Re: CATransform3D perspective question

FROM : Nicko van Someren
DATE : Sat Dec 01 23:43:55 2007

On 1 Dec 2007, at 17:10, John Harper wrote:

> Presumably you did what the samples do and set the perspective 
> matrix as the sublayerTransform property of the superlayer of the 
> layer you're rotating?


Yes, that's right.

> Both sublayerTransform and transform properties are applied relative 
> to the anchor point (typically center, though appkit sets the anchor 
> of layers it creates to the bottom left corner iirc) of the layer 
> the property is set on. So if you have two layers, one with a 
> perspective matrix, and its child rotated around the Y axis by 90°, 
> you will only see the child layer exactly edge on when its center is 
> aligned with the center of its superlayer. Does that explain what 
> you're seeing?


Aha.  Thank you.  That explains what I'm seeing exactly.  The image 
that was being rotated was off-centre in the layer that contained the 
perspective transformation and turning it though 90 degrees left it 
"pointing towards the origin" in the parent layer.  Adding an 
intermediate (empty) layer, moving the perspective transform up to the 
intermediate layer and then centring the later containing the image to 
be rotated in that layer now allows the image to rotate as I would 
expect.

I've filled a bug report on the documentation for this, since there is 
currently no description of the nature of the transformations that 
take place.

Thanks again for the help,

   Nicko