objective-ccocoansimageappkitnsshadow

Odd problem with NSImage -lockFocusFlipped:

I'm using NSImage's -lockFocusFlipped: method to do some drawing into an image. My code looks like this:

NSImage *image = [[NSImage alloc] initWithSize:NSMakeSize(256, 256)];
[image lockFocusFlipped:YES];
NSShadow *shadow = [[NSShadow alloc] init];
[shadow setShadowColor:[NSColor blackColor]];
[shadow setShadowBlurRadius:6.0];
[shadow setShadowOffset:NSMakeSize(0, 3)];
[shadow set];
NSRect shapeRect = NSMakeRect(0, 0, 256, 100);
[[NSColor redColor] set];
NSRectFill(shapeRect);
[image unlockFocus];

This code works to a certain point. I can confirm that the context is indeed flipped because [[NSGraphicsContext currentContext] isFlipped] returns YES, and also because shapeRect is drawn at the right position (using the top left corner as the origin). That said, the NSShadow does not seem to respect the flipped status of the context. Setting the shadow offset to (0, 3) should move the shadow down when the context is flipped, but it actually moves it up (which is what would happen in a standard non-flipped context).

This problem seems specific to -lockFocusFlipped, because when I'm drawing using this same code into a CALayer with a flipped coordinate system, the shadow is drawn just fine (respecting the flip). Documentation on -lockFocusFlipped also seems to be quite vague. This is all it says in the NSImage class documentation:

Prepares the image to receive drawing commands using the specified flipped state.

And I also found this note in the Snow Leopard AppKit Release Notes:

There are cases, for example drawing directly via NSLayoutManager, that require a flipped context. To cover this case, we add

- (void)lockFocusFlipped:(BOOL)flipped;

This doesn't alter the state of the image itself, only the context on which focus is locked. It means that (0,0) is at the top left and positive along the Y-axis is down in the locked context.

None of the docs seem to explain NSShadow's behaviour in this case. And through further testing, it seems NSGradient does not seem to respect the flipped state of the drawing context used by NSImage either.

Any insight is greatly appreciated :-)

Solution

  • From the NSShadow class reference:

    Shadows are always drawn in the default user coordinate space, regardless of any transformations applied to that space. This means that rotations, translations and other transformations of the current transformation matrix (the CTM) do not affect the resulting shadow.

    And that's what flipping ultimately is: Translate up, scale back the other way.

    There's no such statement for NSGradient, so I'd suggest filing a bug about that one.