Drawing an arc on an image inside a canvas - javascript

I am trying to clip and display a very large image inside a canvas div.
Using basic calculations and drawImage I managed to clip the image around the pixel I want and display the clipped image.
An example is here on JSFiddle (displaying image arround eye of the person)
I would like to add an arc which will be over the image around the pixel (the sx, sy pixel I use in the example in drawImage), how should I adjust the coordinates ?
var canvas = document.getElementById('test-canvas');
canvas.width = 500;
canvas.height = 285;
var context = canvas.getContext('2d');
var imageObj = new Image();
imageObj.onload = function () {
//context.drawImage(img,sx,sy,swidth,sheight,x,y,width,height);
context.drawImage(imageObj, 1324 - 250, 1228 - 142.5, 500, 285, 0, 0, 500, 285);
};
imageObj.src = "http://upload.wikimedia.org/wikipedia/en/b/b3/Edvard_Munch_-_Self-Portrait_-_Google_Art_Project.jpg";

An arc is part of a path, which can be either filled or stroke. In order to get your desired result, you need to move to a point on your circle*, create the arc, and then use stroke() (fiddle):
function strokeCircle(ctx, midx, midy, radius){
ctx.moveTo(midx + radius, midy);
ctx.arc(midx, midy, radius, 0, 2 * Math.PI, false);
ctx.stroke();
}
imageObj.onload = function () {
context.drawImage(imageObj, 1324 - 250, 1228 - 142.5, 500, 285, 0, 0, 500, 285);
strokeCircle(context, 250, 142.5, 30);
};
* The correct coordinate depends on your polar coordinates used for the circle. If you draw from 0 to Math.PI, you need to start on the right-most point.

Related

Image transform property not rotating image [duplicate]

I am experimenting with animation in <canvas> and can't work out how to draw an image at an angle. The desired effect is a few images drawn as usual, with one image rotating slowly. (This image is not at the centre of the screen, if that makes any difference).
You need to modify the transformation matrix before drawing the image that you want rotated.
Assume image points to an HTMLImageElement object.
var x = canvas.width / 2;
var y = canvas.height / 2;
var width = image.width;
var height = image.height;
context.translate(x, y);
context.rotate(angleInRadians);
context.drawImage(image, -width / 2, -height / 2, width, height);
context.rotate(-angleInRadians);
context.translate(-x, -y);
The x, y coordinates is the center of the image on the canvas.
It is interesting that the first solution worked for so many people, it didn't give the result I needed.
In the end I had to do this:
ctx.save();
ctx.translate(positionX, positionY);
ctx.rotate(angle);
ctx.translate(-x,-y);
ctx.drawImage(image,0,0);
ctx.restore();
where (positionX, positionY) is the coordinates on the canvas that I want the image to be located at and (x, y) is the point on the image where I want the image to rotate.
I have written a function (based on Jakub's answer) that allows user to paint an image in a X,Y position based on a custom rotation in a custom rotation point:
function rotateAndPaintImage ( context, image, angleInRad , positionX, positionY, axisX, axisY ) {
context.translate( positionX, positionY );
context.rotate( angleInRad );
context.drawImage( image, -axisX, -axisY );
context.rotate( -angleInRad );
context.translate( -positionX, -positionY );
}
Then you can call it like this:
var TO_RADIANS = Math.PI/180;
ctx = document.getElementById("canvasDiv").getContext("2d");
var imgSprite = new Image();
imgSprite.src = "img/sprite.png";
// rotate 45º image "imgSprite", based on its rotation axis located at x=20,y=30 and draw it on context "ctx" of the canvas on coordinates x=200,y=100
rotateAndPaintImage ( ctx, imgSprite, 45*TO_RADIANS, 200, 100, 20, 30 );

Crop an image displayed in a Canvas

I have a canvas tag:
<canvas width="321" height="240" id="img_source"></canvas>
I want to add a crop functionality, so I made a resizeable div that can identify the borders of cropped image through dragging the corners of the div using the mouse. It looks like the image below:
I'm currently using "toDataURL()" to convert the data from the canvass to an image that can be displayed by an <img> tag. My question is, How will I convert to an image only part of the canvas that was identified by the resizeable div?
Use the method getImageData with the selected rectangle coordinates. For example:
let imageData = ctx.getImageData(65, 60, 100, 100);
Then create a secondary canvas with the desired sizes and use putImageData to set the pixels:
let canvas1 = document.createElement("canvas");
canvas1.width = 100;
canvas1.height = 100;
let ctx1 = canvas1.getContext("2d");
ctx1.rect(0, 0, 100, 100);
ctx1.fillStyle = 'white';
ctx1.fill();
ctx1.putImageData(imageData, 0, 0);
Finally use toDataURL to update the image:
dstImg.src = canvas1.toDataURL("image/png");
See the full sample I've prepared for you in CodePen
Create a new canvas at destination size, draw in the cropped image using drawImage() and insert that canvas into the DOM avoiding using img and data-uri:
var ccanvas = document.createElement("canvas"),
cctx = ccanvas.getContext("2d");
ccanvas.width = w;
ccanvas.height = h;
// draw with crop arguments
cctx.drawImage(image_src, x, y, w, h, 0, 0, w, h);
// ^^^^^^^^^^ source region
// ^^^^^^^^^^ dest. region
// insert cropped image somewhere in the DOM tree:
document.body.appendChild(ccanvas);
window.onload = function() {
var img = document.getElementById("image_src");
document.body.appendChild(region2canvas(img, 150, 60, 220, 200));
}
function region2canvas(img, x, y, w, h) {
var ccanvas = document.createElement("canvas"),
cctx = ccanvas.getContext("2d");
ccanvas.width = w;
ccanvas.height = h;
// draw with crop arguments
cctx.drawImage(img, x, y, w, h, 0, 0, w, h);
return ccanvas;
}
<img src="http://i.imgur.com/kWI4Cmz.png" id="image_src">
The key to cropping from one image is that the context's drawImage method allows us to render a cropped section of the source image to the canvas.
context.drawImage(img, sx, sy, sw, sh, dx, dy, dw, dh);
img - Source image object
sx - Source x
sy - Source y
sw - Source width
sh - Source height
dx - Destination x
dy - Destination y
dw - Destination width
dh - Destination height
Create a new canvas, and copy the selected portion to that new canvas, and then get the toDataURL() from that new 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>

Draw multiple image's on canvas with each image rotated

I'm new into HTML5 programming and I wanted to know how to rotate each image when it is added into canvas. Should each of them be placed into a canvas and then rotated? If so how can i add multiple canvas into a single canvas context.
Fiddle : http://jsfiddle.net/G7ehG/
Code
function loadImages(sources, callback) {
var images = {};
var loadedImages = 0;
var numImages = 0;
// get num of sources
for(var src in sources) {
numImages++;
}
for(var src in sources) {
images[src] = new Image();
images[src].onload = function() {
if(++loadedImages >= numImages) {
callback(images);
}
};
images[src].src = sources[src];
}
}
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var sources = {
image1: 'http://farm3.static.flickr.com/2666/3686946460_0acfa289fa_m.jpg',
image2: 'http://farm4.static.flickr.com/3611/3686140905_cbf9824a49_m.jpg'
};
loadImages(sources, function(images) {
context.drawImage(images.image1, 100, 30, 200, 137);
context.drawImage(images.image2, 350, 55, 93, 104);
});
In your comment you mentioned that you know about context.rotate, but you don't want the context to stay rotated. That's not a problem at all. First, calling context.rotate only affects things which are drawn afterwards. Anything drawn before will stay were it was. Second, it can be easily reversed after drawing.
use context.save() to create a snapshot of all current context settings, including current rotation.
use context.rotate(angle) and draw your image. The angle is in Radian. That means a full 360° circle is Math.PI * 2. The point the image will be is rotated around is the current origin of the canvas (0:0). When you want to rotate the image around its center, use context.translate(x, y) to set the origin to where you want the center of the image to be, then rotate, and then draw the image at the coordinates -img.width/ 2, -img.height / 2
use context.restore() to return to your snapshot. Rotation and translation will now be like they were before.
Here is an example function which draws an image rotated by 45° at the coordinates 100,100:
function drawRotated(image, context) {
context.save();
context.translate(100, 100);
context.rotate(Math.PI / 4);
context.drawImage(image, -image.width / 2, -image.height / 2);
context.restore();
}

HTML5 canvas drawImage with at an angle

I am experimenting with animation in <canvas> and can't work out how to draw an image at an angle. The desired effect is a few images drawn as usual, with one image rotating slowly. (This image is not at the centre of the screen, if that makes any difference).
You need to modify the transformation matrix before drawing the image that you want rotated.
Assume image points to an HTMLImageElement object.
var x = canvas.width / 2;
var y = canvas.height / 2;
var width = image.width;
var height = image.height;
context.translate(x, y);
context.rotate(angleInRadians);
context.drawImage(image, -width / 2, -height / 2, width, height);
context.rotate(-angleInRadians);
context.translate(-x, -y);
The x, y coordinates is the center of the image on the canvas.
It is interesting that the first solution worked for so many people, it didn't give the result I needed.
In the end I had to do this:
ctx.save();
ctx.translate(positionX, positionY);
ctx.rotate(angle);
ctx.translate(-x,-y);
ctx.drawImage(image,0,0);
ctx.restore();
where (positionX, positionY) is the coordinates on the canvas that I want the image to be located at and (x, y) is the point on the image where I want the image to rotate.
I have written a function (based on Jakub's answer) that allows user to paint an image in a X,Y position based on a custom rotation in a custom rotation point:
function rotateAndPaintImage ( context, image, angleInRad , positionX, positionY, axisX, axisY ) {
context.translate( positionX, positionY );
context.rotate( angleInRad );
context.drawImage( image, -axisX, -axisY );
context.rotate( -angleInRad );
context.translate( -positionX, -positionY );
}
Then you can call it like this:
var TO_RADIANS = Math.PI/180;
ctx = document.getElementById("canvasDiv").getContext("2d");
var imgSprite = new Image();
imgSprite.src = "img/sprite.png";
// rotate 45º image "imgSprite", based on its rotation axis located at x=20,y=30 and draw it on context "ctx" of the canvas on coordinates x=200,y=100
rotateAndPaintImage ( ctx, imgSprite, 45*TO_RADIANS, 200, 100, 20, 30 );

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