Applying perspective that is always relative to the screen - iphone

Question

I want to ask a question about the perspective that is achieved through CATransform3D.
I know that if you have a view that is 320x480 and then apply this:
CATransform3D perspective = CATransform3DIdentity;
CGFloat zDistance = 1000;
perspective.m34 = 1.0 / -zDistance;
view.layer.sublayerTransform = perspective;
you create a perspective that makes it look like the observer is looking straight at the center of the screen and therefore the same transformation looks different, depending on where the subview that is being transformed is located on the screen. For example, tilting a view looks like this when the view is in the middle of the screen:
And it looks like this if it's in the lower left corner:
Now, my problem is that making the perspective relative to the screen only works if the view I'm transforming is a subview of another view that is 320x480px big. But what if the view I want to transform is a subview of a view that is only 100x100px? Is there a way to make the perspective relative to the whole screen if the superview isn't the size of the screen?
Thanks in advance

Answer 1

According to apple
"The anchorPoint property is a CGPoint that specifies a location within the bounds of a layer that corresponds with the position coordinate. The anchor point specifies how the bounds are positioned relative to the position property, as well as serving as the point that transforms are applied around."
Your perspective should not be relative to the center of the screen or even to the center of your layer by default, is that where you have your anchor point? Aside from that though, what you seem to be asking is how to make your perspective appear to be relative to a different point. The trick is that your perspective is created by multiplying by your perspective matrix. Setting m34 to a small number does nothing magic, you are multiplying by your projection matrix.
YourProjection = {1,0,0 ,0,
0,1,0 ,0,
0,0,1 ,0,
0,0,-1.0/zdistance,1};
Remember that you can combine successive transforms by multiplying them together. Just transform your layer to wherever you want, then apply your Projection matrix, then transform it back, presto, perspective from a different origin.
float x = your coordinates relative to the screen center;
float y = same thing
TranslationMatrix = {1,0,0,0,
0,1,0,0,
0,0,1,0,
x,y,0,1};
ReverseTranslationMatrix = {1,0,0,0,
0,1,0,0,
0,0,1,0,
-x,-y,0,1};
//now just multiply them all together
Final = ReverseTranslation*YourProjection*Translation;
You will need to do the matrix math yourself, hopefully you already have a generic 4x4 column major matrix class that can do multiplication for you, if not i suggest you make one. Also, if you are interested, you might consider reading. This for an explanation of how the matrix you are currently using works, or This for a different take on projection matrices.