Re: [Linux-uvc-devel] frame rate of acquisition

[Laurent P. <lau...@sk...>]

Hi Olivier,

> what is the frame rate you have with the UVC driver ? I have only about
> 7 fps with either the test program or OpenCV and UVC driver. Can I
> change this ? For example, with pwc, we can switch the fps when loading
> the kernel module.

The frame rate should be selectable using VIDIOC_S_PARM, but that's currently 
not implemented. The V4L2 and UVC stream parameters negociation (pixel 
format, frame size, frame rate and compression quality) are not entirely 
compatible, I'm thus trying to find a way to solve the problem.

Basically, UVC negociates streaming parameters by setting the pixel format and 
frame size first, then negociates frame rate and compression quality. V4L2 
uses VIDIOC_S_PARM to set the frame rate and compression quality, and 
VIDIOC_S_FMT to negociate the format. My problem is that if the user selects 
a frame rate/compression quality before negociating the format (which is what 
V4L2 currently does), I will have no way to report a failure before format 
negociation, at which time compression quality/frame rate values might be 
changed by the driver without any notification to the user.

The other issue is that V4L2 reports the maximum buffer size to the user when 
VIDIOC_S_FMT/VIDIOC_TRY_FMT are called, and I need the compression quality to 
compute the buffer size, so I can't report it there if the compression 
quality is changed after pixel format and frame size negociation.

The easiest fix would be to modify V4L2 to negociate compression quality and 
frame rate after format negociation, but that would break backward 
compatibility, so it's probably not a valid solution.

Laurent Pinchart