Wow, Ken! It works like magic (the second method I mean). Thank you
very much. I think I would never guess it myself.
For those who will reuse this code, don't forget to save/restore the
graphics context because CGContextClipToMask modified the clipping
region.
Sorry for late thanking, I had to postpone this task for a while.
Oleg Krupnov.
On Sun, Feb 1, 2009 at 4:03 AM, Ken Ferry <kenfe...@gmail.com> wrote:
> 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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