Question

I have been programming for two years on iOS and never on mac. I am working on a little utility for handling some simple image needs that I have in my iOS development. Anyway, I have working code in iOS that runs perfectly but I have absolutely no idea what equivalents are for mac.

I’ve tried a bunch of different things but I really don’t understand how to start a graphics context on the Mac outside of a “drawRect:” method. On the iPhone I would just use UIGraphicsBeghinImageContext(). I know other post have said to use lockFocus/unlockFocus but I’m not sure how exactly to make that work for my needs. Oh, and I really miss UIImage’s “CGImage” property. I don’t understand why NSImage can’t have one, though it sounds a bit trickier than just that.

Here is my working code on iOS—basically it just creates a reflected image from a mask and combines them together:

    UIImage *mask = [UIImage imageWithContentsOfFile:[[NSBundle mainBundle]pathForResource:@"Mask_Image.jpg" ofType:nil]];
    UIImage *image = [UIImage imageNamed::@"Test_Image1.jpg"];
    UIGraphicsBeginImageContextWithOptions(mask.size, NO, [[UIScreen mainScreen]scale]);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(ctx, 0.0, mask.size.height);
    CGContextScaleCTM(ctx, 1.f, -1.f);
    [image drawInRect:CGRectMake(0.f, -mask.size.height, image.size.width, image.size.height)];
    UIImage *flippedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    CGImageRef maskRef = mask.CGImage;
    CGImageRef maskCreate = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                              CGImageGetHeight(maskRef),
                                              CGImageGetBitsPerComponent(maskRef),
                                              CGImageGetBitsPerPixel(maskRef),
                                              CGImageGetBytesPerRow(maskRef),
                                              CGImageGetDataProvider(maskRef), NULL, false);
    CGImageRef masked = CGImageCreateWithMask([flippedImage CGImage], maskCreate);

    CGImageRelease(maskCreate);

    UIImage *maskedImage = [UIImage imageWithCGImage:masked];
    CGImageRelease(masked);

    UIGraphicsBeginImageContextWithOptions(CGSizeMake(image.size.width, image.size.height + (image.size.height * .5)), NO, [[UIScreen mainScreen]scale]);

    [image drawInRect:CGRectMake(0,0, image.size.width, image.size.height)];
    [maskedImage drawInRect:CGRectMake(0, image.size.height, maskedImage.size.width, maskedImage.size.height)];

    UIImage *anotherImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext(); 

//do something with anotherImage

Any suggestions for achieving this (simply) on the Mac?

Answer 1

Here’s a simple example that draws a blue circle into an NSImage (I’m using ARC in this example, add retains/releases to taste)

NSSize size = NSMakeSize(50, 50);

NSImage* im = [[NSImage alloc] initWithSize:size];
NSBitmapImageRep* rep = [[NSBitmapImageRep alloc]
                            initWithBitmapDataPlanes:NULL
                                          pixelsWide:size.width
                                          pixelsHigh:size.height
                                       bitsPerSample:8
                                     samplesPerPixel:4
                                            hasAlpha:YES
                                            isPlanar:NO
                                      colorSpaceName:NSCalibratedRGBColorSpace
                                         bytesPerRow:0
                                        bitsPerPixel:0];

[im addRepresentation:rep];

[im lockFocus];

CGContextRef ctx = [[NSGraphicsContext currentContext] graphicsPort];
CGContextClearRect(ctx, NSMakeRect(0, 0, size.width, size.height));
CGContextSetFillColorWithColor(ctx, [[NSColor blueColor] CGColor]);
CGContextFillEllipseInRect(ctx, NSMakeRect(0, 0, size.width, size.height));

[im unlockFocus];

[[im TIFFRepresentation] writeToFile:@"/Users/USERNAME/Desktop/foo.tiff" atomically:NO];

The main difference is that on OS X you first have to create the image, then you can begin drawing into it; on iOS you create the context, then extract the image from it.

Basically, lockFocus makes the current context be the image and you draw directly onto it, then use the image.

I’m not completely sure if this answers all of your question, but I think it’s at least one part of it.