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[st33d]

I have a sprite whose handle is at the top left, it holds a rectangular bitmap. However, in Box2D, objects are held at their center. I cannot center the bitmap in the sprite and I cannot attach the sprite to another sprite and center that sprite - I'm chucking around loads of these, a lazy solution is not an option.

I'm guessing that what I need to do is to give a matrix transform to the sprite that will push the handle of it out of the center at a rotated angle.

What the hell do I feed into the matrix class to do this?

[compositeredfox]

You have to create the matrix, translate it to where your sprite is, substract it the pivot's position (where you want the "hinge" to be), rotate the matrix to your wanted angle (in radians), add the pivot's position. Then apply it to your sprite.

Code:

var m:Matrix = new Matrix();
m.translate( spritePosition.x, spritePosition.y ); // Not the actual x and y of the sprite you're applying the matrix to!
m.translate( -pivot.x, -pivot.y ); // x and y of the pivot sprite
m.rotate(whateverRotation); // in Radians!
m.translate( pivot.x, pivot.y);
sprite.transform.matrix = m;

Here's a FLA with an example. The star has its center in its top left, and you can move the pivot and it will rotate correctly around that:
http://dl.dropbox.com/u/64314/forums/pivotedRotation/pivoted_rotation.fla

Here's the SWF:
http://dl.dropbox.com/u/64314/forums/pivotedRotation/pivoted_rotation.swf


Although if you're working with box2d maybe it's better to always center your sprites. Why can't you do that?

[st33d]

I'm rendering the sprites with a combination of copyPixels when they're static and then rendering them as rotated Sprites when they are dynamic.

Thanks for this. It took me a while to realise that pivot was the real-world pivot point and not point on the sprite I was rotating.

There's also a MatrixTransformer class I've found about that does a similar task:

http://jobemakar.blogspot.com/2007/06/rotating-around-point.html

But I think I'm doubtful that it would offer any more optimisation over the code I've got already.

Code:

var pos:b2Vec2 = body.GetPosition();
var angle:Number = body.GetAngle();
matrix.identity();
matrix.tx = -width * 0.5 * Game.SCALE;
matrix.ty = -height * 0.5 * Game.SCALE;
matrix.rotate(angle);
matrix.tx += pos.x * Game.SCALE;
matrix.ty += pos.y * Game.SCALE;
mc.transform.matrix = matrix;

[davidp]

i've been having the same problem lately - this helps a lot!

thanks guys