redrawing. clearRect() does not clear the canvas completely. Non-fill type drawings are not cleared out (eg. rect())
1.To completely clear canvas irrespective of how you draw:
context.clearRect(0, 0, context.canvas.width, context.canvas.height);
context.beginPath();
Pros: Preserves strokeStyle, fillStyle etc.; No lag;
Cons: Unnecessary if you are already using beginPath before drawing anything
2.Using the width/height hack:
context.canvas.width = context.canvas.width;
OR
context.canvas.height = context.canvas.height;
Pros: Works with IE
Cons: Resets strokeStyle, fillStyle to black; Laggy;
I was wondering why a native solution does not exist. Actually, clearRect() is considered as the single line solution because most users do beginPath() before drawing any new path. Though beginPath is only to be used while drawing lines and not closed path like rect().
This is the reason why the accepted answer did not solve my problem and I ended up wasting hours trying different hacks. Curse you mozilla
Chrome responds well to: context.clearRect ( x , y , w , h ); as suggested by #Pentium10 but IE9 seems to completely ignore this instruction.
IE9 seems to respond to: canvas.width = canvas.width; but it doesn't clear lines, just shapes, pictures and other objects unless you also use #John Allsopp's solution of first changing the width.
So if you have a canvas and context created like this:
var canvas = document.getElementById('my-canvas');
var context = canvas.getContext('2d');
You can use a method like this:
function clearCanvas(context, canvas) {
context.clearRect(0, 0, canvas.width, canvas.height);
var w = canvas.width;
canvas.width = 1;
canvas.width = w;
}
Use clearRect method by passing x,y co-ordinates and height and width of canvas. ClearRect will clear whole canvas as :
canvas = document.getElementById("canvas");
ctx = canvas.getContext("2d");
ctx.clearRect(0, 0, canvas.width, canvas.height);
A quick way is to do
canvas.width = canvas.width
Idk how it works but it does!
there are a ton of good answers here.
one further note is that sometimes it's fun to only partially clear the canvas.
that is, "fade out" the previous image instead of erasing it entirely.
this can give nice trails effects.
it's easy. supposing your background color is white:
// assuming background color = white and "eraseAlpha" is a value from 0 to 1.
myContext.fillStyle = "rgba(255, 255, 255, " + eraseAlpha + ")";
myContext.fillRect(0, 0, w, h);
This is what I use, regardless boundaries and matrix transformations:
function clearCanvas(canvas) {
const ctx = canvas.getContext('2d');
ctx.save();
ctx.globalCompositeOperation = 'copy';
ctx.strokeStyle = 'transparent';
ctx.beginPath();
ctx.lineTo(0, 0);
ctx.stroke();
ctx.restore();
}
Basically, it saves the current state of the context, and draws a transparent pixel with copy as globalCompositeOperation. Then, restores the previous context state.
I always use
ctx.fillStyle = "rgb(255, 255, 255)";
ctx.fillRect(0, 0, canvas.width, canvas.height);
For a custom color, and
ctx.clearRect(0, 0, canvas.width, canvas.height);
For making the canvas transparent when clearing
This worked for my pieChart in chart.js
<div class="pie_nut" id="pieChartContainer">
<canvas id="pieChart" height="5" width="6"></canvas>
</div>
$('#pieChartContainer').html(''); //remove canvas from container
$('#pieChartContainer').html('<canvas id="pieChart" height="5" width="6"></canvas>'); //add it back to the container
the shortest way:
canvas.width += 0
I have found that in all browsers I test, the fastest way is to actually fillRect with white, or whataever color you would like. I have a very large monitor and in full screen mode the clearRect is agonizingly slow, but the fillRect is reasonable.
context.fillStyle = "#ffffff";
context.fillRect(0,0,canvas.width, canvas.height);
The drawback is that the canvas is no longer transparent.
private clearCanvas() {
const canvas: HTMLCanvasElement = this.ctx.canvas
this.ctx.save()
this.ctx.setTransform(1, 0, 0, 1, 0, 0)
this.ctx.clearRect(0, 0, canvas.width, canvas.height)
this.ctx.restore()
}
in webkit you need to set the width to a different value, then you can set it back to the initial value
function clear(context, color)
{
var tmp = context.fillStyle;
context.fillStyle = color;
context.fillRect(0, 0, context.canvas.width, context.canvas.height);
context.fillStyle = tmp;
}
A simple, but not very readable way is to write this:
var canvas = document.getElementId('canvas');
// after doing some rendering
canvas.width = canvas.width; // clear the whole canvas
Context.clearRect(starting width, starting height, ending width, ending height);
Example: context.clearRect(0, 0, canvas.width, canvas.height);
I always use this
ctx.clearRect(0, 0, canvas.width, canvas.height)
window.requestAnimationFrame(functionToBeCalled)
NOTE
combining clearRect and requestAnimationFrame allows for more fluid animation if that is what you're going for
If you use clearRect only, if you have it in a form to submit your drawing, you'll get a submit instead the clearing, or maybe it can be cleared first and then upload a void drawing, so you'll need to add a preventDefault at the beggining of the function:
function clearCanvas(canvas,ctx) {
event.preventDefault();
ctx.clearRect(0, 0, canvas.width, canvas.height);
}
<input type="button" value="Clear Sketchpad" id="clearbutton" onclick="clearCanvas(canvas,ctx);">
Hope it helps someone.
This is a Free hand drawing Canvas with a Clear Canvas Button.
See this live example of a canvas which you can draw on and also when required clear it for redrawing clearRect() is used to delete the prersent canvas and fillRect() is used to again draw the initial canvas which was clean and had no drawings on it.
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d"),
painting = false,
lastX = 0,
lastY = 0,
lineThickness = 1;
canvas.width=canvas.height = 250;
ctx.fillRect(0, 0, 250, 250);
canvas.onmousedown = function(e) {
painting = true;
ctx.fillStyle = "#ffffff";
lastX = e.pageX - this.offsetLeft;
lastY = e.pageY - this.offsetTop;
};
canvas.onmouseup = function(e){
painting = false;
}
canvas.onmousemove = function(e) {
if (painting) {
mouseX = e.pageX - this.offsetLeft;
mouseY = e.pageY - this.offsetTop;
// find all points between
var x1 = mouseX,
x2 = lastX,
y1 = mouseY,
y2 = lastY;
var steep = (Math.abs(y2 - y1) > Math.abs(x2 - x1));
if (steep){
var x = x1;
x1 = y1;
y1 = x;
var y = y2;
y2 = x2;
x2 = y;
}
if (x1 > x2) {
var x = x1;
x1 = x2;
x2 = x;
var y = y1;
y1 = y2;
y2 = y;
}
var dx = x2 - x1,
dy = Math.abs(y2 - y1),
error = 0,
de = dy / dx,
yStep = -1,
y = y1;
if (y1 < y2) {
yStep = 1;
}
lineThickness = 4;
for (var x = x1; x < x2; x++) {
if (steep) {
ctx.fillRect(y, x, lineThickness , lineThickness );
} else {
ctx.fillRect(x, y, lineThickness , lineThickness );
}
error += de;
if (error >= 0.5) {
y += yStep;
error -= 1.0;
}
}
lastX = mouseX;
lastY = mouseY;
}
}
var button=document.getElementById("clear");
button.onclick=function clearcanvas(){
canvas=document.getElementById("canvas"),
ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, 250, 250);
canvas.width=canvas.height = 250;
ctx.fillRect(0, 0, 250, 250);}
#clear{border-radius:10px;
font-size:8px !important;
position:absolute;
top:1px;}
#canvas{border-radius:10px}
<link href="https://www.w3schools.com/w3css/4/w3.css" rel="stylesheet"/>
<button id="clear" class="w3-padding w3-xxlarge w3-pink" type="button">Clear Canvas</button>
<canvas id="canvas"></canvas>
There is now a .reset() method which will not only clear the canvas buffer but also completely reset all the properties of the context (styles etc.), reset its transformation matrix, clear its current sub-path, clear its states stack (the one controlled by save() and restore()), and remove all the clipping regions.
context.reset();
// now 'context' is clear as new
Basically, it has the same effects as canvas.width += 0, except that it's more idiomatic.
However it seems that in current Chromium's implementation it's as slow as canvas.width += 0, it also does generate a new buffer instead of simply clearing the previous one (resulting in more memory garbage). Another caveat is that it's currently only supported in Chromium browsers.
Though to polyfill it you can go the Chrome way
if (!CanvasRenderingContext2D.prototype.reset) {
CanvasRenderingContext2D.prototype.reset = function() {
this.canvas.width += 0;
};
}
if (!OffscreenCanvasRenderingContext2D.prototype.reset) {
OffscreenCanvasRenderingContext2D.prototype.reset = function() {
this.canvas.width += 0;
};
}
These are all great examples of how you clear a standard canvas, but if you are using paperjs, then this will work:
Define a global variable in JavaScript:
var clearCanvas = false;
From your PaperScript define:
function onFrame(event){
if(clearCanvas && project.activeLayer.hasChildren()){
project.activeLayer.removeChildren();
clearCanvas = false;
}
}
Now wherever you set clearCanvas to true, it will clear all the items from the screen.
fastest way:
canvas = document.getElementById("canvas");
c = canvas.getContext("2d");
//... some drawing here
i = c.createImageData(canvas.width, canvas.height);
c.putImageData(i, 0, 0); // clear context by putting empty image data
Here is my code. I am uploading a image and using like this :
> var canvas = document.createElement('canvas');
> var context = canvas.getContext('2d');
> context.drawImage(image, 0, 0);
This works fine but if I copy the imagedata like below to another canvas. It loads image partially.
var canvas = document.getElementById('myCanvas');
var context1 = canvas.getContext('2d');
context1.putImageData(context.getImageData(0, 0, image.width, image.height), 0, 0);
I tried same approach by creating two canvas and this code which worked fine but without image.
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(0, 0, 300, 300);
var d = document.getElementById("myCanvas1");
var btx = d.getContext('2d');
var imgData = ctx.getImageData(0, 0, 300, 300);
btx.putImageData(imgData, 0, 0);
If i create two canvas it works fine but not like the above where I am using in memory canvas.
My HTML file has this :
<canvas id="myCanvas" width="960" height="960"></canvas>
<canvas id="myCanvas1" width="960" height="960"></canvas>
When you create canvases, you need to set their width manually. Default canvas size is 300x150. From here HTMLCanvasElement.
Something like this:
var canvas = document.createElement('canvas');
var context = canvas.getContext('2d');
canvas.width = image.width;
canvas.height = image.height;
context.drawImage(image, 0, 0);
To copy one canvas to another use drawImage rather than getImageData, setImageData That way you will use the GPU to move the pixels rather than the CPU. You also have the option to resize, crop, and apply a variety of FXs.
// create a copy of canvasSource
// returns the newly created canvas.
function copyCanvas(canvasSource){
var canvasCopy = document.createElement("canvas");
canvasCopy.width = canvasSource.width;
canvasCopy.height = canvasSource.height;
canvasCopy.getContext("2d").drawImage(canvasSource, 0, 0);
return canvasCopy;
}
I've recently started learning web techniques, so I'm still a newbie regarding pretty much everything. I've stumbled upon this portfolio site, link, which I'm trying to "recreate" as part of my learning process/practice.
Here I'm interested in the background of the page, and how to make that transparent letter on canvas. In my current work, I have a html background image set, fillRect() on a full screen with opacity of 0.9 , and now I don't know how to make use of clearRect().
The question is: Am I on the right track, and is there any way that I'm not aware of, in which I can use clearRect() to clear pixels on canvas in a letter shape? Like, without manually defining a path for clearRect(), but where I would only input a letter and clear pixels in its shape. Sorry if I posted my current code below the wrong way, it's my first time posting here.
var canvas = document.getElementById('layout');
if (canvas.getContext) {
var ctx = canvas.getContext('2d');
}
$(document).ready(function() {
window.addEventListener('resize', setCanvasSize); //false
window.addEventListener('resize', draw); //false
//set canvas size----------------------------------------
setCanvasSize();
function setCanvasSize() {
canvas.width = $(window).width();
canvas.height = $(window).height();
}
//draw---------------------------------------------------
draw();
function draw() {
var x = canvas.width;
var y = canvas.height;
ctx.fillStyle = "white";
ctx.fillRect(0, 0, x, y);
ctx.clearRect(50, 30, 110, 35);
}
});
You can achieve effects such as this using globalCompositeOperation.
Here's an example where some text is used as shape to erase drawn pixels:
var canvas = document.createElement("canvas");
canvas.width = 300;
canvas.height = 200;
var ctx = canvas.getContext("2d");
document.body.appendChild(canvas);
ctx.fillStyle = "rgba(255, 255, 255, 0.8";
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.globalCompositeOperation = "destination-out";
ctx.font = "70pt Arial";
ctx.fillText("Text", 50, 130);
Here's the JSFiddle: http://jsfiddle.net/eSpUv/
The question is best demonstrated with a jsFiddle. Here is is.
So according to this code:
var canvas = document.getElementById('canvas');
canvas.width = 200;
canvas.height = 150;
var context = canvas.getContext('2d');
context.rect(20,20,50,50);
context.stroke();
var image = new Image();
image.src = canvas.toDataURL("image/png");
context.clearRect(0, 0, canvas.width, canvas.height);
context.drawImage(image, 0, 0);
you should see a rectangle drawn to the canvas every time you load the page. However, for some reason the first page load always shows nothing, a "blank"...
This point can be proven by opening the jsFiddle link in separate Chrome "Incognito" tabs. You won't ever see the rectangle load on the first time. Refreshing however it will appear.
And just to prove that you are indeed supposed to be seeing a rectangle on the initial load, try updating the jsFiddle to this:
var canvas = document.getElementById('canvas');
canvas.width = 200;
canvas.height = 150;
var context = canvas.getContext('2d');
context.rect(20,20,50,50);
context.stroke();
//var image = new Image();
//image.src = canvas.toDataURL("image/png");
//context.clearRect(0, 0, canvas.width, canvas.height);
//context.drawImage(image, 0, 0);
and you will see the rectangle on each first-load.
Why does the first load show nothing?
You have to wait for the image to load before you can dray it, and it works on the reload because it gets cached
image.onload = function(){
context.clearRect(0, 0, canvas.width, canvas.height);
context.drawImage(image, 0, 0);
}
http://jsfiddle.net/hJ9WQ/1/