All else aside, yes you can do this in Cocoa and/or Quartz.
Here are a couple different ways. For instruction's sake, here's it done
with a compositing operation, NSCompositeDestinationIn. Result color =
what's already in the context but with additional alpha taken from the new
drawing.
- (void)drawRect:(NSRect)dirtyRect {
NSRect bounds = [self bounds];
NSImage *textLayerImage = [[NSImage alloc] initWithSize:bounds.size]; {
[textLayerImage lockFocus]; {
NSRect textLayerBounds = {NSZeroPoint, [textLayerImage size]};
[@"I am a string" drawAtPoint:textLayerBounds.origin
withAttributes:nil];
NSGradient *alphaGradient = [[NSGradient alloc]
initWithStartingColor:[NSColor blackColor] endingColor:[NSColor clearColor]];
{
[[NSGraphicsContext currentContext] setCompositingOperation:
NSCompositeDestinationIn];
[alphaGradient drawInRect:textLayerBounds angle:0];
} [alphaGradient release];
} [textLayerImage unlockFocus];
[textLayerImage drawInRect:bounds fromRect:NSZeroRect/*whole thing*/
operation:NSCompositeSourceOver fraction:1.0];
} [textLayerImage release];
}
That works, but it loses the LCD antialiasing, as others have noticed. The
text is drawn into a transparent layer, which defeats subpixel rendering.
Also, most people will not understand your code, because of the use of
NSCompositeDestinationIn.
A more understandable method that will also preserve the LCD text is to set
a clip that has partial transparency. This requires mixing CoreGraphics
calls in with the Cocoa.
- (void)drawRect:(NSRect)dirtyRect {
CGContextRef ctx = [[NSGraphicsContext currentContext] graphicsPort];
NSRect bounds = [self bounds];
CGImageRef maskImage = CreateMaskImage(bounds.size); {
CGContextClipToMask(ctx, NSRectToCGRect(bounds), maskImage);
} CFRelease(maskImage);
[@"I am a string" drawAtPoint:[self bounds].origin withAttributes:nil];
}
CGImageRef CreateMaskImage(NSSize size) {
CGImageRef maskImage;
CGContextRef maskContext = CGBitmapContextCreate(NULL, size.width, size.
height, 8/*bitsPerComponent*/, 0/*bytesPerRow - 0 means CG picks*/, [[
NSColorSpace genericGrayColorSpace] CGColorSpace], kCGImageAlphaNone); {
CGGradientRef grayGradient = CGGradientCreateWithColors([[
NSColorSpace genericGrayColorSpace] CGColorSpace], (CFArrayRef)[NSArray
arrayWithObjects:(id)CGColorGetConstantColor(kCGColorWhite), (id)
CGColorGetConstantColor(kCGColorBlack), nil], NULL/*locations - NULL means
colors evenly distribute*/); {
CGContextDrawLinearGradient(maskContext, grayGradient,
CGPointZero, CGPointMake(CGBitmapContextGetWidth(maskContext), 0), 0
/*options*/);
} CFRelease(grayGradient);
maskImage = CGBitmapContextCreateImage(maskContext);
} CFRelease(maskContext);
return maskImage;
}
CoreImage is likely to not perform well for a small task like this.
CoreImage incurs some per-use costs that it makes up for with awesome
per-pixel performance. If you don't have very many pixels and your effect
is simple enough to do with Cocoa/CoreGraphics, the CoreGraphics approach is
likely to perform better.
-Ken
On Thu, Jan 29, 2009 at 12:27 AM, Oleg Krupnov <oleg.krup...@gmail.com>wrote:
> Yeah, the question is however how do I technically (e.g. in Cocoa)
> composite the "appropriate alpha" with an image, whether the
> background image, as you suggest, or with text image, as Ricky
> suggested.
>
> AFAIU, there is not such NSCompositingOperation to do this trick. It
> appears that I need to iterate all pixels of the intermediate bitmap,
> and multiply the transparency value by the mask. This is quite
> low-level, is there a better way?
>
>
> On Thu, Jan 29, 2009 at 8:32 AM, Kyle Sluder <kyle.slu...@gmail.com>
> wrote:
> > On Wed, Jan 28, 2009 at 10:57 AM, Thomas Davie <tom.da...@gmail.com>
> wrote:
> >> This solution will also throw sub-pixel anti-aliasing in the bin.
> >
> > Perhaps the better solution is to draw the text as normal and then
> > re-draw the background with the appropriate alpha on top.
> >
> > --Kyle Sluder
> >
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