具有角度的HTML5画布drawImage

Question

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).

Answer 1

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.

Answer 2

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 );

Answer 3

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.