On 22/01/2013, at 4:35 PM, Andy Lee <[email protected]> wrote:
> On Jan 21, 2013, at 5:12 PM, Graham Cox <[email protected]> wrote:
>> My question is, is there a way to directly convert coordinates between two
>> unrelated layers in a tree, or are these methods implemented by recursion up
>> to a common parent node and then back down to the target layer? If I had
>> some hint of how this was done in the real CALayer architecture it would
>> help me avoid wasting time trying various blind alleys.
>
> Unless and until someone chimes in with a more sophisticated solution, you
> could convert the rect from layer A to the root layer, and from the root
> layer to layer B, then find out if that bogs down performance.
Thanks Andy,
I'm doing something rather like this, and it works well enough. In fact I'm not
sure there really is a need to translate between arbitrary layers across the
tree - 99 times out of 100 I'm just transforming up or down one branch in the
tree, and in practice the tree is usually shallow.
I think I need to change the focus of my question though, because before I can
get to this, I need to work out precisely how I should be manipulating my
transforms. As usual with transforms, what starts off looking straightforward
turns into a mindbending nightmare quite quickly.
My model looks pretty similar to CALayer, so my layer objects have a bounds,
frame, an anchor point, a transform and a position exactly as they are defined
for CALayer.
When a layer draws its sublayer, it applies the following transformations to
the context:
CGPoint anch = layer.anchorPoint;
CGRect br = layer.bounds;
CGPoint pos = layer.position;
CGContextTranslateCTM( ctx, pos.x, pos.y );
CGContextConcatCTM( ctx, layer.transform );
CGContextTranslateCTM(ctx, -(br.origin.x + anch.x * br.size.width),
-(br.origin.y + anch.y * br.size.height));
That works fine.
Where I'm getting into trouble is translating a rect from the layer's local
coordinates back up to super. It works until I have a transform property that
does something. So I'm doing this:
CGRect br = self.bounds;
CGPoint anch = self.anchorPoint;
CGAffineTransform tfm = CGAffineTransformMakeTranslation(
-self.position.x, -self.position.y );
tfm = CGAffineTransformConcat( tfm, self.transform );
//<------ this screws things up if it's not the identity matrix
return CGAffineTransformTranslate( tfm, (br.origin.x + anch.x *
br.size.width), (br.origin.y + anch.y * br.size.height));
To create a transform that supposedly will transform from the superlayer TO the
layer, and I then invert it to go back the other way. It works in the simple
case, but a rotation transform upsets things. I've tried pretty much every
combination of ordering different things in constructing the transform but this
is the "best" result I get. Still wrong though. Funny, every time I think I
have transforms figured out something turns up to show me I don't.
I just need someone to point out my error, then I think I'll be in business.
--Graham
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