I'm trying to identity the center x and y of a circle drawn from an png image source in a canvas, is there a context 2d function that can do this?
Or is there a function that can trace a circle in a png file so that I can identify its coordinates for center x and y?
I just need the logic thanks
There is no native method to identify shapes on an html5 canvas.
Once the pixels are drawn any information about how they were drawn (circle, rectangle, etc) is forgotton.
A method to find your circle
Your circle must be differentiated from the rest of the image.
Is it a unique color? Is the rest of the image transparent?
At the point where you have a differentiation, you can use getImageData to fetch the red, blue, green & alpha information about every pixel on the canvas.
var pixelData = context.getImageData(0,0,canvas.width,canvas.height).data;
This pixelData is one long array with each pixel's color data being in an element:
firstPixelRed=pixelData[0];
firstPixelGreen=pixelData[1];
firstPixelBlue=pixelData[2];
firstPixelAlpah=pixelData[3];
//
secondPixelRed=pixelData[4];
secondPixelGreen=pixelData[5];
secondPixelBlue=pixelData[6];
secondPixelAlpah=pixelData[7];
You can use this pixelData to identify all pixels which are inside your circle.
From these "inside circle" pixels, find their minimumX, minimumY, maximumX & maximumY coordinates.
These minimums & maximums will give you the bounding box of the circle.
topleft = [minumumX,minumimY]
topright = [maximumX,minimumY]
bottomright= [maximumX,maximumY]
bottomleft = [minumumX,maximumY]
The radius of the circle is:
var radius = (maximumX - minimumX) /2;
So the center point of the circle is:
var centerX = minimumX + radius;
var centerY = minimumY + radius;
And you've got your circle with center point [centerX,centerY] with radius!
Related
Im try to drew custom shape, but since I use moveTo ..I cant be filled it, so That my question is there's any way can may be determined points on screen to fill shape? or to do that I most use or drew another real shape in the same block as layer ...
Look at my example here to drew a simple shape:
to I can fill image with blue color Im drew a Fill rectangle, so That is is a true way?
Code for shape for before fill:
var canvas3 = document.getElementById('canvas3');
var c3 = canvas3.getContext('2d');
c3.fillStyle = 'green';
c3.beginPath();
c3.moveTo(10,30);
c3.lineTo(200,30);
c3.moveTo(10,80);
c3.lineTo(200,80);
c3.moveTo(10,30);
c3.lineTo(10,180);
c3.moveTo(200,30);
c3.lineTo(200,180);
//c3.closePath();
c3.fill();
c3.lineWidth = 5;
c3.strokeStyle = 'orange';
c3.stroke();
Code for shape after fill:
var canvas3 = document.getElementById('canvas3');
var c3 = canvas3.getContext('2d');
c3.fillStyle = 'blue';
c3.beginPath();
c3.moveTo(10,30);
c3.fillRect(10,30,190,60);
c3.moveTo(10,30);
c3.lineTo(10,180);
c3.moveTo(10,90);
c3.lineTo(200,90);
c3.moveTo(200,30);
c3.lineTo(200,180);
c3.moveTo(10,30);
c3.lineTo(200,30);
//c3.closePath();
c3.fill();
c3.lineWidth = 5;
c3.strokeStyle = 'orange';
c3.stroke();
and finally which is a best way to I can drew shapes like this?
Note: Im new on html5 canvas and I read from this online book.
is there any way can may be determined points on screen to fill
shape? or to do that I most use or drew another real shape in the same
block as layer
Just draw a shape in the same place. Fill first then stroke afterwards. A little planning may be required with canvas as to in which order to draw things.
You can define objects to hold the geometrical data if you plan to redraw often or move them around. This will certainly simplify the objective later on.
which is a best way to I can drew shapes like this?
In my opinion this code can be drawn much simpler and in fewer lines of codes. There is no need to break up a shape in several parts as in that code if you can draw a shape using a simple method for it. In this case four lines can be replaced with one rectangle.
Knowing how these shapes are drawn internally also helps so we can take advantage of the path a rect() leaves, i.e. closing in upper-left corner so we can continue from there.
For example:
var c3 = c.getContext("2d");
// fill blue box first as it will be at the bottom
c3.rect(10, 30, 190, 50); // x, y, width, height
c3.fillStyle = 'blue';
c3.fill();
// orange stroke
// we know rect() will close at upper-left corner so continue from there with left leg
c3.lineTo(10, 180);
c3.moveTo(200, 80); // create second leg at right separately
c3.lineTo(200, 180);
c3.strokeStyle = "orange";
c3.lineWidth = 5;
c3.lineJoin = c3.lineCap = "round";
c3.stroke();
<canvas id=c height=180></canvas>
An alternative approach would be to fill then build the line path:
var c3 = c.getContext("2d");
c3.fillStyle = 'blue';
c3.fillRect(10, 30, 190, 50); // fillRect does not add to path
// orange stroke
c3.moveTo(10, 180); // create "housing" starting at bottom-left corner
c3.lineTo(10, 30); // upper-left
c3.lineTo(200, 30); // upper-right
c3.lineTo(200, 180); // bottom-right
c3.moveTo(10, 80); // add cross-bar
c3.lineTo(200, 80);
c3.strokeStyle = "orange";
c3.lineWidth = 5;
c3.lineJoin = c3.lineCap = "round";
c3.stroke();
<canvas id=c height=180></canvas>
I have an HTML canvas with a rectangle like this:
var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.rect(20,20,150,100);
ctx.stroke();
Is it possible to center that rectangle in the canvas with css? I want it to stay centered regardless of what the width of the canvas is.
No.
The canvas is just a bunch of pixels. There are no elements for CSS to influence.
If you want to centre something inside the canvas, then you have to pick the right place the put the pixels you are colouring in the first place.
You might be better off using SVG for your graphics, especially if they are simple shapes like rectangles.
An easier way to center the rectangle on canvas:
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var width = 130, height = 80;//Dimensions of rectangle
var x = canvas.Width/2, y = canvas.Height/2;//Center coordinates of rectangle
ctx.rect(x - width/2, y - height/2, width, height);
ctx.stroke();
Refer this for the pictorial representation
I'm working on my own tile bliting engine, this one is using hexagonal tiles - but I think it doesn't differ much from regular tiles.
I have huge x,y array of tiles and they have their x,y coordinates for rendering on canvas, I iterate only the ones that should be visible on canvas in current camera position.
So I'm stuck with scaling and cant resolve this on my own.
Here is my code for drawing tiles on canvas:
public function draw():Void{
clearCanvas(); //Clear canvas (bitmapData)
var _m:Matrix;
iterateTiles(function(_tile:HexTile):Void{ // loop every tile that is visible on screen
_m = new Matrix();
_m.translate(_tile.x + cameraPoint.x,_tile.y + cameraPoint.y);//Get pre calculated tile x,y and add camera x,y
_m.scale(matrixScale, matrixScale);
drawToCanvas(_tile,_m);//Send to draw tile on canvas using Matrix
},true);
}
This works nice and fast but only problem is it scales tiles from left top corner (like regular scale would work)
Before scale
After scale
My question is how to transform tiles to always scale from center. So if tile 10:10 is in center of screen before scaling, then it should
stay there after scaling.
Sorry, I misunderstood the question, but I think I've got it now:
// Scale the distance from the original point to the center of the canvas
var xDistance:Number = ((_tile.x + cameraPoint.x) - xCenter) * matrixScale;
var yDistance:Number = ((_tile.y + cameraPoint.y) - yCenter) * matrixScale;
// Add the distances to the center of the canvas. This is where you want the tile
// to appear.
var x:Number = xCenter + xDistance;
var y:Number = yCenter + yDistance;
// Because the coordinate is going to be scaled, you need to increase it first.
x = (1 / matrixScale) * x;
y = (1 / matrixScale) * y;
_m.translate(x, y);
I have not tested this, I've just drawn it out on graph paper. Let me know if it works.
I suppose this doesn't work because canvas is drawing a bitmap of a vector (and a bitmap is not a path).
Even if it did work, the bitmap is likely always has a rectangular permitter.
Is there any way to leverage something like isPointInPath when using drawImage?
example:
The top canvas is drawn using drawImage and isPointInPath does not work.
The bottom canvas is drawn using arc and isPointInPath works.
a link to my proof
** EDIT **
I draw a circle on one canvas, and use isPointInPath to see if the mouse pointer is inside the circle (bottom canvas in my example).
I also "copy" the bottom canvas to the top canvas using drawImage. Notice that isPointInPath will not work on the top canvas (most likely due to reasons I mentioned above). Is there a work-around I can use for this that will work for ANY kind of path (or bitmap)?
A canvas context has this hidden thing called the current path. ctx.beginPath, ctx.lineTo etc create this path.
When you call ctx.stroke() or ctx.fill() the canvas strokes or fills that path.
Even after it is stroked or filled, the path is still present in the context.
This path is the only thing that isPointInPath tests.
If you want to test if something is in an image you have drawn or a rectangle that was drawn with ctx.fillRect(), that is not possible using built in methods.
Typically you'd want to use a is-point-in-rectangle function that you write yourself (or get from someone else).
If you're looking for how to do pixel-perfect (instead of just the image rectangle) hit detection for an image there are various methods of doing that discussed here: Pixel perfect 2D mouse picking with Canvas
You could try reimplementing ctx.drawImage() to always draw a box behind the image itself, like so (JSFiddle example):
ctx.customDrawImage = function(image, x, y){
this.drawImage(image, x, y);
this.rect(x, y, image.width, image.height);
}
var img1 = new Image();
img1.onload = function(){
var x = y = 0;
ctx.drawImage(img1, x, y);
console.log(ctx.isPointInPath(x + 1, y + 1));
x = 1.25 * img1.width;
ctx.customDrawImage(img1, x, y);
console.log(ctx.isPointInPath(x + 1, y + 1));
Note: you might get side effects like the rectangle appearing over the image, or bleeding through from behind if you are not careful.
To me, isPointInPath failed after canvas was moved. So, I used:
mouseClientX -= gCanvasElement.offsetLeft;
mouseclientY -= gCanvasElement.offsetTop;
I had some more challenges, because my canvas element could be rescaled. So first when I draw the figures, in my case arc, I save them in an array together with a name and draw them:
if (this.coInit == false)
{
let co = new TempCO ();
co.name= sensor.Name;
co.path = new Path2D();
co.path.arc(c.X, c.Y, this.radius, 0, 2 * Math.PI);
this.coWithPath.push(co);
}
let coWP = this.coWithPath.find(c=>c.name == sensor.Name);
this.ctx.fillStyle = color;
this.ctx.fill(coWP.path);
Then in the mouse event, I loop over the items and check if the click event is in a path. But I also need to rescale the mouse coordinates according to the resized canvas:
getCursorPosition(event) {
const rect = this.ctx.canvas.getBoundingClientRect();
const x = ((event.clientX - rect.left ) / rect.width) * this.canvasWidth;
const y = ((event.clientY - rect.top) / rect.height) * this.canvasHeight;
this.coWithPath.forEach(c=>{
if (this.ctx.isPointInPath(c.path, x, y))
{
console.log("arc is hit", c);
//Switch light
}
});
}
So I get the current size of the canvas and rescale the point to the original size. Now it works!
This is how the TempCO looks like:
export class TempCO
{
path : Path2D;
name : string;
}
When rotating a display object (around its center) the visual corner of the element moves (the actual x and y of the "box" remains the same). For example with 45 degrees of rotation the x coordinate will have increased and the y coordinate will have decreased as the top left corner is now at the top center of the "box".
I've tried to use displayObject.getBounds(coordinateSpace).topLeft however this method is simply returning the x and y of the box and thus doesn't change after an object has been rotated.
So, how do you get the x and y of a visual corner of a rotated display object?
Update: this is what I mean with the position of a visual corner after rotation -->
alt text http://feedpostal.com/cornerExample.gif
You simply need to translate the point to its parent's coordinate space.
var box:Shape = new Shape();
box.graphics.beginFill(0xff0099);
box.graphics.drawRect(-50, -50, 100, 100); // ... the center of the rectangle being at the middle of the Shape
addChild(box);
box.x = 100; // note: should be 100 + box.width * .5 in case you want to use the topleft corner to position
box.y = 100;
box.rotation = 45;
// traces the result (Point)
trace( box.parent.globalToLocal(box.localToGlobal(box.getBounds(box).topLeft)) );