AlecMcE.com

I came across this gotcha for the first time today. Praise-be to ASUnit, without which this could have been extremely difficult to unearth:

// given two sprites called "a" and "b"...

a.transform.matrix = new Matrix(-.5, 0, 0, -.5, a.x, a.y);
b.scaleX = b.scaleY = -.5;

trace(a.scaleX, a.scaleY, b.scaleX, b.scaleY); // outputs: 0.5, 0.5, -0.5, -0.5

Both clips have been scaled by -0.5, but because the transform was used to produce the scale for a, the scaleX/Y variables do not reflect the object’s true scale.

Now, just to confuse yourself, add a third sprite to the stage:

// given another sprite called "c"...

c.transform.matrix = new Matrix(-.5, 0, 0, -.5, c.x, c.y);
c.scaleX = c.scaleY = -.5;

Curiouser and curiouser! This needed a more complete experimentation…

The source for this experiment can be found here: scaleAndTransform. The buttons call the following methods:

function onGo(event:Event):void
{
	a.transform.matrix = new Matrix(-.5, 0, 0, -.5, a.x, a.y);

	b.scaleX = b.scaleY = -.5;

	c.transform.matrix = new Matrix(-.5, 0, 0, -.5, c.x, c.y);
	c.scaleX = c.scaleY = -.5;
}

function onResetScale(event:Event):void
{
	a.scaleX = a.scaleY = 1;
	b.scaleX = b.scaleY = 1;
	c.scaleX = c.scaleY = 1;
}

function onResetTransform(event:Event):void
{
	a.transform.matrix = new Matrix(1, 0, 0, 1, a.x, a.y);
	b.transform.matrix = new Matrix(1, 0, 0, 1, b.x, b.y);
	c.transform.matrix = new Matrix(1, 0, 0, 1, c.x, c.y);
}

What Is Going On?

If you apply a transformation matrix then a scale, the magnitude of the scale is applied, but the direction is determined by the product of the transformation and the scalar… which is to say that:

a.realScaleX = (a.transform.matrix.a * a.scaleX > 0 ? 1 : -1) * a.scaleX;

If however you apply a scale then a transformation, the scale is defined by the transformation only!

a.realScaleX = a.transform.matrix.a;

Bug?

Does this qualify as a bug? I haven’t thought about this for long enough. The conditions in which a transformation matrix rotates and skews the MovieClip confuse me; how would I define scaleX and scaleY in that scenario? I need to give this some thought; perhaps you the community can give me some help! I am reluctant to call anything a bug immediately now, after my experience with my The Problem With Vector.<T> post, where I was just plain wrong!

A similar bug exists in Adobe JIRA: UIComponent.scaleX, scaleY differ from transform.matrix.a,d. You’ll need an account to look at this – it’s free, and once you’ve got it you can pester Adobe with your bugs too.