# Transformations using canvas Nikhil
Published on 2021-07-05 10:28:05

### Rotate an Image or Path around its CenterPoint

Steps#1-5 below allow any image or path shape to be moved anywhere on the canvas and rotated to any angle without changing any of the image/path-shape's original point coordinates.

1. Move the canvas [0,0] origin to the shape's center point

context.translate( shapeCenterX, shapeCenterY );

2. Rotate the canvas by the desired angle (in radians)

3. Move the canvas origin back to the top-left corner

context.translate( -shapeCenterX, -shapeCenterY );

4. Draw the image or path-shape using it's original coordinates.

context.fillRect( shapeX, shapeY, shapeWidth, shapeHeight );

5. Always clean up! Set the transformation state back to where it was before #1

Step#5, Option#1: Undo every transformation in the reverse order // undo #3

context.translate( shapeCenterX, shapeCenterY );

// undo #2

// undo #1

context.translate( -shapeCenterX, shapeCenterY );

Step#5, Option#2: If the canvas was in an untransformed state (the default) before beginning step#1, you can undo the effects of steps#1-3 by resetting all transformations to their default state

// set transformation to the default state (==no transformation applied)
context.setTransform(1,0,0,1,0,0)

Example code demo:

// canvas references & canvas styling
var canvas = document.createElement("canvas");
canvas.style.border = '1px solid red';
document.body.appendChild(canvas);
canvas.width = 378;
canvas.height = 256;
var ctx = canvas.getContext("2d");
ctx.fillStyle = 'green';
ctx.globalAlpha = 0.35;
// define a rectangle to rotate
var rect = {
x: 100,
y: 100,
width: 175,
height: 50
};
// draw the rectangle unrotated
ctx.fillRect(rect.x, rect.y, rect.width, rect.height);
// draw the rectangle rotated by 45 degrees (==PI/4 radians)
ctx.translate(rect.x + rect.width / 2, rect.y + rect.height / 2);
ctx.rotate(Math.PI / 4);
ctx.translate(-rect.x - rect.width / 2, -rect.y - rect.height / 2);
ctx.fillRect(rect.x, rect.y, rect.width, rect.height);

### Drawing many translated, scaled, and rotated images quickly

There are many situations where you want to draw an image that is rotated, scaled, and translated. The rotation should occur around the center of the image. This is the quickest way to do so on the 2D canvas. These functions a well suited to 2D games where the expectation is to render a few hundred even up to a 1000+ images every 60th of a second. (dependent on the hardware).

// assumes that the canvas context is in ctx and in scope
function drawImageRST(image, x, y, scale, rotation) {
ctx.setTransform(scale, 0, 0, scale, x, y); // set the scale and translation
ctx.drawImage(image, -image.width / 2, -image.height / 2); // draw the image offset by half its
width and height
}

A variant can also include the alpha value which is useful for particle systems.

function drawImageRST_Alpha(image, x, y, scale, rotation, alpha) {
ctx.setTransform(scale, 0, 0, scale, x, y); // set the scale and translation
ctx.globalAlpha = alpha;
ctx.drawImage(image, -image.width / 2, -image.height / 2); // draw the image offset by half its
width and height
}

It is important to note that both functions leave the canvas context in a random state. Though the functions will not be affected other rendering my be. When you are done rendering images you may need to restore the default transform.

ctx.setTransform(1, 0, 0, 1, 0, 0); // set the context transform back to the default

If you use the alpha version (second example) and then the standard version you will have to ensure that the global alpha state is restored

ctx.globalAlpha = 1;

An example of using the above functions to render some particles and the a few images

// assume particles to contain an array of particles
for (var i = 0; i < particles.length; i++) {
var p = particles[i];
drawImageRST_Alpha(p.image, p.x, p.y, p.scale, p.rot, p.alpha);
// no need to rest the alpha in the loop
}
// you need to reset the alpha as it can be any value
ctx.globalAlpha = 1;
drawImageRST(myImage, 100, 100, 1, 0.5); // draw an image at 100,100
// no need to reset the transform
drawImageRST(myImage, 200, 200, 1, -0.5); // draw an image at 200,200
ctx.setTransform(1, 0, 0, 1, 0, 0); // reset the transform

### Introduction to Transformations

Transformations alter a given point's starting position by moving, rotating & scaling that point.

• Translation: Moves a point by a distanceX and distanceY.
• Rotation: Rotates a point by a radian angle around it's rotation point. The default rotation point in Html Canvas is the top-left origin [x=0,y=0] of the Canvas. But you can reposition the rotation point using translations.
• Scaling: Scales a point's position by a scalingFactorX and scalingFactorY from it's scaling point. The default scaling point in Html Canvas is the top-left origin [x=0,y=0] of the Canvas. But you can reposition the scaling point using translations.

You can also do less common transformations, like shearing (skewing), by directly setting the transformation matrix of the canvas using context.transform.

Canvas actually achieves transformations by altering the canvas' entire coordinate system.

• context.translate will move the canvas [0,0] origin from the top left corner to a new location.
• context.rotate will rotate the entire canvas coordinate system around the origin.
• context.scale will scale the entire canvas coordinate system around the origin. Think of this as increasing the size of every x,y on the canvas: every x*=scaleX and every y*=scaleY.

Canvas transformations are persistent. All New drawings will continue to be transformed until you reset the canvas' transformation back to it's default state (==totally untransformed). You can reset back to default with:

// reset context transformations to the default (untransformed) state
context.setTransform(1,0,0,1,0,0);

### A Transformation Matrix to track translated, rotated & scaled shape(s)

Canvas allows you to context.translate, context.rotate and context.scale in order to draw your shape in the position & size you require.

Canvas itself uses a transformation matrix to efficiently track transformations.

• You can change Canvas's matrix with context. transform
• You can change Canvas's matrix with individual translate, rotate & scale commands
• You can completely overwrite Canvas's matrix with context.setTransform,
• But you can't read Canvas's internal transformation matrix -- it's write-only.

Why use a transformation matrix?

A transformation matrix allows you to aggregate many individual translations, rotations & scalings into a single, easily reapplied matrix.

During complex animations, you might apply dozens (or hundreds) of transformations to a shape. By using a transformation matrix you can (almost) instantly reapply those dozens of transformations with a single line of code.

Some Example uses:

• Test if the mouse is inside a shape that you have translated, rotated & scaled

There is a built-in context.isPointInPath that tests if a point (eg the mouse) is inside a path-shape, but this built-in test is very slow compared to testing using a matrix. Efficiently testing if the mouse is inside a shape involves taking the mouse position reported by the browser and transforming it in the same way that the shape was transformed. Then you can apply hit-testing as if the shape was not transformed.

• Redraw a shape that has been extensively translated, rotated & scaled

Instead of reapplying individual transformations with multiple .translate, .rotate, .scale you can apply all the aggregated transformations in a single line of code.

• Collision test shapes that have been translated, rotated & scaled

You can use geometry & trigonometry to calculate the points that make up transformed shapes, but it's faster to use a transformation matrix to calculate those points.

### A Transformation Matrix "Class"

This code mirrors the native context.translate, context.rotate, context.scale transformation commands. Unlike the native canvas matrix, this matrix is readable and reusable.

Methods:

• translate, rotate, scale mirror the context transformation commands and allow you to feed transformations into the matrix. The matrix efficiently holds the aggregated transformations.
• setContextTransform takes a context and sets that context's matrix equal to this transformation matrix. This efficiently reapplies all transformations stored in this matrix to the context.
• resetContextTransform resets the context's transformation to it's default state (==untransformed).
• getTransformedPoint takes an untransformed coordinate point and converts it into a transformed point.
• getScreenPoint takes a transformed coordinate point and converts it into an untransformed point.
• getMatrix returns the aggregated transformations in the form of a matrix array.

Code:

var TransformationMatrix = (function() {
// private
var self;
var m = [1, 0, 0, 1, 0, 0];
var reset = function() {
var m = [1, 0, 0, 1, 0, 0];
}
var multiply = function(mat) {
var m0 = m * mat + m * mat;
var m1 = m * mat + m * mat;
var m2 = m * mat + m * mat;
var m3 = m * mat + m * mat;
var m4 = m * mat + m * mat + m;
var m5 = m * mat + m * mat + m;
m = [m0, m1, m2, m3, m4, m5];
}
var screenPoint = function(transformedX, transformedY) {
// invert
var d = 1 / (m * m - m * m);
im = [m * d, -m * d, -m * d, m * d, d * (m * m - m * m), d * (m * m - m * m)];
// point
return ({
x: transformedX * im + transformedY * im + im,
y: transformedX * im + transformedY * im + im
});
}
var transformedPoint = function(screenX, screenY) {
return ({
x: screenX * m + screenY * m + m,
y: screenX * m + screenY * m + m
});
}
// public
function TransformationMatrix() {
self = this;
}
// shared methods
TransformationMatrix.prototype.translate = function(x, y) {
var mat = [1, 0, 0, 1, x, y];
multiply(mat);
};
TransformationMatrix.prototype.rotate = function(rAngle) {
var c = Math.cos(rAngle);
var s = Math.sin(rAngle);
var mat = [c, s, -s, c, 0, 0];
multiply(mat);
};
TransformationMatrix.prototype.scale = function(x, y) {
var mat = [x, 0, 0, y, 0, 0];
multiply(mat);
};
multiply(mat);
};
TransformationMatrix.prototype.reset = function() {
reset();
}
TransformationMatrix.prototype.setContextTransform = function(ctx) {
ctx.setTransform(m, m, m, m, m, m);
}
TransformationMatrix.prototype.resetContextTransform = function(ctx) {
ctx.setTransform(1, 0, 0, 1, 0, 0);
}
TransformationMatrix.prototype.getTransformedPoint = function(screenX, screenY) {
return (transformedPoint(screenX, screenY));
}
TransformationMatrix.prototype.getScreenPoint = function(transformedX, transformedY) {
return (screenPoint(transformedX, transformedY));
}
TransformationMatrix.prototype.getMatrix = function() {
var clone = [m, m, m, m, m, m];
return (clone);
}
// return public
return (TransformationMatrix);
})();

Demo:

This demo uses the Transformation Matrix "Class" above to:

• Track (==save) a rectangle's transformation matrix.
• Redraw the transformed rectangle without using context transformation commands.
• Test if the mouse has clicked inside the transformed rectangle.

<!doctype html>
<html>
<style>
body {
background-color: white;
}

#canvas {
border: 1px solid red;
}
</style>
<script>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var cw = canvas.width;
var ch = canvas.height;

function reOffset() {
var BB = canvas.getBoundingClientRect();
offsetX = BB.left;
offsetY = BB.top;
}
var offsetX, offsetY;
reOffset();
window.onscroll = function(e) {
reOffset();
}
window.onresize = function(e) {
reOffset();
}
// Transformation Matrix "Class"
var TransformationMatrix = (function() {
// private
var self;
var m = [1, 0, 0, 1, 0, 0];
var reset = function() {
var m = [1, 0, 0, 1, 0, 0];
}
var multiply = function(mat) {
var m0 = m * mat + m * mat;
var m1 = m * mat + m * mat;
var m2 = m * mat + m * mat;
var m3 = m * mat + m * mat;
var m4 = m * mat + m * mat + m;
var m5 = m * mat + m * mat + m;
m = [m0, m1, m2, m3, m4, m5];
}
var screenPoint = function(transformedX, transformedY) {
// invertvar d =1/(m*m-m*m);
im = [m * d, -m * d, -m * d, m * d, d * (m * m - m * m), d * (m * m - m * m)];
// point
return ({
x: transformedX * im + transformedY * im + im,
y: transformedX * im + transformedY * im + im
});
}
var transformedPoint = function(screenX, screenY) {
return ({
x: screenX * m + screenY * m + m,
y: screenX * m + screenY * m + m
});
}
// public
function TransformationMatrix() {
self = this;
}
// shared methods
TransformationMatrix.prototype.translate = function(x, y) {
var mat = [1, 0, 0, 1, x, y];
multiply(mat);
};
TransformationMatrix.prototype.rotate = function(rAngle) {
var c = Math.cos(rAngle);
var s = Math.sin(rAngle);
var mat = [c, s, -s, c, 0, 0];
multiply(mat);
};
TransformationMatrix.prototype.scale = function(x, y) {
var mat = [x, 0, 0, y, 0, 0];
multiply(mat);
};
multiply(mat);
};
TransformationMatrix.prototype.reset = function() {
reset();
}
TransformationMatrix.prototype.setContextTransform = function(ctx) {
ctx.setTransform(m, m, m, m, m, m);
}
TransformationMatrix.prototype.resetContextTransform = function(ctx) {
ctx.setTransform(1, 0, 0, 1, 0, 0);
}
TransformationMatrix.prototype.getTransformedPoint = function(screenX, screenY) {
return (transformedPoint(screenX, screenY));
}
TransformationMatrix.prototype.getScreenPoint = function(transformedX, transformedY) {
return (screenPoint(transformedX, transformedY));
}
TransformationMatrix.prototype.getMatrix = function() {
var clone = [m, m, m, m, m, m];
return (clone);
}
// return public
return (TransformationMatrix);
})();
// DEMO starts here// create a rect and add a transformation matrix
// to track it's translations, rotations & scalings
var rect = {
x: 30,
y: 30,
w: 50,
h: 35,
matrix: new TransformationMatrix()
};
// draw the untransformed rect in black
ctx.strokeRect(rect.x, rect.y, rect.w, rect.h);
// Demo: label
ctx.font = '11px arial';
ctx.fillText('Untransformed Rect', rect.x, rect.y - 10);
// transform the canvas & draw the transformed rect in red
ctx.translate(100, 0);
ctx.scale(2, 2);
ctx.rotate(Math.PI / 12);
// draw the transformed rect
ctx.strokeStyle = 'red';
ctx.strokeRect(rect.x, rect.y, rect.w, rect.h);
ctx.font = '6px arial';
// Demo: label
ctx.fillText('Same Rect: Translated, rotated & scaled', rect.x, rect.y - 6);
// reset the context to untransformed state
ctx.setTransform(1, 0, 0, 1, 0, 0);
// record the transformations in the matrix
var m = rect.matrix;
m.translate(100, 0);
m.scale(2, 2);
m.rotate(Math.PI / 12);
// use the rect's saved transformation matrix to reposition,
// resize & redraw the rect
ctx.strokeStyle = 'blue';
drawTransformedRect(rect);
// Demo: instructions
ctx.font = '14px arial';
ctx.fillText('Demo: click inside the blue rect', 30, 200);
// redraw a rect based on it's saved transformation matrix
function drawTransformedRect(r) {
// set the context transformation matrix using the rect's saved matrix
m.setContextTransform(ctx);
// draw the rect (no position or size changes needed!)
ctx.strokeRect(r.x, r.y, r.w, r.h);
// reset the context transformation to default (==untransformed);
m.resetContextTransform(ctx);
}
// is the point in the transformed rectangle?
function isPointInTransformedRect(r, transformedX, transformedY) {
var p = r.matrix.getScreenPoint(transformedX, transformedY);
var x = p.x;
var y = p.y;
return (x > r.x && x < r.x + r.w && y > r.y && y < r.y + r.h);
}
// listen for mousedown events
canvas.onmousedown = handleMouseDown;

function handleMouseDown(e) {
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation(); // get mouse position
mouseX = parseInt(e.clientX - offsetX);
mouseY = parseInt(e.clientY - offsetY);
// is the mouse inside the transformed rect?
if (isPointInTransformedRect(rect, mouseX, mouseY)) {
alert('You clicked in the transformed Rect');
}
}
// Demo: redraw transformed rect without using
// context transformation commands
function drawTransformedRect(r, color) {
var m = r.matrix;
var tl = m.getTransformedPoint(r.x, r.y);
var tr = m.getTransformedPoint(r.x + r.w, r.y);
var br = m.getTransformedPoint(r.x + r.w, r.y + r.h);
var bl = m.getTransformedPoint(r.x, r.y + r.h);
ctx.beginPath();
ctx.moveTo(tl.x, tl.y);
ctx.lineTo(tr.x, tr.y);
ctx.lineTo(br.x, br.y);
ctx.lineTo(bl.x, bl.y);
ctx.closePath();
ctx.strokeStyle = color;
ctx.stroke();
}
</script>
<body>
<canvas id="canvas" width=512 height=250></canvas>
</body>
</html>

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