On Aug 25, 2010, at 9:45 AM, Annick Fron wrote:
> Hi,
>
> What is the usage of groupWhile: in cairo ?
> I tried it without success.
Sorry for the belated reply Annick, I saw this, meant to respond ASAP,
and then got distracted.
A simple example might look something like this:
p := Pixmap extent: 400 @ 400.
p graphicsContext newCairoContextWhile:
[:cr |
cr groupWhile:
[cr source: ColorValue blue.
cr paint].
cr paint].
p asImage
Inspect that and you'll see a blue image. The example is overly trite,
there is no real value in doing a groupWhile: there. But the possibly
missed part, is that you need to do something with the result of
groupWhile: to actually see anything. The beginning action of a
groupWhile: is temporarily retarget the cr at a second/temporary
surface of the same type as the original, the blocks actions are
executed, and then the cr is returned to the original surface AND the
temporary surface is set as the current source. So in the example
above, that's where the second paint call, outside of the block comes
in. I've used a group to paint a blue source, but it's not been
painted onto my current surface. You can use this pattern, with paint,
to basically get double buffering, showing the result of a lot of
complex drawing operations in one single final paint. But you're not
limited to paint'ing the result of a groupWhile:
p := Pixmap extent: 100 @ 100.
p graphicsContext newCairoContextWhile:
[:cr |
cr groupWhile:
[cr source: ColorValue blue.
cr circleCenter: p extent half radius: p width half.
cr fill].
cr rectangle: (p bounds insetBy: 6).
cr strokeWidth: 3.
cr stroke].
p asImage
In this example, we use the group to draw a blue filled circle, which
is then set as the source. But rather than just painting that, we then
create a rectangle and stroke the rectangle with the blue circle. So
it plays in basically building composition stacks.
groupWhile: is just a handy helper btw, the real meat of the
functionality are found in the methods #pushGroup, #popGroup, and
#popGroupToSource.
HTH
--
Travis Griggs
Objologist
"The best way to know you have a mind is to change it" -Judge Pierre
Leval
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