The "nearly" OpenGL coordinate system is quite tricky when it comes
to absolute positionning.
The attached compo will do the job you're looking for
It looks a bit heavy for such a common task, but to that's the best
way i've found.
First, we try to 'attach' our image image to the left edge.
in the QC coordinates system, the width of the view is always 2. The
origin is at the center of the screen
we always have the left edge of the view at X = -1, and the right
edge of the view at X=1.
If you set the X position of your billboard to -1, you have the
center of the image aligned on the left edge,
because X define the position of the center of your billboard. To
correct this, you just have to add the (width of the image) / 2. You
achieve this using an "image dimensions" and "Math" patch.
OK, we're done for the X coord, now Y:
unlike the width, the height of the viewport depends on the aspect
ratio of your viewport. If you're viewport is twice as high as it is
wide, the height of the patch is 4 (2x width). The "height" output of
the "Rendering Destination Dimensions" patch give what you're looking
for. Using that, you know that the y coordinate of the bottom and top
edges are respectively is -height / 2 and height / 2
As before, you need to compensate the fact that image are postioned
by their center, you need to substract one half of the image height.
You're done.
Note this works only if you render your billboard at scale 1 (i.e. by
biding the image "width" output to your Billboard's "width" input
--
Xavier Michelon
On May 18, 2005, at 18:45, patrick wrote:
That's excellent news!
Perhaps I could do it all in a single composition file; but one of my
major challenges is using the co-ordinate system that Quartz Composer
(OpenGL?) uses. I so far haven't really found any good way of
positioning something exactly in the upper-left corner of the screen.
I have a square sprite whose X and Y co-ordinates are set to (-0.84,
-0.6) which seems pretty close to the upper left, but not exactly.
Plus, these co-ordinates are not very intuitive to me. Is there any
way I can do a weird-coordinate-to-pixel conversion?
Thanks,
Patrick
On 5/16/05, Pierre-Olivier Latour <email@hidden> wrote:
Provided that I'm not worried about supporting lower-end systems and
anything before Tiger, would it be a bad approach to load a Quartz
composition multiple times on the screen and getting each instance to
do all of the transition work, or would it be considerably better
(from a memory and processor usage point of view) to do everything
programmatically using OpenGL calls to load the images onto planes on
which the transitions will occur?
Assuming you cannot or do not want to put everything into a single
composition file, you can definitely create multiple QCRenderers
from the
same composition and mix their drawing with your custom OpenGL
code. The
overhead of a QCRenderer is very low.
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