To control a USB video camera from the computer it is connected to, the USB Video Class (UVC) standard defines several sets of controls that a camera can declare to support. The computer can then command the camera via the UVC protocol.

Showmewebcam's default behaviour is to advertise all controls as available. The uvc-gadget program on the Pi then translates incoming control read or write requests to the V4L interface for the camera module attached to the Pi. The command chain is quite long, as you can see:

• Computer
• (UVC control protocol)
• uvc-gadget
• (V4L2 control protocol)
• Pi V4L Driver
• (MMAL protocol)
• Pi GPU
• (MIPI CSI)
• Camera module

Therefore, there's unfortunately no direct mapping of the controls you can manipulate on your computer to what happens on the camera, although everybody involved along the chain is doing their best.

Showmewebcam's default is to mark the following controls as exposed in the UVC descriptor (taken from uvc-gadget documentation on UVC controls):

Camera Controls (values 2, 0, 0):

Processing controls (values 255, 6):

Byte 1:

Byte 2:

You can add the following parameters to cmdline.txt on the boot volume (either by mounting /boot read-write on the Pi, or after flashing while preparing the SD card on the computer) as follows:

usb_f_uvc.camera_terminal_controls=0,0,0 usb_f_uvc.processing_unit_controls=0,0

The previous example sets all controls to disabled, and should thus be safe. The parameters directly correspond to the bmControls bitfields in the descriptor, and are explained below. To compute a new value, you sum up the control bits' "value" column you want to enable.

For the default of usb_f_uvc.camera_terminal_controls=2,0,0 above, you sum only bit D1 (Auto-Exposure), i.e. 2 for byte 1, and leave the other bytes 0.

Say, you want to disable the Hue, White Balance controls. This gives you: usb_f_uvc.processing_unit_controls=59,6 (1+2+8+16+32=59, 2+4=6)

Perhaps stating the obvious, you CANNOT add controls to the camera this way.

!!! Only controls properly mapped in V4L and uvc-gadget can possibly work. !!!

And even then, there's a good chance your control might not work due to camera and/or driver quirks.

source: USB Device Class Definition for Video Devices, Revision 1.1 Again shout-out to the uvc-gadget documentation on UVC controls

This setting corresponds to the UVC_VC_INPUT_TERMINAL structure's bmControls field. It is a bitmap type. A bit set to 1 indicates that the mentioned Control is supported for the video stream.

Byte 1:

Byte 2:

Byte 3:

This setting corresponds to the UVC_VC_PROCESSING_UNIT structure's bmControls field. It is a bitmap type. A bit set to 1 indicates that the mentioned Control is supported for the video stream.

Byte 1:

Byte 2:

Byte 3: (NOT SUPPORTED BY KERNEL)

For reference only, the UVC descriptor for processing controls supports this third byte, which is not present in the kernel's uvc gadget implementation. As it is not relevant to webcam operation, no further work was undertaken to add support. To reiterate: The commandline flag usb_f_uvc.processing_unit_controls=<byte1>,<byte2> only takes two values!