IPhone UIViewContentModeRedraw & UIScrollView zooming is pixellated

Dear Experts,
I have a UIView that draws its content using Quartz2D operations (CGContextAddLineToPoint() etc) inside its drawRect: method. This view is a subview of a scrollview. Everything is fine until I zoom. It seems that the content is magnified as an image, so it looks pixellated. I thought that the solution to this would be to set the view's contentMode to UIViewContentModeRedraw, but this doesn't seem to have helped: my debugging suggests that drawRect is not being called when the UIScrollView changes the magnification.
I think that manual changes to the view's transform property have the same effect, i.e. they also don't cause drawRect to be invoked and the result looks pixelated.
In test code I can get decent results by instead doing the magnification on the graphics context inside drawRect: and calling setNeedsDisplay to ensure that drawRect is called. But I don't think I can wire this hack up to the UIScrollView.
Does anyone else have experience of zooming Quartz2D content?
Thanks for any suggestions -- Phil.

fwiw, I'm seeing the same issue when animating UIView frame resizes. My backing CGPathRef with a 1 pixel stroke border looks stretched during the animation.

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