How to add shadow to a group of CALayer using CATransform3D - objective-c

Question

How can I add shadow to a group of CALayers?
I have a "FoldingView"-class which maintans several "SliceView"-classes. Each sliceview's layer will be givven a CATransform3D where I'm using the perspective property (.m34 = 1.0 / -1000).
How can I add a shadow with good visual logic? This is what I've been thinking so far:
I could get the path of each slice and combine these to get a shadow path.
I don't know how the get a path of CALayer when the layer is using CATransform3D
It might work visually, but I'm afraid it won't be totally right if the light is supposed to come from top left.
I could just apply standard CALayer shadow to all layers
It does not look good due to the shadow is overlapping each other
If anyone has any other suggestions or know how to code idea number 1 I'll be very happy! Here is a link to the sample project you see screenshot from.
Download zipped application with code

Answer 1

This seems to work as wanted. This is done per sliceview.
- (UIBezierPath *)shadowPath
{
if(self.progress == 0 && self.position != VGFoldSliceCenter)
{
UIBezierPath *path = [UIBezierPath bezierPath];
return path;
}
else
{
CGPoint topLeft = pointForAnchorPointInRect(CGPointMake(0, 0), self.bounds);
CGPoint topRight = pointForAnchorPointInRect(CGPointMake(1, 0), self.bounds);
CGPoint bottomLeft = pointForAnchorPointInRect(CGPointMake(0, 1), self.bounds);
CGPoint bottomRight = pointForAnchorPointInRect(CGPointMake(1, 1), self.bounds);
CGPoint topLeftTranslated = [self.superview convertPoint:topLeft fromView:self];
CGPoint topRightTranslated = [self.superview convertPoint:topRight fromView:self];
CGPoint bottomLeftTranslated = [self.superview convertPoint:bottomLeft fromView:self];
CGPoint bottomRightTranslated = [self.superview convertPoint:bottomRight fromView:self];
UIBezierPath *path = [UIBezierPath bezierPath];
[path moveToPoint:topLeftTranslated];
[path addLineToPoint:topRightTranslated];
[path addLineToPoint:bottomRightTranslated];
[path addLineToPoint:bottomLeftTranslated];
[path closePath];
return path;
}
}