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    <p>Your confusion is quite understandable.</p> <p>In truth, a positive angle represents a rotation from the positive X axis toward the positive Y axis. A negative angle represents a rotation from the positive X axis toward the negative Y axis.</p> <p>The “native” Core Graphics coordinate system is modeled after the standard <a href="http://en.wikipedia.org/wiki/Cartesian_coordinate_system" rel="noreferrer">Cartesian coordinate system</a>, in which the Y axis increases <strong>upward</strong> on the page. In this system, a positive angle represents a counter-clockwise rotation:</p> <p><img src="https://i.stack.imgur.com/B2Juq.png" alt="normal coordinate system"></p> <p>So if you create your own <code>CGContext</code> (for example, by using <code>CGBitmapContextCreate</code> or <code>CGPDFContextCreate</code>), rotations will work as you expect.</p> <p>However, computer systems have historically used a coordinate system in which the Y axis increases <strong>downward</strong> on the page. In a flipped coordinate system like this, a positive angle represents a clockwise rotation:</p> <p><img src="https://i.stack.imgur.com/W7mBB.png" alt="flipped coordinate system"></p> <p>Notice that in both coordinate systems, a positive angle rotates from the positive X axis toward the positive Y axis.</p> <p>It turns out that UIKit flips the coordinate system of the graphics contexts that it creates for you. This includes the graphics context it sets up before sending you <code>drawRect:</code> and the graphics context it sets up in <code>UIGraphicsBeginImageContext</code>. (The <a href="http://developer.apple.com/library/ios/DOCUMENTATION/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_overview/dq_overview.html#//apple_ref/doc/uid/TP30001066-CH202-CJBBAEEC" rel="noreferrer"><em>Quartz 2D Programming Guide</em></a> explains this.) You can check this by looking at the current transform matrix (using <code>CGContextGetCTM</code>). You will find that it has a <code>-1</code> in its <code>d</code> element, meaning that the Y axis is flipped.</p> <p>A <code>UIView</code> also uses a flipped coordinate system for laying out its subviews, which affects the meaning of the <code>UIView</code> <code>transform</code> property.</p>
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    1. COthx a lot, appreciate it!
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    2. CObut in a flipped coordinate system like that, a positive angle represents a clockwise rotation, but the documents say "In iOS, a positive value specifies counterclockwise rotation." So i think it's still contradictive. Is the document wrong?
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    3. COIt's wrong sometimes. It depends on the context in which you use the transform.
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