html5 canvas not drawing triangle - javascript

Hi i am trying to practice HTML 5 canvas. I just developed the code to make a triangle in a canvas. I am getting the canvas element using javascript and trying to draw a triangle in it. I don't know whats going wrong my javascript file is not getting linked or grabbing the canvas.
Html file code:
<!doctype html>
<html lang="en">
<head>
<meta charset= "UTF-8">
<style>
#canvas {
width: 400px;
height: 200px;
border: 1px solid red;
}
</style>
<script src="canvas.js"></script>
</head>
<body>
<canvas id="tcanvas">
</canvas>
</body>
</html>
Javascript code to draw in the canvas:
window.onload=init;
function draw() {
var canvas = document.getElementById('tcanvas');
var context=canvas.getContext("2d");
canvas.fillstyle="#0000FF";
context.lineWidth=3;
context.beginPath();
context.moveTo(50,100);
context.lineTo(150,100);
context.lineTo(100,50);
context.lineTo(50,100);
context.fill();
context.closePath();
}
draw();

init is undefined which means that your draw function is never invoked.
Check working example below which invokes draw on window.onload.
function draw() {
let canvas = document.getElementById('tcanvas');
let context = canvas.getContext("2d");
canvas.fillstyle = "#0000FF";
context.lineWidth = 3;
context.beginPath();
context.moveTo(50, 100);
context.lineTo(150, 100);
context.lineTo(100, 50);
context.lineTo(50, 100);
context.fill();
context.closePath();
}
window.onload = draw;
#canvas {
width: 400px;
height: 200px;
border: 1px solid red;
}
<canvas id="tcanvas"></canvas>

It does, you just have the init that is not defined and breaks up your code.
Possibly is defined elsewhere BUT contains error/s?
Another possible cause can be a canvasjs missing from where you linked it in your html, we cannot check that :)
Browser's console (F12) is your friend in situations like this.
Here is quick working fiddle
Note that this is the preferred way to show your code since it's more testable and easier to jump onto by others.
<canvas id="tcanvas">
function draw() {
var canvas = document.getElementById('tcanvas');
var context=canvas.getContext("2d");
canvas.fillstyle="#0000FF";
context.lineWidth=3;
context.beginPath();
context.moveTo(50,100);
context.lineTo(150,100);
context.lineTo(100,50);
context.lineTo(50,100);
context.fill();
context.closePath();
}
#canvas {
width: 400px;
height: 200px;
border: 1px solid red;
}

Related

globalAlpha not working with globalCompositeOperation on Safari

I am trying to render two rectangles one on the top of other inside canvas element with globalCompositeOperation set to 'source-out' and globalAlpha set to 0.2. However globalAlpha is not working on Safari and the outer rectangle in getting rendered with opacity 1.
Below is the code.
const el = document.getElementById('canvas');
const ctx = el.getContext('2d');
ctx.globalCompositeOperation = 'source-out';
ctx.beginPath();
ctx.rect(200,200,400,400);
ctx.closePath();
ctx.fill();
ctx.globalAlpha = 0.2;
ctx.beginPath();
ctx.rect(0,0,800,800);
ctx.closePath();
ctx.fill();
body {
background-color: white;
margin: 0;
}
.canvas-container {
position: relative;
width: 75%;
height: 0;
margin: auto;
margin-top : 50px;
padding-bottom: 75%;
}
.canvas-container .canvas{
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
<!DOCTYPE HTML>
<html>
<head>
<link rel="stylesheet" href="main.css">
</head>
<body>
<div class="canvas-container">
This is canvas container
<canvas id="canvas" class="canvas" width="800" height="800"></canvas>
</div>
</body>
</html>
I can definitely reproduce the bug, and being a bug, the only true solution is to report it in the hope it gets fixed.
Now, there are a lot of workarounds available.
The first one is to use a semi-transparent fillStyle rather than globalAlpha:
const el = document.getElementById('canvas');
const ctx = el.getContext('2d');
ctx.beginPath();
ctx.rect(100,100,200,200);
ctx.fill();
ctx.globalCompositeOperation = 'source-out';
ctx.fillStyle = "rgba(0,0,0,0.2)";
ctx.beginPath();
ctx.rect(0,0,400,400);
ctx.fill();
canvas { border: 1px solid; }
<canvas id="canvas" class="canvas" width="400" height="400"></canvas>
But this may not be really practical, for instance if you have complex gradients or even patterns as fillStyle.
A second workaround is to use the fillRect() method, which is not affected by the bug.
const el = document.getElementById('canvas');
const ctx = el.getContext('2d');
ctx.beginPath();
ctx.rect(100,100,200,200);
ctx.fill();
ctx.globalCompositeOperation = 'source-out';
ctx.globalAlpha = 0.2;
ctx.fillRect(0,0,400,400);
canvas { border: 1px solid; }
<canvas id="canvas" class="canvas" width="400" height="400"></canvas>
But this will obviously only work for rectangles, and not for any other shapes.
So a third workaround, which is probably the best one, consists in drawing the other way: first draw the semi-transparent contour, then the hole:
const el = document.getElementById('canvas');
const ctx = el.getContext('2d');
// first the contour
ctx.globalAlpha = 0.2;
ctx.beginPath();
ctx.rect(0,0,400,400);
ctx.fill();
// now the hole
ctx.globalCompositeOperation = 'destination-out';
ctx.globalAlpha = 1;
ctx.beginPath();
ctx.rect(100,100,200,200);
ctx.fill();
canvas { border: 1px solid; }
<canvas id="canvas" class="canvas" width="400" height="400"></canvas>
But if for some reasons you can't even do that, then a last resort workaround is to use a second canvas on which you'll draw your shape and do the compositing through drawImage(), which isn't affected either by this bug:
const el = document.getElementById('canvas');
const ctx = el.getContext('2d');
const copy_ctx= el.cloneNode().getContext("2d");
ctx.beginPath();
ctx.beginPath();
ctx.rect(100,100,200,200);
ctx.fill();
copy_ctx.beginPath();
copy_ctx.rect(0,0,400,400);
copy_ctx.fill();
// we can set the alpha either here or while drawing
// on copy_ctx, it doesn't change much, both work
ctx.globalAlpha = 0.2;
ctx.globalCompositeOperation = 'source-out';
ctx.drawImage(copy_ctx.canvas,0,0);
canvas { border: 1px solid; }
<canvas id="canvas" class="canvas" width="400" height="400"></canvas>

Audio Visualizer with JavaScript/HTML5 not Visualizing

Hello: I have a questions regarding my implementation of the visualization of audio (songs):
Currently, the HTML file and CSS file display what I want (a black box with a blue/cyan box underneath, as well as the audio.controls displaying), and the audio plays as it should. However, the script.js JavaScript files do not work to visualize the audio that is playing. Is this an issue with the linking of the JavaScript file, my use of the Web Audio API, or something else? Here is my code:
HTML - index.html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE-edge">
<link rel="stylesheet" type="text/css" href="./style.css">
<script type="text/javascript" src="./script.js"></script>
<title>Audio Visualizer</title>
</head>
<body>
<div id="mp3_player">
<div id="audio_box"></div>
<canvas id="analyser_render"></canvas>
</div>
</body>
</html>
CSS style.css
#mp3_player {
width: 500px;
height: 60px;
background: #000;
padding: 5px;
margin: 50px auto;
}
<code>#mp3_player > div > audio{
width: 500px;
background: #000;
float: left;
}
<code>#mp3_player > canvas{
width: 500px;
height: 30px;
background: #002D3C;
float: left;
}
JavaScript script.js
var audio = new Audio();
audio.src = 'gypsyheart.mp3';
audio.controls = true;
audio.loop = true;
audio.autoplay = false;<code>
var canvas, ctx, source, analyser, fbc_array, bars, bar_x, bar_width, bar_height;
window.addEventListener("load", initMp3Player, false);
function initMp3Player(){
document.getElementById('audio_box').appendChild(audio);
context = new AudioContext();
analyser = context.CreateAnalyser();
canvas= document.getElementById('analyser_render');
ctx = canvas.getContext('2d');
source = context.createMediaElementSource(audio);
source.connect(context.destination);
frameLooper();
}
function frameLooper(){
window.requestAnimationFrame(frameLooper);
fbc_array = new Uint8Array(analyser.frequencyBinCount);
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = '#00CCFF';
bars = 100;
for (var i = 0; i < bars; i++) {
bar_x = i*3;
bar_height = -(fbc_array[i] / 2);
bar_width = 2
ctx.fillRect(bar_x, canvas.height, bar_width, bar_height);
}
}
FILE TREE
_folder includes (with no sub-folders):_
gypsyheart.mp3
index.html
style.css
script.js
Thank you for helping me out. All comments and suggestions are appreciated. If you have a different idea on how to approach this problem of visualizing (simply) music, I'd love to hear it, and am open to different methods of solving this problem! Apologies for any formatting errors here (first time using Stack Overflow)!
-Quin

Clear button not working with my javascript and hmtl5 canvas

I'am very new at this and I'm trying to create a signature drawing program in html, canvas and javascript.
Im trying to get the clear button working with this function;
function clearCanvas() {
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
But it does not seem to work. I don't really know where in the js script to put the function for it to be executed in the html code.
this is the html code:
<!DOCTYPE html>
<html lang="sv">
<head>
<meta charset="utf-8">
<script type="text/javascript" src="uppgift-6.js"></script>
<title>Uppgift6</title>
<style type="text/css">
#canvas { position: relative; }
#bildruta { border: 2px solid #000; }
body {font-family: Calibri}
h2 {font-size: 150%; color: BLACK; background-color: BEIGE; padding: 20px; margin: 5px auto; text-align: center;}
</style>
</head>
<body>
<h2>Signatur</h2>
<div id="canvas">
<canvas id="bildruta" width="600" height="400"></canvas>
</div>
<button type="button" onclick="clearCanvas()"> Clear signature </button>
</body>
</html>
Grateful for all help that i can get!
Your ID is not correct. Change your id or change the JS line like this:
function clearCanvas() {
var canvas = document.getElementById('bildruta'); // that's the correct ID
var context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
}
And it works.
var canvas = document.getElementById('canvas') points to this div <div id="canvas"> not to your canvas.
It should be document.getElementById('bildruta')

JQuery / Javascript - Mouse Location completely off

I'm trying to make a 'draw' app, which allows the user to draw something by dragging their mouse over a canvas.
I want to draw a 'pixel' where the user drags their mouse, however: The location of the mouse and the location where the 'pixels' get drawn are not the same.
I've searched all over the internet but I don't know what the difference is with my version and with theirs.
Image:
HTML
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Drawer</title>
<script src="jquery-2.2.1.min.js"></script>
<script src="scripting/draw.js"></script>
</head>
<body>
<canvas id="drawGround" style="width: 500px; height: 500px; margin: 0 auto; border: 1px solid black;"></canvas>
</body>
</html>
Code:
$(document).ready(function() {
var drawSize = 3;
var canDrawCanvas = false;
const canvasID = "#drawGround";
document.onmouseup = function() {
canDrawCanvas = false;
};
$(canvasID).mousedown(function() {
canDrawCanvas = true;
});
$(canvasID).mousemove(function(event) {
if (canDrawCanvas) handleDrawing(event);
});
function handleDrawing(mouseEvent) {
var canvas = document.getElementById(canvasID.substring(1));
var context = canvas.getContext('2d');
context.fillRect(mouseEvent.pageX, mouseEvent.pageY, drawSize, drawSize);
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<canvas id="drawGround" style="width: 500px; height: 500px; margin: 0 auto; border: 1px solid black;"></canvas>
Instead of scaling the canvas object with the "style" attribute, you should use the "width" and "height"-attributes in the canvas-element.
<canvas id="drawGround" width="500px" height="500px" style="margin: 0 auto; border: 1px solid black;"></canvas>
Then use the offsetX and offsetY parameters instead of pageX and pageY to get the tip position of the mouse:
function handleDrawing(mouseEvent) {
var canvas = document.getElementById(canvasID.substring(1));
var context = canvas.getContext('2d');
context.fillRect(mouseEvent.offsetX, mouseEvent.offsetY, drawSize, drawSize);
}

Why html5 canvas circle x doent corresponding to the X i given

My problem is I try to paint circles in an X position of a canvas and always painted with a difference of more than 100px I give. I see no pattern in terms of pixel added. The position of X given is that I need regarding this painting canvas where I do not need 100px or more. Leave a basic code sample.
Style
.defaul_paint
{
margin:1px auto;
overflow:hidden;
text-align:center;
float:right;
background:#394147;
max-width:660px;
width:660px;
height: 30px;
margin-bottom: 2px;
}
.paint{
margin:1px auto;
overflow:hidden;
text-align:center;
float:right;
max-width:660px;
height: 340px;
margin-bottom: 2px;
background:#394147;
}
JavaScript
function paint(xx, yy, radio, begin, grades, booleanVar, context) {
context.arc(xx, yy, radio, begin, (Math.PI * 2), booleanVar);
circle = new Circle(xx, yy, radio);
circles.push(circle);
}
function testing() {
var canvas, ctx;
canvas = $('.paint')[0];
ctx = canvas.getContext('2d');
ctx.fillStyle = '#FFF';
ctx.beginPath();
paint(47, 5, 5, 0, Math.PI * 2, true, ctx);
ctx.closePath();
ctx.fill();
}
HTML
<article class="defaul_paint">
<canvas class='paint'>
</canvas>
</article ">
Don't use CSS to resize your canvas--this will cause your pixels to be stretched.
Instead, resize the canvas element itself. This will add pixels to the canvas.
You don't show your Circle "class" here's a example code: http://jsfiddle.net/m1erickson/VkV9n/
<!doctype html>
<html>
<head>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
window.onload = function() {
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
function Circle(x,y,radius,fill){
this.x=x;
this.y=y;
this.radius=radius;
this.fill=fill;
}
Circle.prototype.draw=function(){
ctx.beginPath();
ctx.arc(this.x,this.y,this.radius,0,Math.PI*2);
ctx.closePath();
if(ctx.fillStyle!=this.fill){ctx.fillStyle=this.fill;}
ctx.fill();
};
var c1=new Circle(150,150,100,"skyblue");
c1.draw();
}; // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

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