Develop Reference

Draw fixed boxes on HTML canvas image [duplicate]

I'm tyring to rotate a diagram in canvas around its center while keeping the letters upright. I'm trying to use ctx.rotate(#) but it's rotating the entire diagram using what appears to be the left side of the canvas as the center.
The following link offers a visual: I want it to look like the green, not the red, as it currently does with my code. Visual Explanation
The following is the JSFiddle: http://jsfiddle.net/ddxarcag/143/
And my code is below:
<script>
$(document).ready(function () {
init();
function init() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
draw(ctx);
}
function draw(ctx) {
// layer1/Line
ctx.rotate(00);
ctx.beginPath();
ctx.moveTo(75.1, 7.7);
ctx.lineTo(162.9, 7.7);
ctx.stroke();
function WordSelector1() {
var word = ['A', 'B', 'C'];
var random = word[Math.floor(Math.random() * word.length)];
return random;
}
var x = WordSelector1();
// layer1/P
ctx.font = "12.0px 'Myriad Pro'";
ctx.rotate(0);
ctx.fillText(x, 60.0, 10.0);
}
});
</script>
Any help would be much appreciated. Thanks!

Drawing rotated graphs in canvas is somewhat easy once you know how to move the origin (0,0) to another point and then rotating the axes around it.
I added some comments in the code as not to repeat code and explanations.
I also moved the functions out of the $(document).ready and changed some numbers for more rounded values.
$(document).ready(function () {
init();
});
function init() {
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
draw(ctx);
}
function draw(ctx) {
ctx.font = "12.0px 'Myriad Pro'";
var angle = Math.random()*150; //Run more times to see other angles
//This translates the 0,0 to the center of the horizontal line
ctx.translate(100, 100);
//This draws the original straight line
ctx.beginPath();
ctx.moveTo(-50, 0); //The coordinates are relative to the new origin
ctx.lineTo(50, 0);
ctx.stroke();
//Draw the first letter
var x = WordSelector1();
ctx.fillText(x, -60, 0);
//This section draws the rotated line with the text straight
//Rotate the canvas axes by "angle"
ctx.rotate(angle * Math.PI/180);
ctx.beginPath();
ctx.moveTo(-50, 0); //The relative coordinates DO NOT change
ctx.lineTo(50, 0); //This shows that the axes rotate, not the drawing
ctx.stroke();
var x = WordSelector1();
ctx.translate(-60,0); //The origin must now move where the letter is to be placed
ctx.rotate(-angle * Math.PI/180); //Counter-rotate by "-angle"
ctx.fillText(x, 0, 0); //Draw the letter
}
function WordSelector1() {
var word = ['A', 'B', 'C'];
var random = word[Math.floor(Math.random() * word.length)];
return random;
}
canvas{
border: 1px solid;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<canvas id="canvas" width="200" height="200"></canvas>
One warning: after everything is drawn the axes end parallel to the canvas borders because it was rotated by -angle, but the origin is where the last letter was placed. You may want to use ctx.save() and ctx.restore() to avoid having to revert translations and rotations.

Text over background (Canvas/JS)

I'm trying to place text over a background color. I think the issue is that the "fillStyle" is being applied to both the text and the background. I want the text to be black. What am I doing wrong here?
Below is my code:
var canvas = document.createElement("canvas");
canvas.width = 200;
canvas.height = 200;
var ctx = canvas.getContext("2d");
ctx.fillText("hello", 0, 0);
ctx.fillStyle = "#E7E0CA";
ctx.fillRect(0, 0, 200, 200);
var img = document.createElement("img");
img.src = canvas.toDataURL("image/png");
document.body.appendChild(img);
Here's a link to the fiddle: https://jsfiddle.net/jessecookedesign/9rsy9gjn/36/

Unlike HTML where you define a list of what you want to appear, when working with a canvas it's like you're painting. So each "drawing" operation you do (like fillRect or fillText) will go on top of any existing content and cover it up.
Similarly since you're actually painting, rather than defining objects, you need to set the fill style before drawing. Using the analogy, you need to select the color of paint you'll use before you put paint to canvas.
Finally, the fillText method takes a position as the start of the baseline of the text. Since (0, 0) is the top left of the canvas, your text will get drawn above the bounds of the canvas an not be visible, so you need to move it down e.g. fillText("Hello World", 10, 100);
Correcting for those issues you get something like the following (and skipping the steps involved in converting to an img tag):
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
// Draw a black background
context.fillStyle = "black";
context.fillRect(0, 0, 200, 200);
// Draw the text
context.fillStyle = "#E7E0CA";
context.fillText("Hello world", 10, 100);
<canvas id="canvas" width="200" height="200"></canvas>

Wrong order - You're drawing the rectangle over the text.
The text has the same color as the rectangle
There's no font specified
The position (0,0) is out of bounds
Try it like this:
var ctx = canvas.getContext("2d");
ctx.fillStyle = "#E7E0CA";
ctx.fillRect(0, 0, 200, 200);
ctx.fillStyle = "black";
ctx.font="20px Georgia";
ctx.fillText("hello",10,30);

There are several issues:
You need to first fill the background, then fill the text.
Your text is above the canvas area — try a lower position.
This code has the correct order, and a position for the text that isn’t outside the canvas bounds.
var ctx = canvas.getContext("2d");
ctx.fillStyle = "#E7E0CA";
ctx.fillRect(0, 0, 200, 200);
ctx.fillStyle = "#000000";
ctx.fillText("hello", 10, 10);
With the changed order, of course you need to choose a new color for the text, in this case "#000000".
Alternatively, you could save and restore your canvas state:
var ctx = canvas.getContext("2d");
ctx.save();
ctx.fillStyle = "#E7E0CA";
ctx.fillRect(0, 0, 200, 200);
ctx.restore();
ctx.fillText("hello", 10, 10);

Whenever you access canvas of html page,
whatever you draw first, will display first.
so if you want to display your colored box first fill it first, then write your text by providing color ,font and position of text. for example,
<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "#E7E0CA";//your rect color
ctx.fillRect(0, 0, 200, 200);//your rect size
ctx.fillStyle = "#000000";//color for your text
ctx.font="30px Arial";//font style and size
ctx.fillText("hello world",25,50);//text and location
</script>

How can I detect if a polygon(drawn in canvas) is clicked by me？ [duplicate]

I've created a grid of several distorted rectangles made with Bezier curves. Each rectangle has its own color on the picture.
Let's say, I want to add hover effect for each of these rectangles, therefore I need to know its dimensions. Since I can fill or stroke the figure I assume that there is some way to get them, but I'm not sure.
Here is the example of the rectangles:
So the question is, is there some method in the canvas API with which I can achieve the desired effect?

Yes you can use isPointInPath(Path2D, x, y) method.
Note that if you don't use the Path2D object, you can also call it just with isPointInPath(x, y), but then it will check on the currently being drawn path (declared with beginPath()).
var ctx = canvas.getContext('2d');
var myPath = new Path2D();
myPath.bezierCurveTo(50, 100, 180, 10, 20, 10);
myPath.lineTo(50, 100);
function draw(hover) {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = hover ? 'red' : 'green';
ctx.fill(myPath);
}
canvas.onmousemove = function(e) {
var x = e.clientX - canvas.offsetLeft,
y = e.clientY - canvas.offsetTop;
var hover = ctx.isPointInPath(myPath, x, y)
draw(hover)
};
draw();
<canvas id="canvas"></canvas>

Reveal image on canvas with draw/strokes/paint

I want to achive the following:
Draw a bg-image to the canvas (once or if needed repeatedly)
The image should not be visible at the beginning
While i "paint" shapes to the canvas the bg-image should get visible where the shapes were drawn
The parts of the image that will be revealed shall be "painted" (like with a brush) so i want to use strokes.
What i tried:
- Do not clear the canvas
- Paint rects to the canvas with globalCompositeOperation = 'destination-in'
This works, the rectangles reveal the image but i need strokes
If i use strokes they are ignored with 'destination-in' while i see them with normal globalCompositeOperation.
Is this intended that the strokes are ignored? Is there a workaround like somehow converting the stroke/shape to a bitmap? Or do i have have to use two canvas elements?
In OpenGL i would first draw the image with its rgb values and with a = 0 and then only "paint" the alpha in.

You can solve it by these steps:
Set the image as a pattern
Set the pattern as fillStyle or strokeStyle
When you now fill/stroke your shapes the image will be revealed. Just make sure the initial image fits the area you want to reveal.
Example showing the principle, you should be able to adopt this to your needs:
var ctx = canvas.getContext("2d"),
img = new Image,
radius = 40;
img.onload = setup;
img.src = "http://i.imgur.com/bnAEEXq.jpg";
function setup() {
// set image as pattern for fillStyle
ctx.fillStyle = ctx.createPattern(this, "no-repeat");
// for demo only, reveals image while mousing over canvas
canvas.onmousemove = function(e) {
var r = this.getBoundingClientRect(),
x = e.clientX - r.left,
y = e.clientY - r.top;
ctx.beginPath();
ctx.moveTo(x + radius, y);
ctx.arc(x, y, radius, 0, 2*Math.PI);
ctx.fill();
};
}
<canvas id=canvas width=900 height=600></canvas>
Hope this helps!

Alternative solution:
Put the image as a normal image on your website
add a canvas and use CSS positioning to place it right above the image
Fill the canvas with the color you use as the page background
have your paint tools erase the canvas when you draw. By the way, you can set context.globalCompositionOperation = 'destination-out' to turn all drawing operations into an eraser.
Here is an example. As you can see, the alpha properties of your tools are respected.
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//prepare canvas
ctx.fillStyle = '#ffffff'
ctx.fillRect(0, 0, 120, 120);
//prepare a 30% opacity eraser
ctx.globalCompositeOperation = 'destination-out';
ctx.lineWidth = 5;
ctx.strokeStyle = 'rgba(0, 0, 0, 0.3)';
// make random strokes around cursor while mouse moves
canvas.onmousemove = function(e) {
var rect = this.getBoundingClientRect();
var x = e.clientX - rect.left;
var y = e.clientY - rect.top;
ctx.beginPath();
ctx.moveTo(x + Math.random() * 33 - 16, y + Math.random() * 33 - 16);
ctx.lineTo(x + Math.random() * 33 - 16, y + Math.random() * 33 - 16);
ctx.stroke();
}
<span>Move your mouse:</span>
<div>
<img src='https://upload.wikimedia.org/wikipedia/commons/thumb/6/61/HTML5_logo_and_wordmark.svg/120px-HTML5_logo_and_wordmark.svg.png' style='position:absolute'>
<canvas id='canvas' width=120 height=120 style='position:absolute'></canvas>
</div>

Html5 Canvas: alternative for drawImage()

Since I faced some rounding issues* with the default scale()-function of canvas I've implemented my own matrix-transformations for canvas. This is working fine with a sole exception, I cannot rotate an image because the drawImage() function can only be parameterized with the top left corner of the picture.
Is there any other method to draw an image on a canvas? A method that can be parameterized with at least the coordinates for the top, left and the bottom right corner?, so that I can manually rotate the coordinates?
*The issue is a one-pixel-gap between shapes after scaling by a factor < 1.

Based off of your fiddle in the comments you could use an in memory canvas as a back buffer to draw what you need to at normal size, then scale the context of your main canvas and use drawImage to draw the scaled result.
Live Demo
var canvas = document.getElementById('c');
var context = canvas.getContext('2d');
var backBuffer = document.createElement("canvas"),
bCtx = backBuffer.getContext("2d");
paint = function (x, y, scale) {
bCtx.clearRect(0,0,backBuffer.width,backBuffer.height);;
bCtx.beginPath();
bCtx.rect(x, y, 30, 30);
bCtx.fillStyle = 'black';
bCtx.fill();
bCtx.beginPath();
bCtx.rect(x + 30, y, 30, 30);
bCtx.fillStyle = 'black';
bCtx.fill();
context.save();
context.scale(scale,scale);
context.drawImage(backBuffer,0,0);
context.restore();
}
paint(10, 10, 1);
paint(10, 70, .66);