clearRect function doesn't clear the canvas

Question

I'm using this script on the body onmousemove function:

function lineDraw() {
    // Get the context and the canvas:
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
    // Clear the last canvas
    context.clearRect(0, 0, canvas.width, canvas.height);
    // Draw the line:
    context.moveTo(0, 0);
    context.lineTo(event.clientX, event.clientY);
    context.stroke();
}

It's supposed to clear the canvas each time I move the mouse around, and draw a new line, but it isn't working properly. I'm trying to solve it without using jQuery, mouse listeners or similar.

Here is a demo: https://jsfiddle.net/0y4wf31k/

Dextroglucose

Answer 1

You should use "beginPath()". That is it.

function lineDraw() {   
    var canvas=document.getElementById("myCanvas");
    var context=canvas.getContext("2d");
    context.clearRect(0, 0, context.width,context.height);
    context.beginPath();//ADD THIS LINE!<<<<<<<<<<<<<
    context.moveTo(0,0);
    context.lineTo(event.clientX,event.clientY);
    context.stroke();
}

Lidialidice

Would like to add that this goes for draw methods such as rect and arc as well. – Staples

It's old but... closePath is useless and misleading here, it's just a lineTo(previousStartingPointOfThePath) so for a single line, it won't do anything, and it doesn't tell at all that you finished your path declaration. – Ginzburg

Thanks @kaiido, I edited the question. I think the edit is in the spirit of the answer. Hope you don't mind aviomaksim – Galenism

Thank you! that is crazy, why does the clear fail unless you do a beginPath? it makes no sense. – Skinflint

Answer 2

Be advised that ctx.clearRect() does not work properly on Google Chrome. I spent hours trying to solve a related problem, only to find that on Chrome, instead of filling the rectangle with rgba(0, 0, 0, 0), it actually fills the rectangle with rgba(0, 0, 0, 1) instead!

Consequently, in order to have the rectangle filled properly with the required transparent black pixels, you need, on Chrome, to do this instead:

ctx.fillStyle = "rgba(0, 0, 0, 0)";
ctx.fillRect(left, top, width, height);

This should, of course, work on all browsers providing proper support for the HTML5 Canvas object.

Hamadryad

Answer 3

And beginPath() and stroke() we need to use. This code works same as clearRect(...)

ctx.beginPath();
ctx.fillStyle = "rgba(0, 0, 0, 255)";
ctx.fillRect(0, 0, canvas.width, canvas.height);    
ctx.stroke();

Rheumatic

why do we need this? what if one has a colored background? – Frizette

Answer 4

If you're getting a blacked out screen using this method

ctx.beginPath()
ctx.fillStyle = "rgba(0, 0, 0, 255)"
ctx.fillRect(0, 0, canvas.width, canvas.height)    
ctx.stroke();

then switch all the 0's in rgba to 255

ctx.beginPath();
ctx.fillStyle = "rgba(255, 255, 255, 255)";
ctx.fillRect(0, 0, canvas.width, canvas.height);    
ctx.stroke();

Adala

Answer 5

Try with context.canvas.width = context.canvas.width:

function lineDraw() {   
    var canvas=document.getElementById("myCanvas");
    var context=canvas.getContext("2d");
    //context.clearRect(0, 0, context.width,context.height);
    context.canvas.width = context.canvas.width;
    context.moveTo(0,0);
    context.lineTo(event.clientX,event.clientY);
    context.stroke();
}

Demo HERE

Catchfly

or simply myCanvas.width = myCanvas.width;. – Exercise