I want to clear the canvas with condition on each iteration, may be performance. And, the condition is to only want to clear the line and not the arc.
I already tried chatGPT, save and restore method in JS for save the previous canvas, but it didn't work for me.
This line of code uses the clearRect method of the 2D rendering context to clear the canvas by specifying the x, y, width, and height. But, its clear the whole canvas which I don't want as I mention earlier.
For the better context of my question, you can see this image.
Any answer will be appreciated.
const canvas = document.getElementById('cvs')
const ctx = canvas.getContext('2d')
const CH = (canvas.height = 400)
const CW = (canvas.width = 400)
canvas.style.background = 'black'
const vs = () => {
let radius = 50;
let i = 0;
setInterval(() => {
let x = Math.cos(i) * radius + CH / 2;
let y = Math.sin(i) * radius + CW / 2;
if(i>2*Math.PI)return clearInterval();
/* if I add `ctx.clearRect(0, 0, CW, CH);` it's clear the whole canvas, I don't want that type of behavior */
ctx.beginPath();
ctx.arc(x,y,1,0,2*Math.PI)
ctx.closePath();
ctx.strokeStyle = "white";
ctx.stroke();
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(x, y);
ctx.closePath();
ctx.strokeStyle = "red";
ctx.stroke();
i += 0.01;
}, 10)
}
vs();
body{
display:grid;
place-items:center;
min-height:100vh;
background:gray;
}
<canvas id="cvs"></canvas>
I already tried chatGPT, save and restore method in JS for save the previous canvas, but it didn't work for me.
Those store-restore the context state, color in use and the like. If you look into the box in the top-right, coincidentally you will see a link "Temporary policy: ChatGPT is banned" - that has a reason.
What you need is storing-restoring the bitmap data, getImageData() and putImageData() are the methods for that. If you're concerned about performance (though it feels a bit early), the arc() call can be skipped too, as with this step-size you won't end up with a dotted circle (of course the strokeRect() I'm using instead could be then replaced with direct pixel manipulation, but a cool thing is that it provides anti-aliasing):
const canvas = document.getElementById('cvs')
const ctx = canvas.getContext('2d')
const CH = (canvas.height = 400)
const CW = (canvas.width = 400)
canvas.style.background = 'black'
const vs = () => {
let radius = 50;
let i = 0;
let backup = false;
setInterval(() => {
let x = Math.cos(i) * radius + CH / 2;
let y = Math.sin(i) * radius + CW / 2;
if (backup) ctx.putImageData(backup, 0, 0);
if (i > 2 * Math.PI) return clearInterval();
ctx.strokeStyle = "white";
ctx.strokeRect(x, y, 2, 2);
/* ctx.beginPath();
ctx.arc(x,y,1,0,2*Math.PI)
ctx.closePath();
ctx.strokeStyle = "white";
ctx.stroke();*/
backup = ctx.getImageData(0, 0, x + 2, y + 2); // 0 or 1 will have some red pixels
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(x, y);
ctx.closePath();
ctx.strokeStyle = "red";
ctx.stroke();
i += 0.01;
}, 10)
}
vs();
body {
display: grid;
place-items: center;
min-height: 100vh;
background: gray;
}
<canvas id="cvs"></canvas>
am having trouble doing something in Canvas - I want to create a grid that only exists within that shape. I am able to create the gird to fit the shape if it is a square but if the shape is an unusual shape I do not know how to create the grid.
Here is what I have so far: I only want the grid to exist within the shape
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(75, 50);
ctx.lineTo(40, 65);
ctx.lineTo(100, 105);
ctx.lineTo(200, 15);
ctx.fillStyle = 'green';
ctx.fill();
function drawGrid(context) {
context.globalCompositeOperation = 'destination-out ';
for (var x = 40.5; x < 300; x += 10) {
context.moveTo(x, 0);
context.lineTo(x, 300);
}
for (var y = 0.5; y < 501; y += 10) {
context.moveTo(0, y);
context.lineTo(300, y);
}
context.strokeStyle = "#ddd";
context.stroke();
}
drawGrid(ctx)
https://jsfiddle.net/fom9gtb6/
context.globalCompositeOperation = 'destination-out ';
should be
context.globalCompositeOperation = 'destination-out';
You can use context.globalCompositeOperation = 'source-atop';
From MDN
The new shape is only drawn where it overlaps the existing canvas content.
This way the strokeStyle is actually used for the grid instead of erasing parts of the shape, which destination-out will do.
I want to draw one circle and a character with shadow on a canvas in a HTML page while loading the page and recreate the image on a button click. I am using this code:
window.onload = function() {
draw();
};
function draw(){
var c = document.getElementById("canvas");
var ctx = c.getContext("2d");
ctx.clearRect(0, 0, c.width, c.height);
var width = c.width;
var height = c.height;
//DRAW A CIRCLE
var centerX = Math.floor((Math.random() * width));
var centerY = Math.floor((Math.random() * height));
var radius = Math.floor(Math.random() * 50);
var color = '#f11';
ctx.fillStyle = color;
ctx.beginPath();
ctx.arc(centerX, centerY, radius, 0, Math.PI * 2, false);
ctx.closePath();
ctx.fill();
//DRAW A CHARACTER WITH SHADOW
var c = "S";
ctx.font = "300% Verdana";
ctx.shadowBlur = 20;
ctx.shadowColor = "black";
ctx.shadowOffsetX = 20;
ctx.shadowOffsetY = 20;
ctx.fillStyle = "#111";
ctx.fillText(c, 10, 90);
}
In HTML I am calling draw function onclick() event of a button named Refresh.
For the first time it is giving desired output by drawing one circle and a character with shadow. As I click on the Refresh button it is drawing both the objects with shadow. I dont want to draw shadow of the circle. Can anyone please tell me the mistake I'm doing here.
You may want to use the CanvasRenderingContext2D.save() method :
window.onload = function() {
draw();
};
document.getElementById("canvas").addEventListener('click', draw);
function draw(){
var c = document.getElementById("canvas");
var ctx = c.getContext("2d");
ctx.clearRect(0, 0, c.width, c.height);
var width = c.width;
var height = c.height;
//DRAW A CIRCLE
var centerX = Math.floor((Math.random() * width));
var centerY = Math.floor((Math.random() * height));
var radius = Math.floor(Math.random() * 50);
var color = '#f11';
ctx.fillStyle = color;
ctx.beginPath();
ctx.arc(centerX, centerY, radius, 0, Math.PI * 2, false);
ctx.closePath();
ctx.fill();
//DRAW A CHARACTER WITH SHADOW
//save the actual context
ctx.save();
var c = "S";
ctx.font = "300% Verdana";
ctx.shadowBlur = 20;
ctx.shadowColor = "black";
ctx.shadowOffsetX = 20;
ctx.shadowOffsetY = 20;
ctx.fillStyle = "#111";
ctx.fillText(c, 10, 90);
//restore it
ctx.restore();
}
canvas{border:1px solid;}
<canvas id="canvas" width="400" height="200"></canvas>
I am trying to draw circles using a for loop. It works just fine accept it double draws the last circle.
See this jsfiddle for the example.
If I comment out the last
context.stroke();
in the second 'for' loop the circles display correctly. If I leave it in it double draws the last circle making it look bold.
What am I doing wrong?
Ken
The duplicate is caused by the extension lines you are drawing after the circles. Add a context.beginPath() call in the last for loop:
for(var j = 0; j < circle_Count + 1; j++) {
context.beginPath();
...
Working fiddle: http://jsfiddle.net/kwwqw5n2/3/
You have to close paths.
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var box_Height = 50;
// Make Top Rect
context.fillStyle = "#F3E2A9";
context.fillRect(1, 1, canvas.width-1, box_Height-1);
context.strokeRect(0.5, 0.5, canvas.width-1, box_Height);
//Define the circles
var centerY = 25;
var radius = 10;
var circle_Count = 3;
var distance_Between = canvas.width / (circle_Count+1);
//Draw three white circles.
for(var i=0;i<circle_Count;i++){
context.beginPath();
context.arc(distance_Between * (i+1), centerY, radius, 0, 2 * Math.PI, true);
context.fillStyle = 'white';
context.lineWidth = 1;
context.strokeStyle = '#000000';
context.fill();
context.stroke();
context.closePath();
}
//Define the Extension Lines
var Ext_Line_Start_X = 0;
var Ext_Line_Start_Y = box_Height + 4; //The plus is the Gap
var Ext_Line_Length = 60;
//Draw Extension Lines
for(var j=0;j<circle_Count+1;j++){
context.beginPath();
context.moveTo(distance_Between * j + 0.5, Ext_Line_Start_Y);
context.lineTo(distance_Between * j + 0.5, Ext_Line_Start_Y + Ext_Line_Length);
context.stroke();
context.closePath();
}
I'm currently using HTML5's canvas to render a number of strings using the fillText method. This works fine, but I'd also like to give each string a 1px black outer stroke. Unfortunately the strokeText function seems to apply an inner stroke. To counter this, I've written a drawStrokedText function that achieves the effect I'm after. Unfortunately it's horrible slow (for obvious reasons).
Is there a fast, cross-browser way of achieving a 1px outer stroke using native canvas functionality?
drawStrokedText = function(context, text, x, y)
{
context.fillStyle = "rgb(0,0,0)";
context.fillText(text, x-1, y-1);
context.fillText(text, x+1, y-1);
context.fillText(text, x-1, y);
context.fillText(text, x+1, y);
context.fillText(text, x-1, y+1);
context.fillText(text, x+1, y+1);
context.fillStyle = "rgb(255,255,255)";
context.fillText(text, x, y);
};
Here's an example of the effect at work:
What's wrong with stroke? Since half the stroke will be outside of the shape, you can always draw the stroke first with a line width of double what you want. So if you wanted a 4px outer stroke you could do:
function drawStroked(text, x, y) {
ctx.font = '80px Sans-serif';
ctx.strokeStyle = 'black';
ctx.lineWidth = 8;
ctx.strokeText(text, x, y);
ctx.fillStyle = 'white';
ctx.fillText(text, x, y);
}
drawStroked("37°", 50, 150);
Which makes:
live fiddle here: http://jsfiddle.net/vNWn6/
IF that happens to not look as accurate at smaller text rendering scales, you can always draw it large but scale it down (in the above case you'd do ctx.scale(0.25, 0.25))
Simon's answer is a good solution, yet it may have mitering glitches in some cases, especially with capital 'M', 'V', & 'W':
drawStroked("MVW", 50, 150);
http://jsfiddle.net/hwG42/1/
In this case, it's best to utilize:
ctx.miterLimit=2;
http://jsfiddle.net/hwG42/3/
Best of luck!
The above answers are great, using some of these solutions* and some of my own ideas, I made a quick reference and some creative alternatives in the below fiddle.
*All credits given where due in the fiddle code
drawStrokedText ( text, x, y );
drawShadowedText ( text, x, y, shadowBlur);
drawGlowingText ( text, x, y, glowColorHex, glowDistance);
drawBlurredText ( text, x, y, blurAmount);
drawReflectedText ( text, x, y, reflectionScale, reflectionOpacity);
https://jsfiddle.net/vtmnyea8/
// Author: Aaron Edmistone
// Text effects using HTML5 Canvas with 2D Context.
// https://stackoverflow.com/questions/7814398/a-glow-effect-on-html5-canvas
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext('2d');
// prepare the presentation of the canvas
ctx.fillStyle = 'black';
ctx.fillRect(0,0,250,450);
ctx.fillStyle = 'gray';
ctx.fillRect(250,0,250,450);
ctx.fillStyle = 'white';
ctx.fillRect(500,0,250,450);
ctx.fillStyle = '#0066CC';
ctx.fillRect(750,0,250,450);
// prepare the font and fill
ctx.font = "80px Sans-serif";
ctx.fillStyle = "white";
function drawStrokedText(text, x, y)
{
// using the solutions from #Simon Sarris and #Jackalope from
// https://stackoverflow.com/questions/7814398/a-glow-effect-on-html5-canvas
ctx.save();
ctx.strokeStyle = 'black';
ctx.lineWidth = 8;
ctx.lineJoin="round";
ctx.miterLimit=2;
ctx.strokeText(text, x, y);
ctx.fillText(text, x, y);
ctx.restore();
}
function drawShadowedText(text, x, y, shadowBlur = 3)
{
ctx.save();
ctx.shadowBlur = shadowBlur;
ctx.shadowColor = "#000000";
ctx.shadowOffsetX = 4;
ctx.shadowOffsetY = 4;
ctx.fillText(text, x, y);
ctx.restore();
}
function drawGlowingText(text, x, y, glowColorHexString, glowDistance = 10)
{
ctx.save();
ctx.shadowBlur = glowDistance;
ctx.shadowColor = glowColorHexString;
ctx.strokeText(text, x, y);
for(let i = 0; i < 3; i++)
ctx.fillText(text, x, y); //seems to be washed out without 3 fills
ctx.restore();
}
function drawBlurredText(text, x, y, blur = 5)
{
//using technique from https://www.html5rocks.com/en/tutorials/canvas/texteffects/
ctx.save();
let width = ctx.measureText(text).width + blur * 2;
ctx.shadowColor = ctx.fillStyle;
ctx.shadowOffsetX = width + x + ctx.canvas.width;
ctx.shadowOffsetY = 0;
ctx.shadowBlur = blur;
ctx.fillText(text, -width + -ctx.canvas.width, y);
ctx.restore();
}
function drawReflectedText(text, x, y, reflectionScale = 0.2, reflectionAlpha = 0.10)
{
ctx.save();
ctx.fillText(text, x, y);
ctx.scale(1, -reflectionScale);
ctx.globalAlpha = reflectionAlpha;
ctx.shadowColor = ctx.fillStyle;
ctx.shadowBlur = 15;
ctx.fillText(text, x, -(y * (1 / reflectionScale)));
ctx.restore();
}
for(let i = 0; i < 4; i++)
{
drawStrokedText ("MVW", 20 + i * 250, 80 * 1);
drawShadowedText ("MVW", 20 + i * 250, 80 * 2, 3);
drawGlowingText ("MVW", 20 + i * 250, 80 * 3, "#FF0000", 10);
drawBlurredText ("MVW", 20 + i * 250, 80 * 4, 5);
drawReflectedText ("MVW", 20 + i * 250, 80 * 5, 0.5, 0.5);
}
<canvas id="myCanvas" width="1000" height="500"></canvas>
Output of the fiddle:
What it supports:
Outline text
Shadow text
Glowing text
Blurred text
Reflected text
Some performance metrics:
Considering using this in a game or at high frame rates?
Check out this jsperf using the above methods.
https://jsperf.com/various-text-effects-html5-2d-context
For a smooth shadow you can try this
ctx.beginPath();
ctx.fillStyle = 'white';
ctx.font = "bold 9pt Tahoma";
ctx.shadowBlur = 3;
ctx.textAlign = "center";
ctx.shadowColor = "#000000";
ctx.shadowOffs = 0;
ctx.fillText('www.ifnotpics.com', 100, 50);
ctx.closePath();