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Creating an NSImage from an NSView

There are many different techniques for creating an NSImage from an NSView - finally I found one that works well!

There are various techniques:

  • lockFocus on the image, and then drawRect: the view

But this only draws the root view, not subviews.

  • lockFocus on the image, and then drawRect: the view and draw subviews

But its nigh on impossible to actually get the transformations correct for adjusting to the subview co-ordinate systems.

  • lockFocus on the view, and use initWithFocusedViewRect:

But this only gets the visible parts of the view.

  • use dataWithPDFInsideRect

But this is quite slow and has some drawing artefacts when the resulting image is displayed on the screen.

I finally found Gerd Knops’ post which yields a good result.

- (NSImage *)imageWithSubviews
 NSSize mySize = self.bounds.size;
 NSSize imgSize = NSMakeSize( mySize.width, mySize.height );
 NSBitmapImageRep *bir = [self bitmapImageRepForCachingDisplayInRect:[self bounds]];
 [bir setSize:imgSize];
 [self cacheDisplayInRect:[self bounds] toBitmapImageRep:bir];
 NSImage* image = [[[NSImage alloc]initWithSize:imgSize] autorelease];
 [image addRepresentation:bir];
 return image;

I am combining this with Matt Gemmell’s MGViewAnimation with good results for use in alpha fading views under 10.5.

Posted Tuesday, April 21, 2009. Permalink. 1 Comments.


I found an interesting bug in cacheDisplayInRect and focus ring drawing when the view you are imaging is enclosed in an NSScrollView/NSClipView (I'm not sure which, since I never use one without the other). In this case, if there is an active (firstResponder) control with a focus ring, the focus ring is clipped incorrectly. The view draws highlighted as normal, but the focus ring draws partially clipped. Eventually I determined that the offset of the clip was the same as the offset of the enclosing NSScrollView/NSClipView in the window.

I developed the following work around, which increases the bounds of the desired cached area by the appropriate offset. rdar://problem/6987153.

- (NSImage *)imageWithSubviews
 NSRect realBounds = self.bounds;
 NSSize realSize = realBounds.size;
 NSRect fakeBounds = realBounds;

 NSScrollView* hackScrollView = self.enclosingScrollView;

 NSPoint offset = NSZeroPoint; 
 if ( hackScrollView ) {
  NSPoint botLeft;
  botLeft.x = NSMinX( hackScrollView.bounds );
  botLeft.y = hackScrollView.isFlipped ? NSMaxY( hackScrollView.bounds ) : NSMinY( hackScrollView.bounds );
  offset = [hackScrollView convertPoint:botLeft toView:hackScrollView.window.contentView];
 fakeBounds.origin.x -= offset.x;
 fakeBounds.origin.y -= offset.y;
 fakeBounds.size.width += offset.x;
 fakeBounds.size.height += offset.x;
 NSSize fakeSize = fakeBounds.size;
 NSBitmapImageRep *bir = [self bitmapImageRepForCachingDisplayInRect:fakeBounds];
 [bir setSize:fakeSize];
 [self cacheDisplayInRect:fakeBounds toBitmapImageRep:bir];
 NSImage* image = [[[NSImage alloc] initWithSize:realSize] autorelease];
 [image lockFocus];
 verify( [bir drawAtPoint:fakeBounds.origin] );
 [image unlockFocus];
 return image;

Posted Tuesday, June 23, 2009 05:38 AM by Peter N Lewis.

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