using HTML5 Canvas - rotate image about arbitrary point

Question

Rotate the dial on top of a semi circular(Northern Hemisphere) image as background. range could be 0 - 180 degrees. on input to the method that does canvas transformation, the dial would rotate and stop over the matched value. Here's what I was trying based on help and sample passed on by phrogz

Answer 1

In general, what you want to do is:

1. Transform the context to the point on the canvas that the object should rotate about.
2. Rotate the context.
3. Transform the context by the negative offset within the object for the center of rotation.
4. Draw the object at 0,0.

In code:

ctx.save();
ctx.translate( canvasRotationCenterX, canvasRotationCenterY );
ctx.rotate( rotationAmountInRadians );
ctx.translate( -objectRotationCenterX, -objectRotationCenterY );
ctx.drawImage( myImageOrCanvas, 0, 0 );
ctx.restore();

Here's a working example showing this in action. (The math for the rotation was just experimentally hacked to find a swing amount and offset in radians that fit the quickly-sketched gauge dial.)

As may be evident, you can substitute the translate call in step #3 above with offsets to the drawImage() call. For example:

ctx.drawImage( myImageOrCanvas, -objectRotationCenterX, -objectRotationCenterY );

Using context translation is recommended when you have multiple objects to draw at the same location.