[gst-devel] How to control pipeline frame rate when my camera needs some time to get one frame data

[Stefan Kost]

Edward Hervey schrieb:
> On Wed, 2009-11-18 at 16:24 +0800, Zhao, Halley wrote:
>> But the timestamp will be mess.
>>
>> You could get some frames in a short time, then idle some time, then
>> another some frames.
> 
>   What thiagoss recomended is correct (you basically end up with one
> thread dedicated to capture the image and the other threads doing the
> extra processing).
>   Timestamping is done at the source element (i.e. the timestamp will be
> the moment at which the image is captured).

Unfortunately not entierly correct. The timestamp is the time when the capture
thread actualy gets scheduled :/ Sometimes using the timestampo provided by v4l2
itself would be better, but this would need skew slaving ...

Running the capture thread with SCHED_RR helps a lot.

Stefan

> 
>   If you were to give the gst-launch pipeline you're testing with we
> could give you more info.
> 
> 
>     Edward
> 
>>  
>>
>>  
>>
>>                                    
>> ______________________________________________________________________
>> From:thiagossantos at gmail.com [mailto:thiagossantos at gmail.com] 
>> Sent: 2009年11月18日7:19
>> To: Discussion of the development of GStreamer
>> Subject: Re: [gst-devel] How to control pipeline frame rate when my
>> camera needs some time to get one frame data
>>
>>
>>  
>>
>>  
>>
>> On Tue, Nov 17, 2009 at 8:11 PM, Zhao, Halley <halley.zhao at intel.com>
>> wrote:
>>
>> Hi all:
>>
>> My camera needs ~20ms to get one frame of data, and a video sink in
>> the pipeline needs ~4ms to render it.
>>
>> But when I set pipeline fps to 35, I only got 20 fps (calculate by GST
>> CLOCK). CPU is not busy at this time, only 60%.
>>
>>  
>>
>> I guess the reason is following:
>>
>> When we got nth frame at time 0, gst framework schedule to get next
>> frame after 30ms (1000/35), and when the camera src element got
>> scheduled to create one frame, it needs 20ms to get the data.
>>
>> Then the time between n frame and n+1 frame is 30+20=50ms, so I just
>> got 20 fps.
>>
>> Is my assumption right?
>>
>> If yes, is there any way to adjust it?
>>
>>
>> You can speed it up by adding a queue right after your source element.
>> It will create another thread to push data after it, the thread used
>> to request a frame from the camera will quickly return to the source
>> and it will request another one.
>>  
>>
>>
>>          
>>         
>>         I had though to create another thread inside camera source,
>>         and prepare the frame data in a buffer before it is schedule.
>>         
>>         However, I think it is hack, and make timestamp differ. I
>>         think it’s better settled inside gst framework. 
>>         
>>          
>>         
>>          
>>         
>>         ZHAO, Halley (Aihua)
>>         
>>         Email: halley.zhao at intel.com
>>         
>>         Tel: +86(21)61166476
>>         
>>         iNet: 8821-6476
>>         
>>         SSG/OTC/Moblin 3W038 Pole: F4
>>         
>>         
>>          
>>         
>>         
>>         
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>>
>> -- 
>> Thiago Sousa Santos
>>
>>
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