UVC frame interval inconsistency

UVC frame interval inconsistency

[Rémi Denis-Courmont]

   Hello guys,



I have been reworking the V4L2 input in VLC and I hit what looks like a

weird bug in the UVC driver. I am using a Logitech HD Pro C920 webcam.



By default, VLC tries to find the highest possible frame rate (actually

smallest frame interval in V4L2), then the largest possible resolution at

that frame rate.



When enumerating the frame sizes and intervals on the device, the winner

is 800x600 at 30 f/s. But when setting 30 f/s with VIDIOC_S_PARM, the

system call returns 24 f/s. Does anyone know why it is so? Is this a

firmware bug or what?



-- 

Rémi Denis-Courmont

Sent from my collocated server

Re: UVC frame interval inconsistency

[Laurent Pinchart]

Hi Rémi,

On Wednesday 11 April 2012 12:27:08 Rémi Denis-Courmont wrote:
>    Hello guys,
> 
> I have been reworking the V4L2 input in VLC and I hit what looks like a
> weird bug in the UVC driver. I am using a Logitech HD Pro C920 webcam.
> 
> By default, VLC tries to find the highest possible frame rate (actually
> smallest frame interval in V4L2), then the largest possible resolution at
> that frame rate.
> 
> When enumerating the frame sizes and intervals on the device, the winner
> is 800x600 at 30 f/s. But when setting 30 f/s with VIDIOC_S_PARM, the
> system call returns 24 f/s. Does anyone know why it is so? Is this a
> firmware bug or what?

The frame sizes and intervals returned by the uvcvideo driver during 
enumeration come directly from the values advertised by the device. When you 
set a frame rate using VIDIOC_S_PARM, the driver then negotiates the value 
with the device, and returns the frame rate it received from the device to the 
application. The device is free to adjust the frame rate (based on current 
lightning conditions for instance, if auto-exposure is turned on).

-- 
Regards,

Laurent Pinchart