Combining Images with UIImage & CGContext – (Offscreen drawing)

Georg Tremmel — Wed, 20 Jan 2010 09:25:49 +0000

In Cocoa NSImage has a lockFocus method, that allows to draw images offscreen and combine them into one.

[img lockFocus];
//...
[img unlockFocus];

On the iPhone, UIImage lacks the lockFocus methods, instead the following:

// Create new offscreen context with desired size
UIGraphicsBeginImageContext(CGSizeMake(64.0f, 64.0f));
 
// draw img at 0,0 in the context
[img drawAtPoint:CGPointZero];
 
// draw another at 0,0 in the context, maybe with an alpha value
[another drawAtPoint:CGPointZero];
 
// ... and other operations
 
// assign context to UIImage
UIImage *outputImg = UIGraphicsGetImageFromCurrentImageContext();
 
// end context
UIGraphicsEndImageContext();

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Combining Images with UIImage & CGContext – (Offscreen drawing)