Re: CGShading

FROM : R. Scott Thompson
DATE : Wed Oct 13 04:02:18 2004

> I tried that, and was never able to reproduce the result that OpenGL
> gives; compositing multiple CGShadings produces something
> similar-but-not-quite the same. Additionally, the clip path in Quartz
> isn't antialiased, which pretty much ruins any sort of compositing
> tricks for me.
>
> See old thread:
> http://www.cocoabuilder.com/archive/message/cocoa/2003/12/9/92421
>
> Of course, if you can draw a quad (or just a triangle) via CGShading
> with different colors at each corner that interpolate like OpenGL,
> please post the code!


That you can not do (at least not with arbitrary colors).  If you let
me specify the colors then I can do it, but if you want to be able to
pick any old colors then it won't work.  In particular,  if you wanted
to do a rect with black on one corner and three linearly independent
colors (like red, green, and blue) on the other three then it can't be
done with CGShading. That's where I was confused by John's example.  I
assumed the colors at the corners could be specified arbitrarily, but
with his shading code they are interdependent.

> Bonus points if you can do it with antialiased edges.


Are you sure the clipping path in Quartz is not anti-aliased.

Hang on...

The clipping path is antialiased. I just wrote a small test app that
drew a circular path, clipped to it, and then filled a rectangle over
the same area.  The edges of the circle were antialised:

// code before this translates the origin to the center of a view and
scales to a reasonable pixel size
CGContextAddArc(inContext, 0, 0, 1, 0, 2 * pi, true);    
CGContextClip(inContext);
CGContextFillRect(inContext, CGRectMake(-1, -1, 2, 2));


> Twilight Zone points if you can do it in one pass ;)


One pass with CG, or one pass in general?  Does it count if I use my
own code to scan convert the triangle into a CGImage and then draw the
resulting image with CG?  I could really use those twilight zone
points.

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