Re: Drawing on Another App's CGDisplayCapture
Re: Drawing on Another App's CGDisplayCapture
- Subject: Re: Drawing on Another App's CGDisplayCapture
- From: Mike Paquette <email@hidden>
- Date: Thu, 22 Mar 2007 09:44:40 -0700
On Mar 22, 2007, at 9:15 AM, Ben H Kram wrote:
My group films lectures for Distance Education.
I wrote a little app that monitors Keynote and writes a timestamp
to a file for every slide viewed.
This is a great aid to our producers when they time sync the
presentation to video.
The videographers really would like some indication that this is
running when they see the first couple of slides from behind the
camera.
I was hoping to create a cue-mark superimposed on an upper corner
of the screen to show this.
I am assuming that Keynote uses CGDisplayCapture() to grab the
display. If I try to do the same when Keynote is in presentation
mode, I get a kCGErrorNoneAvailable, which I would expect.
Is there any way to write on another app's CGDisplayCaptured
screen? Perhaps at a lower level in the API?
That's an interesting application. Neat idea.
Alas, writing to another app's captured screen is fraught with
peril. If the app is simply drawing to display memory, there's
nothing to guarantee that what you write won't be overwritten.
If the capturing app is using OpenGL in a full screen context, there
is even more confusion possible, as OpenGL may put the framebuffer at
a different address than another program might see, and as the OpenGL
context flushes, the active framebuffer may change (page flipping, or
full screen buffer swaps happening). Full screen OpenGL contexts
also may alter pixel addressing through address coherency tricks like
microtiling. The exact device state at any time isn't' visible to
other processes, so there would be no easy way to synchronize your
drawing with the capturing app's drawing.
Mike Paquette
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