[FFmpeg-devel] probable dshow bug or strangeness

Don Moir donmoir at comcast.net
Mon Mar 10 22:27:48 CET 2014

----- Original Message ----- 
From: "Roger Pack" <rogerdpack2 at gmail.com>
To: "FFmpeg development discussions and patches" <ffmpeg-devel at ffmpeg.org>
Sent: Monday, March 10, 2014 2:58 PM
Subject: Re: [FFmpeg-devel] probable dshow bug or strangeness

> On 3/7/14, Don Moir <donmoir at comcast.net> wrote:
>> I am posting here since not quite sure and need some advice.
>> I got word that some captures devices were not working. Everything
>> enumerated and went thru but no display.
>> You get packets with zero size and then no decode success.
>> I traced this into dshow_pin.c and function libAVMemInputPin_Receive. In
>> this function there is:
>> buf_size = IMediaSample_GetActualDataLength(sample);
>> In this case, buf_size is always zero. I think it is supposed to be set
>> somewhere with IMediaSample_SetActualDataLength and could be a coding issue
>> with driver.
>> This apparently happens enough where I can't just blame the driver if thats
>> where it lies. Maybe it is supposed to be set in dshow_pin.c somewhere but
>> don't know.
>> When this happens (buf_size == 0), the real data length appears to be
>> returned by  IMediaSample_GetSize(sample); This is supposed to be the actual
>> buffer length and not necessarily the data length. When I change it to use
>> IMediaSample_GetSize it works perfect.
> Are you sure it's not just re-using the previous frame's worth of data?

I think I am sure about that. It plays normal if I use GetSize and nothing with GetActualDataLength.

> Maybe you should just ignore frames with size 0?
> Just wondering.

GetActualDataLength is always zero in this case. I think yes you should ignore it but as a quick fix I used:

buf_size = GetActualDataLength(sample);
if (!buf_size)
    buf_size = GetSize(sample);

If you completely ignore nothing will display and basically same thing.

The above is not correct and something else is going on. Still looking into it but have to do some other things first. If you have 
any ideas let me know. 

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