Question

Please note that this question is about CGLayer (which you typically use to draw offscreen), it is not about CALayer.

In iOS, what’s the correct code to save a CGLayer as a PNG file? Thanks!

Again, that’s CGLayer, not CALayer.

Note that you CAN NOT use UIGraphicsGetImageFromCurrentImageContext.

(From the documentation, “You can call UIGraphicsGetImageFromCurrentImageContext only when a bitmap-based graphics context is the current graphics context.”)

Note that you CAN NOT use renderInContext:. renderInContext: is strictly for CALayers. CGLayers are totally different.

So, how can you actually convert a CGLayer to a PNG image? Or indeed, how to render a CGLayer in to a bitmap in some way (of course you can then easily save as an image).

---

Later … Ken has answered this difficult question. I will paste in a long example code that may help people. Thanks again Ken! Amazing!

-(void)drawingExperimentation
{
// this code uses the ASTOUNDING solution by KENNYTM -- Oct/Nov2010
//
// create a CGLayer for offscreen drawing
// note. for "yourContext", ideally it should be a context from your screen, ie the
// context you "normally get" in one of your drawRect routines associated with
// drawing to the screen normally.
// UIGraphicsGetCurrentContext() also normally works but you could have colorspace woes

// so create the CGLayer called notepad...

CGLayerRef notepad = CGLayerCreateWithContext(yourContext,CGSizeMake(1500,1500), NULL); 
CGContextRef notepadContext = CGLayerGetContext(notepad);

// you can for example write an image in to notepad
CGImageRef imageExamp = [[UIImage imageWithContentsOfFile:
    [[NSBundle mainBundle] pathForResource:@"smallTestImage" ofType:@"png"] ] CGImage];
CGContextDrawImage( notepadContext, CGRectMake(100,100, 50,50), imageExamp);

// setting the colorspace may or may not be relevant to you
CGContextSetFillColorSpace( notepadContext, CGColorSpaceCreateDeviceRGB() );

// you can draw to notepad as much as you like in the normal way
// don't forget to push it's context on and off your work space so you can draw to it

UIGraphicsPushContext(notepadContext);

// set the colors
CGContextSetRGBFillColor(notepadContext, 0.15,0.25,0.35, 0.45);
// draw rects
UIRectFill(CGRectMake(x,y,w,h));
// draw ovals, filled stroked or whatever you wish
UIBezierPath* d = [UIBezierPath bezierPathWithOvalInRect:CGRectMake(x,y,w,h)];
[d fill];
// draw cubic and other curves
UIBezierPath *longPath;
longPath.lineWidth = 42;
longPath.lineCapStyle = kCGLineCapRound;
longPath.lineJoinStyle = kCGLineJoinRound;
[longPath moveToPoint:p];
[longPath addCurveToPoint:q controlPoint1:r controlPoint2:s];
[longPath addCurveToPoint:a controlPoint1:b controlPoint2:c];
[longPath addCurveToPoint:m controlPoint1:n controlPoint2:o];
[longPath closePath];
[longPath stroke];

UIGraphicsPopContext();

// so now you have a nice CGLayer.

// how to save it to a file?

// you can save it to a file using the amazing KENNY-TM-METHOD !!!

UIGraphicsBeginImageContext( CGLayerGetSize(notepad) );
CGContextRef rr = UIGraphicsGetCurrentContext();
CGContextDrawLayerAtPoint(rr, CGPointZero, notepad);
UIImage* ii = UIGraphicsGetImageFromCurrentImageContext();
NSData* pp = UIImagePNGRepresentation(ii);
[pp writeToFile:@"foo.png" atomically:YES];
UIGraphicsEndImageContext();

// you may prefer to look at it like this:

UIGraphicsBeginImageContext( CGLayerGetSize(notepad) );
CGContextDrawLayerAtPoint(UIGraphicsGetCurrentContext(), CGPointZero, notepad);
[UIImagePNGRepresentation(UIGraphicsGetImageFromCurrentImageContext()) writeToFile:@"foo.png" atomically:YES];
UIGraphicsEndImageContext();

// there are three clever steps in the KENNY-TM-METHOD:
// - start a new UIGraphics image context
// - CGContextDrawLayerAtPoint which can, in fact, draw a CGLayer
// - just use the usual UIImagePNGRepresentation to convert to a png

// done!  a miracle

// if you are testing on your mac-simulator, you'll find the file
// simply in the main drive directory
return;
}

Answer 1

For iPhone OS, it should be possible to draw a CGLayer on a CGContext and then convert into a UIImage, which can then be encoded into PNG and saved.

CGSize size = CGLayerGetSize(layer);
UIGraphicsBeginImageContext(size);
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextDrawLayerAtPoint(ctx, CGPointZero, layer);
UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
NSData* pngData = UIImagePNGRepresentation(image);
[pngData writeToFile:... atomically:YES];
UIGraphicsEndImageContext();

(not tested)