Re: To Draw | ! To Draw in an Expose Handler

On 11/30/07, Richard Boaz <riboaz xs4all nl> wrote:
> Sorry, I can't think of a single application responsible for making its
> own drawings by hand where option 2 is not crazy hard.  (If anybody has an
> example of this in real-world application, please post?)

My 2p: Most applications let you zoom the display, or at least have a
main window which is a viewport into a much larger document. There's
no way you can keep a pixmap for the whole thing (imagine a word
processor, or an image editor with the display zoomed in 10 times), so
you will always have to have some mechanism for clipping your document
view against a viewport.

Once you have this, it's trivial to clip against each expose event
rather than the viewport, and once you do that, you don't need a
backing pixmap. No pixmap means less memory use (X server memory is
still constrained, so this is a win), and the slight loss in speed is
more than made up for (IMO) by the drop in latency due to not having
to manage a large memory object.

My app, for example, is an image processing package. The images are
very expensive to generate, so I keep parts of them around (in a tile
cache) on the client side. On an expose, I extract the set of rects
for the damaged area from the image and paint them to the screen, then
I loop over all the image annotations and for each one test for
bounding-box-of-expose-intersects-bounding-box-of-annotation. If there
is an intersection, I redraw that annotation.

I will only ever have a few hundred annotations, so I can just loop
over a list. If I had 900m objects (!!!!eeek) then yes, I'd need
either a very fast culling algorithm (quad trees or some such I
suppose) or a cache of pre-rendered views I could paste from. Even in
this case I would still compose the window in expose: add rubber-band
objects, annotations, axies, etc. But I think very few applications
have to deal with datasets of this size.

(though my image processing app can do multi-gigabyte images, so maybe
that is of a similar scale, if you consider each pixel to be a
separate object)


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