Conference bgsdev::multimedia_services

497.0. "multimedia services" by CSC32::T_HINTON () Wed Mar 19 1997 17:20

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\<s2><xxwkvm >unix 4.0b cust is having problems w/ mulimedia services           
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Will post the following note to the BGSDEV::MULTIMEDIA_SERVICES notes
conference to see if anyone can give us some ideas about what might cause
the hang situation that he is seeing.  What follows is what will be posted:

    I have a customer who has taken the viddualrecord.c program and
    modified it to act as a backend to a frontend program which runs
    in another process and has a gui user interface.  He is using shared
    memory to effect the communication between the 2 processes.  This
    arrangement works fine for a while but at various points the video
    card appears to stop streaming and his backend program hangs at the
    mmeWaitForCallbacks call.  He is aware that this is probably caused
    by a problem that he has introduced but he would like to know if
    anyone has any ideas what would cause the AV321 card to stop streaming.
    My guess would be that he has overwritten some memory somewhere that
    is essential to the proper operation of the card.  I suggested that
    he start pulling things out of the code until it stops failing and
    he is in the process of doing this but he would like to know if
    anyone has had similar experiences and what circumstances might
    cause the card to stop streaming.  He is running MME V2.2 under
    Digital Unix 4.0b and has a AV321 full video supreme card in his alpha
    workstation.

    Another question that he has regards the recording and displaying of
    black and white video.  He is currently using color because it can
    be recorded and displayed using dual buffers, but he would like to
    use black and white because of the space savings and his application
    only needs black and white information.  However, black and white
    seems to be limited to only one buffer so that he cannot record and
    display simultaneously.  Is this actually true, and if so, has anyone
    found a workaround?

The bug is someplace in their programs, but I could only
begin to guess where.  They need to debug it.  One thing
to watch for is that the MMS library is not thread safe,
so all calls must be protected by a mutex.  The whole
scheme will require careful design and verification by
the programmer to make sure that they don't introduce
races.

The easiest way to record black and white is to capture
YUV and through away the U&V (chrominance) compenents.
In other words, only keep every other pixel.  I'm not
sure why dual buffers are required.  Why can't the same
buffer be used for the screen and the file if the formats
are the same?  Dual capture is for capturing two 
*different* formats simultaneously.

Tom