Conference bgsdev::open3d

1231.0. "true color capture; using pseudo color default question?" by SAYER::ELMORE (Steve elmore@mail.dec.com 412-364-5893) Sat Feb 01 1997 18:08

    I am not quite sure where to post this question.  I am trying here
    first, but perhaps the X conference is better.  If someone would kindly
    point me to the right place, if not here, I would be grateful.
    
    I received the following mail from a medical imaging OEM.  I don't have
    the expertise to help them.  They are trying to mix pseudo and true
    color together but capture only true color.  Please read on:
    
    
>     Hi.
>
>     We spoke briefly in the hallway while you were at Picker MR
>     a few days ago. 
>
>     I have a problem with a system running Dec UNIX 4.0a and X/Motif.
>     (The problem is not OS or system specific, but it might make a
>     difference later.) 
>
>     The system has a 24 bit color card and the J300 video capture card.
>     We have set up our /usr/lib/X11/xdm/Xservers file to start the system
>     in pseudo color mode even though it is true color capable. We then
>     have some windows on the screen that use true color X visuals and
>     some
>     that use pseudo color. This works OK and saves memory and time when
>     the windows that use the default (pseudo color) visual are moved
>     around.
>
>     The problem comes up when we do video capture. That routine reads
>     pixels from the root window in order to capture the entire screen.
>     The
>     root window defaults to a pseudo color visual,  so the video capture
>     data is only 8 bits deep and is often the wrong value. Somehow we
>     need to
>     read the data from a source that has the full 24 bit pixels. 
>
>     Video capture works OK if we don't force the default visual to be
>     pseudo color, since then the root window comes out as true color and
>     the captured data looks fine. However, then we pay a price in
>     performance and memory usage since all our pixmap data triples in
>     size (from 8 to 24 bits). This can be 100s of megabytes in some
>     cases.
>
>     So, my problem is:
>
>        How can I read 24 bit pixel data from the frame buffer for video
>        capture when the X root window is set up with an 8 bit pseudo
>        color
>        visual? Is there a way to read directly from the frame buffer and
>        bypass X altogether ?
>
>
>     I hope this explanation makes some sense. Please let me know if you
>     have any ideas about this, or other sources I might try for help.
>
>       Thanks.
>               Don Russell
    

    Actually the right place is the Multimedia Services conference at
    BGSDEV::Multimedia_Services.
    
    The frame buffer of the J300 is entirely separate from the graphics
    frame buffer and you can certainly capture directly from it in any of
    the supported formats.  If the bundled applications don't meet their
    needs, they could certainly modify one of the example applications in
    the developer's subset to meet their needs.
    
    Tom

    Thanks...moved to the BGSDEV::Multimedia_Services conference.
    
    --Steve