Javascript - How to clear context data? - javascript

How do I empty the data in my ctx?
Basically the second stroke() should do anything, but draws another triangle. How do I clear this triangle from ctx?
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.save();
ctx.beginPath();
ctx.moveTo(20, 20);
ctx.lineTo(20, 100);
ctx.lineTo(70, 100);
ctx.closePath();
ctx.stroke();
ctx.stroke();//First stroke
ctx.restore();//Does not empty line paths?
ctx.strokeStyle = "green";
ctx.stroke();//Seccond stroke, same triangle drawn, but green (I do not want to draw the triangle)
https://jsfiddle.net/dh3e5wg6/

Related

HTML Canvas globalCompositeOperation without affecting previous layers

I have these 4 layers.
What I'm trying to do is put the red and blue layer into one mask. But I don't want the purple or orange layer to be affected by this mask (only the red and blue). I manage to make it work for the orange but not for the purple layer
See my code
var canvas = document.querySelector('canvas');
canvas.height = window.innerHeight
canvas.width = window.innerWidth
var ctx = canvas.getContext('2d');
//this should'nt be affected by the mask
ctx.beginPath()
ctx.fillStyle = 'purple';
ctx.rect(0, 50, 100, 100);
ctx.fill()
//this is the mask
ctx.beginPath()
ctx.rect(10, 10, 70, 70);
ctx.fillStyle = 'green';
ctx.fill()
ctx.globalCompositeOperation = 'source-atop';
//this need to be inside the mask
ctx.beginPath()
ctx.fillStyle = 'blue';
ctx.rect(10, 10, 100, 100);
ctx.fill()
//this need to be inside the mask
ctx.beginPath()
ctx.fillStyle = 'red';
ctx.rect(50, 40, 100, 100);
ctx.fill()
ctx.globalCompositeOperation = 'source-over'; //reset
//this should'nt be affected by the mask
ctx.beginPath()
ctx.fillStyle = 'orange';
ctx.rect(200, 40, 100, 100);
ctx.fill()
And the fiddle https://jsfiddle.net/ws3b4q95/4/
Canvas doesn't know about shapes as objects, it only cares about pixels. So the purple rectangle can't be excluded from your mask, because everyting that's already drawn on the canvas, will be part of the mask.
Instead you should draw the rectangle after you've applied the mask, and use destination-over operation:
//this need to be inside the mask
ctx.beginPath()
ctx.fillStyle = 'red';
ctx.rect(50, 40, 100, 100);
ctx.fill()
//this should'nt be affected by the mask
ctx.globalCompositeOperation = 'destination-over';
ctx.beginPath()
ctx.fillStyle = 'purple';
ctx.rect(0, 40, 100, 100);
ctx.fill()
This is nice summary from Mozilla about composite operations: MDN web docs: CanvasRenderingContext2D.global .CompositeOperation

Using Clip in canvas causing Pixels

I am trying to using context.clip() to clip a draw arc from other one and fill the clipped result.
But when i clip section & fill it, it gives pixelated fill .
var ctx = document.getElementById("canvas").getContext("2d");
var x = 150 ;
var y = 150 ;
var r = 100 ;
ctx.save() ;
ctx.translate(x,y) ;
ctx.beginPath() ;
ctx.arc(0,0,r,0,2*Math.PI);
ctx.closePath() ;
ctx.fillStyle = "cyan" ;
ctx.fill() ;
ctx.lineWidth = 10;
ctx.stroke();
ctx.restore() ;
ctx.save() ;
ctx.clip() ;
ctx.translate(x,y);
ctx.beginPath();
ctx.moveTo(r,-r-10);
ctx.arc(0,-r-10,r,0,Math.PI*2);
ctx.closePath();
ctx.fillStyle = "#f2f2f2";
ctx.fill();
ctx.lineWidth = 1;
ctx.stroke();
ctx.restore();
https://jsfiddle.net/x0d0n40z/1/
An alternative approach which eliminates the need for clip()/save()/restore() is to use a few steps of compositing.
Clipping mask is anti-aliased in some browsers while in other not. To obtain consistency (and in some cases also performance since save-clip-restore are relative expensive operations) using composition is preferred if possible.
In this case:
Fill main arc in target color
Define a clipping arc
Change composite mode to destination-out and fill (will cut main)
Change composite mode to source-atop and stroke (will outline cut)
Change composite mode to source-over and stroke outline of main circle
Example
Update: Simplified steps (with the last step merged into the process, ref. comments). I also chose to demonstrate use of the Path2D since we can reuse the object without interfering with the path on the ordinary context -
var ctx = c.getContext("2d"),
p = new Path2D(), // this will store main shape for reuse
x = 75, y = 75, radius = 70;
// main arc
p.arc(x, y, radius, 0, 6.28); // store to path object
ctx.fillStyle = "cyan";
ctx.fill(p); // fill path object
// clip top arc
ctx.globalCompositeOperation = "source-atop";
ctx.arc(x, y - radius, radius, 0, 6.28);
ctx.fillStyle = "#09f";
ctx.fill();
ctx.lineWidth = 5;
ctx.stroke();
// stroke main arc
ctx.globalCompositeOperation = "source-over";
ctx.stroke(p); // stroke path object
body {background:#e9e9e9}
<canvas id=c></canvas>
Old version:
var ctx = c.getContext("2d"),
x = 75, y = 75, radius = 70;
// main arc
ctx.arc(x, y, radius, 0, 6.28);
ctx.fillStyle = "cyan";
ctx.fill();
// clipping arc
ctx.beginPath();
ctx.arc(x, y - radius, radius, 0, 6.28);
// cut step
ctx.globalCompositeOperation = "destination-out";
ctx.fill();
// stroke gap step
ctx.globalCompositeOperation = "source-atop";
ctx.lineWidth = 10;
ctx.stroke();
// stroke whole outline
ctx.globalCompositeOperation = "source-over";
ctx.beginPath();
ctx.arc(x, y, radius, 0, 6.28);
ctx.lineWidth = 5;
ctx.stroke();
// if you want to color the clip then use this:
ctx.globalCompositeOperation = "destination-atop";
ctx.fillStyle = "#09f";
ctx.fill();
body {background:#e9e9e9}
<canvas id=c></canvas>
The problem is that the clip boundary is not being anti alised.
To solve you can render the shape without using the clip. The ctx.arc method lets you set the start and end angles so you can get the inset by filling two arcs.
You will need to get the angles where the clip circle and the inset circle intercept.
For this case it is very simple. First get the distance between the circles, and the angle from one to the other. This works only for two circles of same radius.
var c = {x:?,y:?}; // circle one location
var c1 = {x:?,y:?}; // circle two location
var radius = ?; // radius of both
var angle = Math.atan2(c1.y - c.y, c1.x - c.x); // get the angle from one to the next
var dist = Math.hypot(c1.x - c.x, c1.y - c.y); // get the distance. NOTE IE does not have hypot so do it the normal way with Math.sqrt....
Now you have the angle and distance the intercepts are a simple relationship between the distance and the radius
var iAngle = Math.acos(dist / 2 / radius); // the angle from the line between the circles
// to the intercepts
Now you have that angle you can draw the two arcs
ctx.beginPath();
ctx.arc(c.x,c.y,radius,angle - iAngle , angle + iAngle); // first arc
ctx.arc(c1.x,c1.y, radius, angle + Math.PI - iAngle, angle + Math.PI + iAngle); // second arc
ctx.fill();
ctx.stroke();
There is not much you can do to prevent the jaggies from effecting the clip area. Another way to achieve clipping is to use ctx.globalCompositeOperation to render a mask. You can mask in and out, and many more options. This will be a better solution when the clipping area becomes more complex.
I finally figured the right way to correct the bug .
Heres the clean result of what i wanted https://jsfiddle.net/x0d0n40z/6/
Code :
var ctx = document.getElementById("canvas").getContext("2d");
var r = 50
x = ctx.canvas.width/2;
y = ctx.canvas.height/2;
var offset = 60;
ctx.save();
ctx.setTransform(1,0,0,1.5,x,y);
ctx.beginPath();
ctx.arc(0,0,r,0,2*Math.PI);
ctx.stroke();
ctx.clip();
ctx.beginPath();
ctx.arc(0,0,r,0,2*Math.PI,false);
ctx.fillStyle = "cyan";
ctx.fill();
ctx.setTransform(1, 0, 0, 1, x, y);
ctx.beginPath();
ctx.arc(0,-offset,r,0,2*Math.PI,false);
ctx.fillStyle = "#f2f2f2";
ctx.fill();
ctx.lineWidth = 1 ;
ctx.stroke();
ctx.setTransform(1,0,0,1.5,x,y);
ctx.beginPath();
ctx.arc(0,0,r,0,2*Math.PI,false);
ctx.lineWidth = 3 ;
ctx.stroke();
ctx.restore();
Source from were i learned to use clip : http://www.html5canvastutorials.com/advanced/html5-canvas-clipping-region-tutorial/

HTML5 Canvas not clipping correctly?

I'm wondering if the following code yields appropriate behavior. I feel like the top left square should still be green. that is, if I clip one area, ten restore, then paint in a second area, the canvas paints in BOTH areas. Why?
https://jsfiddle.net/s6t8k3w3/
var my_canvas = document.getElementById('canvas');
var ctx = my_canvas.getContext("2d");
//Make Clipping Path 1
ctx.save();
ctx.rect(20, 20, 100, 100);
ctx.clip();
//Fill Background with Green
ctx.fillStyle = 'rgba(0,255,0,1)';
ctx.fillRect(0, 0, my_canvas.width, my_canvas.height);
//Close Clipping Path 1
ctx.restore();
//Open Clipping Path 2
ctx.save();
ctx.rect(50, 50, 100, 100);
ctx.clip();
//Fill background with Blue
ctx.fillStyle = 'rgba(0,0,255,1)';
ctx.fillRect(0, 0, my_canvas.width, my_canvas.height);
//Draw Line
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(500, 500);
ctx.stroke();
//CloseClipping Path 2
ctx.restore();
You're not actually opening a second clipping path with that second ctx.save(); to do that, you need to call ctx.beginPath()

html5 canvas stroke twice drawing first stroke again

I want to draw a line with one color, and the next line with a different color. But when I call stroke() the second time, the first line is draw again! How can I avoid it? Here's my code:
var canv = document.getElementById("canvas");
var ctx = canv.getContext("2d");
ctx.moveTo(0,0);
ctx.lineTo(100,100);
ctx.strokeStyle = "#FF0000";
ctx.stroke();
ctx.moveTo(100,100);
ctx.lineTo(100,200);
ctx.strokeStyle = "#999999";
ctx.stroke();
Thanks in advance!
Just insert a beginPath() in there:
var canv = document.getElementById("canvas");
var ctx = canv.getContext("2d");
ctx.moveTo(0,0);
ctx.lineTo(100,100);
ctx.strokeStyle = "#FF0000";
ctx.stroke();
cxt.beginPath(); // <---
ctx.moveTo(100,100);
ctx.lineTo(100,200);
ctx.strokeStyle = "#999999";
ctx.stroke();
That will reset your path. A stroke just strokes what exists on the path but does not clear it. You will have to manually reset the path for each new shape you want to draw.
The advantage is that you can reuse the path for fill, clip and point testing. The disadvantage is that it's easy to forget sometimes.

html5 canvas stroke as clear line

i currently have the following:
ctx.save();
ctx.translate(459,258);
ctx.rotate(sec * Math.PI/36);
ctx.lineWidth = 10;
ctx.beginPath();
ctx.moveTo(135,0);
ctx.lineTo(210,0);
ctx.strokeStyle = 'rgba(0,153,255)';
ctx.stroke();
ctx.restore();
drawing a circle of lines (stroke). I want to convert it into a clearRect so that instead of drawing this circle of lines, i want it to erase a circle in the same shape... how can i do this?
i tried doing this but it did not work:
ctx.save();
ctx.translate(459,258);
ctx.rotate(sec * Math.PI/36);
ctx.lineWidth = 10;
ctx.beginPath();
ctx.moveTo(135,0);
ctx.lineTo(210,0);
ctx.clear();
ctx.restore();
thanks.

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