UIImage vs NSImage: Drawing to an off screen image in iOS

Question

In mac osx (cocoa), It is very easy to make a blank image of a specific size and draw to it off screen:

NSImage* image = [[NSImage alloc] initWithSize:NSMakeSize(64,64)];
[image lockFocus];
/* drawing code here */
[image unlockFocus];

However, in iOS (cocoa touch) there does not seem to be equivalent calls for UIImage. I want to use UIImage (or some other equivalent class) to do the same thing. That is, I want to make an explicitly size, initially empty image to which I can draw using calls like UIRectFill(...) and [UIBezierPath stroke].

How would I do this?

score 9 · Accepted Answer · answered Mar 22 '12 at 19:59

CoreGraphics is needed here, as UIImage does not have high level functions like what you explained..

UIGraphicsBeginImageContext(CGSizeMake(64,64));

CGContextRef context = UIGraphicsGetCurrentContext();
// drawing code here (using context)

UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

sch · Answer 2 · 2012-03-22T20:04:31.217

3

You can do that as follows:

UIGraphicsBeginImageContext(CGSizeMake(64, 64));
//Drawing code here
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Here is Apple's Graphics and Drawing reference.

edited Mar 22 '12 at 20:04

answered Mar 22 '12 at 19:57

sch

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But don't forget to end the image context! – Richard J. Ross III Mar 22 '12 at 19:59
Ok I assume that this is equivalent to creating the `NSImage` and calling `lockFocus`, but how would I unlock focus? – mtmurdock Mar 22 '12 at 20:01
1

Using the function `UIGraphicsEndImageContext();`. – sch Mar 22 '12 at 20:02

score 0 · Answer 3 · answered Mar 22 '12 at 19:59

0

Something like this:

UIGraphicsBeginImageContextWithOptions(mySize, NO, 0.0f);       
CGContextRef context = UIGraphicsGetCurrentContext();
UIGraphicsPushContext(context);

[myImage drawInRect:myImageRect];
[myText drawAtPoint:myOrigin withFont:myFont];

UIGraphicsPopContext();
UIImage *myNewImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

answered Mar 22 '12 at 19:59

Dave Batton

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Notice you'll want to use UIGraphicsBeginImageContextWithOptions rather than UIGraphicsBeginImageContext, so that you draw in the scale supported by the device. Using UIGraphicsBeginImageContext will look poor on a retina display. – Dave Batton Mar 22 '12 at 20:02
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`UIGraphicsPushContext(context);` and `UIGraphicsPopContext();` can be removed because `UIGraphicsBeginImageContextWithOptions(mySize, NO, 0.0f);` and `UIGraphicsEndImageContext();` already do that. – sch Mar 22 '12 at 20:11

UIImage vs NSImage: Drawing to an off screen image in iOS

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