Sets whether the polarity of the y axis is inverted when drawing an image.
YES if you want the image data to be inverted before drawing; otherwise, NO.
If flag is YES, the y-axis of the image's internal coordinate system is inverted, with the origin in the upper-left corner and the positive y axis extending downward. This method affects only the coordinate system used internally by the image and the orientation of the image when it is drawn; it does not affect the coordinate system used to specify the position of an image in a view. This method does not cause the receiver to recache itself.
If you set flag to YES and then lock focus and draw into the image, the content you draw is cached in the inverted (flipped) orientation. Changing the value for flag does not affect the orientation of the cached image.