Re: drawing in a separate thread
Re: drawing in a separate thread
- Subject: Re: drawing in a separate thread
- From: Duncan <email@hidden>
- Date: Sat, 3 May 2008 07:36:29 -0400
On May 3, 2008, at 5:00 AM, Graham Cox <email@hidden> wrote:
One thing I realised after the previous posting is that the
performance hit I'm seeing is because unlike the usual drawRect:
case, I wasn't doing any clipping to the update area, so the entire
view was getting repainted rather than just some small part of it -
adding a clipRect: call substantially improved things.
However it also made me realise that just doing the drawing on a
second thread is not enough - my graphics code makes quite a bit of
use of the view's -needsToDrawRect: method, which of course when
it's called by the thread is just not going to be valid, as
drawRect: has been and gone. I'm not sure how to tackle this - it
would seem I'd have to cache the result of -
getRectsBeingDrawn:count: each time I queue a draw from the main
thread and somehow make these rects available to clients of the view
when they are drawn from the second thread (by overriding
needsToDrawRect: for example). This seems complicated and somewhat
hackish - or I redesign the drawing code to have these rects passed
down as a parameter for every draw, which is equally awkward.
If you're going to do your drawing in a separate thread, you'll need
to remember WHAT to draw. I wouldn't call it "hackish", I'd call it
the cost of doing business that way.
If you implement a job queue, add the list of dirty rectangles to each
job object. -drawRect would be where you'd add your jobs to the queue.
It cold call getRectsBeingDrawn:count to get the list of dirty rects,
then add that list to the job object. Then when the worker thread
picks up a job, it would get the list of dirty rectangles from the job
object instead of calling getRectsBeingDrawn:count. That should be
pretty straightforward.
One thing you may face with rendering in a separate thread: At some
point, you may need to wait for rendering to complete before your app
can go on to it's next task. Say, for example, you need to show the
user the results of their changes, then ask them what to do next. In
that case, you'd need a way to wait until drawing completes. You could
add something like a flushRendering call (much like glFlush in OpenGL)
that wold block until all the rendering jobs were completely, or
waitUntilJobComplete, which could wait until a specific rendering job
was complete
I'm not yet convinced that drawing on a second thread will really
give any benefit, so I'm reluctant to change things dramatically to
allow it when I may end up abandoning the whole idea.
Be useful to hear from anyone who's done something like this to a)
convince me it's worth it and b) hear about any solutions to the
update rects issue.
Using a separate thread for time-consuming drawing does make the user
experience much better. My app does OpenGL rendering of large, complex
3D fractals, and I ended up pulling out the drawing code into a
separate thread. That's even hairier than what you're trying to do,
because I use NSOpenGLViews, and they are not particularly well set up
for multi-threaded use.
What I do is to display a progress bar at the bottom of my 3D view
window. As rendering progresses, I update the progress bar. I return
control to the user immediately, so I avoid the spinning wait cursor.
If the user then does something that invalidates the current rendering
task, I set a "terminate rendering" flag and wait for the rendering
thread to stop, then submit a new rendering job.
Duncan C
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