[Libav-user] How can we set frame size for live stream?

[Carl Eugen Hoyos]

Wenpeng Zhou <zhou.wenpeng at ...> writes:

> Input #0, dshow, from 'video=Mercury USB2.0 Camera':
>   Duration: N/A, start: 58890.453000, bitrate: N/A
>     Stream #0:0: Video: mjpeg, yuvj422p, 1280x720, 
> 30 tbr, 10000k tbn, 30 tbc
> 
> I hope to change frame size from 1280x720 to 640x480. 

If your question is:
"How can I convince the decoder to output the frames 
that where originally encoded with a size of 1280x720 
as 640x480, ie how can the decoder do a resize?"
Then the answer is: This is generally impossible, a 
decoder outputs the frame that was encoded, there is 
libswscale (or the scale filter that uses libswscale) to 
do the resizing.

There is an exception though:
You can use -lowres 1 with some decoders (for example mjpeg 
iirc) to decode with lower resolution (and quality!) and 
faster speed to "half" (actually quarter) resolution, in 
your case that would be 640x360.
(I don't think this is what you are searching for, but it 
is useful on very low-end hardware.)

Carl Eugen