The correct scale of taken photo and overlay image in IOS 4 on iPhone 4?

I am making a Camera application with overlay frame. That's what I did:

• All navigation bar, tool bar and control buttons are hidden
• The 开发者_开发技巧preview rectangle is rescaled with the function self.cameraViewTransform = CGAffineTransformScale(self.cameraViewTransform, 1, 1.24299);
• The preview is set to Full Screen (640 x 960) with the above transformation
• Overlay Frame (320 x 480) is set in ImagePickerController's cameraOverlayView property

After picture is taken, here is what I did: - Resize and draw the taken image (1936 x 2592) to 640 x 960 in currentContext - Draw the overlay frame (320 x 480) to that 640 x 960 currentContext

But I found that: - scale is not that right - the frame appeared moved towards the top edge for about 2x pixels

Is there any hint/solution about this issue ?

1936x2592 gives you aspect ratio of 0.74
640x960 gives you aspect ratio of 0.66

640x856 (320x428) would be the correct image size to preserve aspect ratio.

hope it helps...