I have been experimenting with canvas and things were going pretty well. However when it came time to rotate an object I had the hardest time getting it to rotate. When I finally did I found that depending on the order the objects being initiated it. The object with the rotation attributes would get passed on to objects being called after it.
All my objects were put into their own functions, and then called by a draw function. I am curious if this is always true and I have to be conscious of my objects order of initiation or if there is something I am missing.
Below if you move Obj1 and Obj2 before or after each other you will see the results are different. My goal is to have Obj1 before Obj2 and only have Obj1 rotated.
window.onload = function(){
var canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d'),
x = 470,
y = 260;
canvas.width = 500;
canvas.height = 500;
function Obj1(){
ctx.beginPath();
ctx.translate(100,0);
ctx.rotate(20 * Math.PI / 180);
ctx.fillRect(x, y + 10, 20, 45);
ctx.fillStyle = 'blue';
ctx.fill();
ctx.closePath();
}
function Obj2(){
ctx.beginPath();
ctx.rect(0,300,100, 200);
ctx.fillStyle = "black";
ctx.fill();
ctx.closePath();
}
function draw(){
ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
Obj1();
Obj2();
}
draw();
}
<canvas id="canvas"></canvas>
You need to reset the rotation matrix after rotating the object
Whats happening is that the transforms are cascading .
Rendering happens as a stack not a per element basis .
The order of operations is
TRANSFORM
DRAW
RESET
DRAW
This is also a recursive process .
You can say rotate the world scene / view port and also individual object .
TRANSFORM
TRANSFORM
DRAW
RESET
TRANSFORM
DRAW
RESET
RESET
If you post a simple code example I can review it for you
Have a look at
https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Transformations
function Obj1(){
ctx.save();
ctx.beginPath();
ctx.translate(100,0);
ctx.rotate(20 * Math.PI / 180);
ctx.fillRect(x, y + 10, 20, 45);
ctx.fillStyle = 'blue';
ctx.fill();
ctx.closePath();
ctx.restore();
}
I believe off the top of my head .
Related
I'm trying to draw a pair of skis (two rectangles) on a skier (a square) at varying rotations. I don't quite understand how you line up rotated elements on a canvas as you rotate the whole canvas, not just the object.
At the moment I have this:
ctx.fillStyle = 'rgba(0,0,0,0.8)';
ctx.fillRect(0, 0, W, H);
ctx.fillStyle = skiier.color;
ctx.fillRect(skiier.x, skiier.y, skiier.width, skiier.height);
ctx.fillStyle = '#00f';
var angle = 20;
ctx.rotate(angle*Math.PI/180);
ctx.fillRect(skiier.x, skiier.y,100,10);
ctx.fillRect(skiier.x, skiier.y + 20,100,10);
ctx.rotate(-angle*Math.PI/180);
Which gives me this:
But what I'd like to do is the following:
Bearing in mind the x and y coords of the skier is constantly changing, how can I adjust and position the skis relative to him?
I have a demo here if it helps:
http://codepen.io/EightArmsHQ/pen/cfa7052ed205b664b066450910c830c5?editors=001
You should consider the context save, restore and translate as follows:
// ...
var angle = 20;
ctx.save(); // save the state of the ctx
ctx.translate(skiier.x, skiier.y); // translate your context point to be the same as skiier.
ctx.rotate(angle * Math.PI / 180);
ctx.fillRect(-25, 0, 100, 10); // You can draw from new context point.
ctx.fillRect(-25, 20, 100, 10); // Same here.
// Instead of rotating back, use restore()...
// Useful to restore all ctx options as they were before the save()
ctx.restore();
To rotate around a point: you need to translate context to the point, rotate context, translate context back.
For example...
ctx.fillStyle = 'rgba(0,0,0,0.8)';
ctx.fillRect(0, 0, W, H);
ctx.fillStyle = skiier.color;
ctx.fillRect(skiier.x, skiier.y, skiier.width, skiier.height);
ctx.fillStyle = '#00f';
var angle = 20;
ctx.translate(skiier.x, skiier.y);
ctx.rotate(angle*Math.PI/180);
ctx.translate(-skiier.x, -skiier.y);
ctx.fillRect(skiier.x, skiier.y,100,10);
ctx.fillRect(skiier.x, skiier.y + 20,100,10);
ctx.rotate(-angle*Math.PI/180);
So I've been fiddling with the canvas element, and I seem to have run into a situation that is highly irritating, yet I haven't been able to find a solution.
Say that two polygons are drawn on a canvas, and that they should be touching each other. Where one polygon is drawn like this:
ctx.beginPath();
ctx.moveTo(oX,oY);
ctx.lineTo(oX=oX+k,oY=oY-h);
ctx.lineTo(oX=oX+k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY-h);
ctx.fill();
A simple version is implemented in this fiddle.
As you can probably see there is a thin line between these shapes. How can I avoid it? I have tried the solutions here, but they don't really seem to refer to this case, because I'm dealing with diagonal lines.
One solution
You could always use the stroke-line trick, but depending on your goal:
If it is to show many polygons next to each other, you could look at the polygons as simple squares.
Draw them in as such in an off-screen canvas next to each other. This will produce a result with no gaps.
Then transform the main canvas into the position you want those polygons to appear. Add rotation and/or skew depending on goal.
Finally, draw the off-screen canvas onto the main canvas as an image. Problem gone.
This will give you an accurate result with no extra steps in stroking, and the calculations for the boxes becomes very simple and fast to do (think 2d grid).
You have to use an off-screen canvas though. If you transform main canvas and draw in the shapes you will encounter the same problem as already present. This is because each point is transformed and if there is need for interpolation it will be calculated for each path shape separately. Drawing in an image will add interpolation on the whole surface, and only where there are gaps (non-opaque alpha). As we already are "gap-free" this is no longer a problem.
This will require an extra step in planning to place them correctly, but this is a simple step.
Example
Step 1 - draw boxes into an off-screen canvas:
This code draws on the off-screen canvas resulting in two boxes with no gap:
(the example uses an on-screen to show result, see next step for usage of off-screen canvas)
var ctx = document.querySelector("canvas").getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 50, 50);
ctx.fillRect(60, 10, 50, 50);
<canvas/>
Step 2 - transform main canvas and draw in off-screen canvas
When drawn into main canvas with transformation set, the result will be (pseudo-random transformation just to show):
var ctx = document.querySelector("canvas").getContext("2d");
// off-screen canvas
var octx = document.createElement("canvas").getContext("2d");
octx.fillStyle = "red";
octx.fillRect(10, 10, 50, 50);
octx.fillRect(60, 10, 50, 50);
// transform and draw to main
ctx.translate(80, 0);
ctx.rotate(0.5, Math.PI);
ctx.transform(1, 0, Math.tan(-0.5),1, 0,0); // skew
ctx.drawImage(octx.canvas, 0, 0);
<canvas />
Step 3 (optional) - Interaction
If you want to interact with the boxes you simply apply the same transform, then add path for a box and hit-test it against the mouse position. Redraw a single state, erase by clearing and draw back the off-screen canvas on top:
var ctx = document.querySelector("canvas").getContext("2d");
// off-screen canvas
var octx = document.createElement("canvas").getContext("2d");
octx.fillStyle = "red";
octx.fillRect(10, 10, 50, 50);
octx.fillRect(60, 10, 50, 50);
// allow us to reuse some of the steps:
function getTransforms() {
ctx.setTransform(1,0,0,1,0,0);
ctx.translate(80, 0);
ctx.rotate(0.5, Math.PI);
ctx.transform(1, 0, Math.tan(-0.5),1, 0,0); // skew
}
function clear() {
ctx.setTransform(1,0,0,1,0,0);
ctx.clearRect(0,0,300,150);
}
function redraw() {
ctx.drawImage(octx.canvas, 0, 0);
}
getTransforms();
redraw();
ctx.canvas.onmousemove = function(e) {
var r = this.getBoundingClientRect(),
x = e.clientX - r.left, y = e.clientY - r.top;
// box 1 (for many, use array)
ctx.beginPath();
ctx.rect(10, 10, 50, 50);
clear(); // these can be optimized to use state-flags
getTransforms(); // so they aren't redraw for every move...
redraw();
// just one box check here
if (ctx.isPointInPath(x, y)) {
ctx.fill();
}
};
<canvas />
Yes, it's annoying when filled polygons result in that tiny gap. It's especially common on diagonals that should theoretically meet.
A common workaround is to put a half-pixel, same-colored stroke around the polygons:
//Some basic setup ...
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var oX = 50;
var oY = 50;
var h = 33;
var k = 50;
ctx.fillStyle = 'red';
ctx.strokeStyle='red';
ctx.lineWidth=0.50;
//Draw one polygon
ctx.beginPath();
ctx.moveTo(oX,oY);
ctx.lineTo(oX=oX+k,oY=oY-h);
ctx.lineTo(oX=oX+k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY-h);
ctx.fill();
ctx.stroke();
//Draw another polygon
oX = oX+k;
oY = oY+h;
ctx.beginPath();
ctx.moveTo(oX,oY);
ctx.lineTo(oX=oX+k,oY=oY-h);
ctx.lineTo(oX=oX+k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY-h);
ctx.fill();
ctx.stroke();
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
//Some basic setup ...
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var oX = 50;
var oY = 50;
var h = 33;
var k = 50;
ctx.fillStyle = 'red';
ctx.strokeStyle='red';
ctx.lineWidth=0.50;
//Draw one polygon
ctx.beginPath();
ctx.moveTo(oX,oY);
ctx.lineTo(oX=oX+k,oY=oY-h);
ctx.lineTo(oX=oX+k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY-h);
ctx.fill();
ctx.stroke();
//Draw another polygon
oX = oX+k;
oY = oY+h;
ctx.beginPath();
ctx.moveTo(oX,oY);
ctx.lineTo(oX=oX+k,oY=oY-h);
ctx.lineTo(oX=oX+k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY+h);
ctx.lineTo(oX=oX-k,oY=oY-h);
ctx.fill();
ctx.stroke();
#canvas{border:1px solid red;}
<canvas id="canvas" width=300 height=300></canvas>
I want to animate an Arc on Canvas, and it works (with a really basic animation, interval), but the outcome is very pixelated/edgy. On the left side I draw an arc (animated), on the right side without animation (smooth).
JsFiddle: http://jsfiddle.net/C8CXz/2/
function degreesToRadians (degrees) {
return degrees * (Math.PI/180);
}
function radiansToDegrees (radians) {
return radians * (180/Math.PI);
}
var canvas = document.getElementById('circle');
var ctx = canvas.getContext('2d');
var start = 0, end = 0;
var int = setInterval(function(){
end++;
ctx.beginPath();
ctx.arc(80, 80, 50, degreesToRadians(0)-Math.PI/2, degreesToRadians(end)-Math.PI/2, false);
ctx.lineWidth = 10;
ctx.stroke();
if(end >= 360) {
clearInterval(int);
}
}, 10);
ctx.beginPath();
ctx.arc(220, 80, 50, degreesToRadians(0)-Math.PI/2, degreesToRadians(360)-Math.PI/2, false);
ctx.lineWidth = 10;
ctx.stroke();
(raw simple code, dont mind the sloppiness)
You need a:
ctx.clearRect(0, 0, w, h);
In each draw loop.
Basically, you are drawing the same arc over itself hundreds of times. The edge pixels that are only partially black are bing darkened over and over until they are completely black.
Things like this are way nearly all canvas animations clear the canvas and draw fresh for each iteration.
Try clearing the drawing rectangle on every frame
ctx.clearRect(x,y,width,height);
http://jsfiddle.net/C8CXz/3/
I found that I first need the clear the canvas.
ctx.clearRect(0, 0, canvas.width, canvas.height);
I have to draw a stroked fill on canvas. For this I call ctx.fill and ctx.stroke separately. Due to this, the shadow of the stroke draws on top of the fill which I want to avoid.
Could somebody please tell if there is a way to avoid this?
Here is my code:
ctx1.fillStyle = "blue";
ctx1.strokeStyle = "black";
ctx1.shadowColor = "rgba(0,255,0, 1)";
ctx1.shadowOffsetX = 50;
ctx1.shadowOffsetY = 50;
ctx1.lineWidth = "20";
ctx.beginPath();
ctx.moveTo(300, 100);
ctx.lineTo(400, 100);
ctx.lineTo(400, 200);
ctx.lineTo(300, 200);
ctx.closePath();
ctx1.fill();
ctx1.stroke();
http://jsfiddle.net/abWYZ/3/
Every time you perform a draw action on the context the shadow is also drawn. The way canvas works is every thing that's drawn is placed on top of what was previously there. So whats happening is the fill is performed, making a shadow of it, and then the stroke is drawn, which makes a shadow on top of all previous drawn objects.
Here is one possible solution.
Live Demo
// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
ctx.save();
ctx.fillStyle = "blue";
ctx.strokeStyle = "black";
ctx.lineWidth="20";
ctx.beginPath();
ctx.moveTo(300, 100);
ctx.lineTo(400, 100);
ctx.lineTo(400, 200);
ctx.lineTo(300, 200);
ctx.closePath();
ctx.shadowColor = "rgba(0,255,0, 1)";
ctx.shadowOffsetX = 50;
ctx.shadowOffsetY = 50;
ctx.stroke();
ctx.fill();
// clear the shadow
ctx.shadowColor = 0;
ctx.shadowOffsetX = 0;
ctx.shadowOffsetY = 0;
// restroke w/o the shadow
ctx.stroke();
If you use an approach like this I suggest making a function called toggleShadow or something along those lines allowing you to control when the shadows are drawn.
I placed an text on html5 canvas object using fillText option, question is I need to move the text position or change the color of the text that is already rendered.
Shortly I need to know how to Manipulate particular child of canvas element
This will move a small circle over your canvas
var can = document.getElementById('canvas');
can.height = 1000; can.width = 1300;
var ctx = can.getContext('2d');
var x = 10, y = 100;
ctx.fillStyle = "black";
ctx.fillRect(700, 100, 100, 100);
function draw() {
ctx.beginPath();
ctx.arc(x, y, 20, 0, 2 * Math.PI);
ctx.fillStyle = 'rgba(250,0,0,0.4)';
ctx.fill();
x += 2;
ctx.fillStyle = "rgba(34,45,23,0.4)";
ctx.fillRect(0, 0, can.width, can.height);
requestAnimationFrame(draw);
//ctx.clearRect(0,0,can.width,can.height);
}
draw();
<canvas id="canvas" style="background:rgba(34,45,23,0.4)"></canvas>
I think there is no object model behind the canvas, so you cannot access a "child object" like a "text object" and change it.
What you can do is that you draw the text again with a different color that overwrites the "pixels" of the canvas.
If you want to move the text, first you have to either clear the canvas or re-draw the text with a background/transparent color to get rid of the text in the previous position. Then you can draw the text in the new position.
I've never tried it but I think this would be the way to do it.
var canvas = document.getElementById("canvas"); //get the canvas dom object
var ctx = canvas.getContext("2d"); //get the context
var c = { //create an object to draw
x:0, //x value
y:0, //y value
r:5; //radius
}
var redraw = function(){ // this function redraws the c object every frame (FPS)
ctx.clearRect(0, 0, canvas.width, canvas.height); // clear the canvas
ctx.beginPath(); //start the path
ctx.arc(c.x, c.y, c.r, 0, Math.PI*2); //draw the circle
ctx.closePath(); //close the circle path
ctx.fill(); //fill the circle
requestAnimationFrame(redraw);//schedule this function to be run on the next frame
}
function move(){ // this function modifies the object
var decimal = Math.random() // this returns a float between 0.0 and 1.0
c.x = decimal * canvas.width; // mulitple the random decimal by the canvas width and height to get a random pixel in the canvas;
c.y = decimal * canvas.height;
}
redraw(); //start the animation
setInterval(move, 1000); // run the move function every second (1000 milliseconds)
Here is a fiddle for it.
http://jsfiddle.net/r4JPG/2/
If you want easing and translations, change the move method accordingly.
Hope it is allowed to advertise somebody's project.
Take a look at http://ocanvas.org/ you can get inspiration there.
It is object like canvas library. Allows you to handle events, make animations etc.
<html>
<head>
<title>Canvas Exam</title>
</head>
<body>
<canvas id="my_canvas" height="500" width="500" style="border:1px solid black">
</canvas>
<script>
var dom=document.getElementById("my_canvas");
var ctx=dom.getContext("2d");
var x1=setInterval(handler,1);
var x=50;
var y=50;
r=40;
function handler()
{
ctx.clearRect(0,0,500,500);
r1=(Math.PI/180)*0;
r2=(Math.PI/180)*360;
ctx.beginPath();
//x=x*Math.random();
x=x+2;
r=r+10*Math.random();
ctx.arc(x,y,r,r1,r2);
ctx.closePath();
ctx.fillStyle="blue";
ctx.fill();
ctx.stroke();
if(x>400)
{
x=50;
y=y+10;
}
r=40;
}
</script>
</body>
</html>