On Jan 31, 2007, at 08:19 AM, Steve Christensen wrote:
I'm not sure I understand the double counting part, but I
understand that once an operation has been performed on a context
that the resulting pixels take into account the pixel's previous
alpha value as well as that of the context's alpha. All I'm saying
is that I'd like to be able to do things like this:
Just to this instead:
CGContextSetRGBStrokeColor(theContext, r, g, b, 0.5);
CGContextStrokeRect(theContext, theRect);
That will get you exactly what you are looking for.
In this case, I'd expect to see a rectangle drawn at half the
current context alpha. I might have, for example, just drawn an
image at an arbitrary alpha on top of some background, then wanted
to draw a border at half that alpha.
And then here you'd get one drawn at 1/4 alpha.
I could obviously just carry around an alpha value to be used for
all the drawing and then use that for calculating relative alpha
values. But since the context must already know what its current
alpha value is, it seems reasonable to be able to just ask when
needed.
The context alpha is basically applied to all pixels, along with
their own alpha value. Imagine the context alpha as a big switch that
says "how much of this context's pixels do I let bleed through" and
each pixel's alpha as "how much of the previous pixel do I let bleed
through".
What makes this more interesting is that CGContextSetAlpha() only
works until the next call of the same (or a GState restore that
restores a previous setting of it). So it's not quite one or the
other. And the inherent danger of having a "getter" and doing math on
it is that you can set a value, it gets clipped, and when you get it
you get a value that is not what you expect.
--
Reality is what, when you stop believing in it, doesn't go away.
Failure is not an option. It is a privilege reserved for those who try.
David Duncan
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