iOS to Mac GraphicContext Explanation/Conversion

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. Anywa

Answer 1

Well, here's the note on UIGraphicsBeginImageContextWithOptions:

UIGraphicsBeginImageContextWithOptions

Creates a bitmap-based graphics context with the specified options.

The OS X equivalent, which is also available in iOS (and UIGraphicsBeginImageContextWithOptions is possibly a wrapper around) is CGBitmapContextCreate:

Declared as:

CGContextRef CGBitmapContextCreate (
   void *data,
   size_t width,
   size_t height,
   size_t bitsPerComponent,
   size_t bytesPerRow,
   CGColorSpaceRef colorspace,
   CGBitmapInfo bitmapInfo
);

Although it's a C API, you could think of CGBitmapContext as a subclass of CGContext. It renders to a pixel buffer, whereas a CGContext renders to an abstract destination.

For UIGraphicsGetImageFromCurrentImageContext, you can use CGBitmapContextCreateImage and pass your bitmap context to create a CGImage.

Answer 2

Here is a Swift (2.1 / 10.11 API-compliant) version of Cobbal's answer

    let size = NSMakeSize(50, 50);
    let im = NSImage.init(size: size)

    let rep = NSBitmapImageRep.init(bitmapDataPlanes: nil,
        pixelsWide: Int(size.width),
        pixelsHigh: Int(size.height),
        bitsPerSample: 8,
        samplesPerPixel: 4,
        hasAlpha: true,
        isPlanar: false,
        colorSpaceName: NSCalibratedRGBColorSpace,
        bytesPerRow: 0,
        bitsPerPixel: 0)

    im.addRepresentation(rep!)
    im.lockFocus()

    let rect = NSMakeRect(0, 0, size.width, size.height)
    let ctx = NSGraphicsContext.currentContext()?.CGContext
    CGContextClearRect(ctx, rect)
    CGContextSetFillColorWithColor(ctx, NSColor.blackColor().CGColor)
    CGContextFillRect(ctx, rect)

    im.unlockFocus()

Answer 3

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.

Answer 4

Swift3 version of Cobbal's answer

   let size = NSMakeSize(50, 50);
    let im = NSImage.init(size: size)

    let rep = NSBitmapImageRep.init(bitmapDataPlanes: nil,
                                    pixelsWide: Int(size.width),
                                    pixelsHigh: Int(size.height),
                                    bitsPerSample: 8,
                                    samplesPerPixel: 4,
                                    hasAlpha: true,
                                    isPlanar: false,
                                    colorSpaceName: NSCalibratedRGBColorSpace,
                                    bytesPerRow: 0,
                                    bitsPerPixel: 0)


   im.addRepresentation(rep!)
    im.lockFocus()

    let rect = NSMakeRect(0, 0, size.width, size.height)
    let ctx = NSGraphicsContext.current()?.cgContext
    ctx!.clear(rect)
    ctx!.setFillColor(NSColor.black.cgColor)
    ctx!.fill(rect)

    im.unlockFocus()