Re: Drawing partially transparent gradient

On Sep 6, 2009, at 4:39 PM, Christiaan Hofman wrote:

> I am trying to draw an NSGradient starting from a light gray (though  
> that's irrelevant) to clearColor over a (light) background. However,  
> for some reason the gradient becomes dark in the middle, while near  
> the ends it looks as I expected. I already tried setting the  
> compositeOperation explicitly to NSCompositeSourceOver, but that did  
> not help (other operations are certainly wrong). Also playing with  
> the CG blend mode got me nowhere.

Without actually trying this out myself, here's my hunch: [NSColor  
clearColor] is defined as 0.0 alpha of black, not white:

   NSLog(@"clearColor: %@", [NSColor clearColor]);

   clearColor: NSCalibratedWhiteColorSpace 0 0

The first '0' means it's 0.0 for the white value. So the gradient is  
not transitioning just the alpha value, but the white value as well.  
The effect you're seeing is probably the same as what you'd see in  
Photoshop.

My guess is that creating an NSColor with 0.0 for alpha and the same  
white value as your start color will produce the result you want.

      - Scott