Re: [PATCH v3 04/11] media: v4l2-ctrl: Document V4L2_CID_CAMERA_SENSOR_ROTATION

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 9/12/19 10:10 PM, Jacopo Mondi wrote:
> Add documentation for the V4L2_CID_CAMERA_SENSOR_ROTATION camera
> control. The newly added read-only control reports the camera device
> mounting rotation.
> 
> Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
> ---
>  .../media/uapi/v4l/ext-ctrls-camera.rst       | 117 ++++++++++++++++++
>  1 file changed, 117 insertions(+)
> 
> diff --git a/Documentation/media/uapi/v4l/ext-ctrls-camera.rst b/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> index f879dcc9409c..74991522ca3a 100644
> --- a/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> +++ b/Documentation/media/uapi/v4l/ext-ctrls-camera.rst
> @@ -542,6 +542,123 @@ enum v4l2_scene_mode -
>  
>  
>  
> +``V4L2_CID_CAMERA_SENSOR_ROTATION (integer)``
> +    This read-only control describes the sensor orientation expressed as
> +    rotation in counterclockwise degrees along the axis perpendicular to the
> +    device mounting plane, and directed away from the sensor lens. Possible
> +    values for the control are 90, 180 and 270 degrees. To compensate the device

compensate -> compensate for

> +    mounting rotation on the captured images, a rotation of the same amount of
> +    degrees, in the same counterclockwise rotation direction should be applied
> +    along the axis directed from the observer to the captured image when
> +    displayed on a screen.

Is this right? Shouldn't that be "in the clockwise direction"? If the sensor is
mounted 90 degrees counterclockwise, then I need to rotate by 90 degrees clockwise
to compensate for that, right?

> +
> +    To better understand the effect of the sensor rotation on the acquired
> +    images when displayed on a screen, it is helpful to consider a fictional
> +    scan-out sequence of the sensor's pixels, assuming the pixel array having
> +    its top-left pixel at position (0, 0) with values on the 'x' axis increasing
> +    towards the right direction, and values on the 'y' axis increasing towards
> +    the bottom. The effect of sensor rotation could be easily visualized
> +    considering the sequence of captured pixels.
> +
> +    Assuming the following scene has to be captured::
> +
> +                o
> +               -|-
> +               / \
> +
> +    An upright mounted sensor has its pixel array displaced as follow::
> +
> +                                      x
> +            (0,0)---------------------->
> +              ! 0,0 0,1 0,2 ... 0,line-len

Isn't that 0,0 ... 0,num-col?
line-len is a weird name, shouldn't that be num-lines?

line-len sounds like it is the same as num-col.

I'm totally confused.

> +              ! 1,0 1,1 1,2 ...
> +              ! ...
> +              ! ...
> +              ! (num-col,0)...  (num-col,line-len)
> +            y V
> +
> +
> +    Assuming pixels are scanned out from (0,0) to (num-col,line-len)
> +    progressively::
> +
> +             (0,0) ---->-------------> (0,line-len)---!
> +             !------------------------------------<a--!
> +             V
> +             (1,0) ---->-------------> (1,line-len)---!
> +             !------------------------------------<---!
> +             V
> +             (...) .-->--------------> ( ,,,, )    ---!
> +             !------------------------------------<---!
> +             V
> +             (num-col,0)------------->(num-col,line-len)
> +
> +
> +    If a rotation of 90 degrees counterclockwise along the axis perpendicular to
> +    the sensor's lens and directed towards the scene to be captured is applied
> +    to the sensor, the pixel array would then be rotated as follows::
> +
> +            x ^  0,line-len,,,(num-col,line-len
> +              !  ....
> +              !  0,2 1,2      ...
> +              !  0,1 1,1      ...
> +              !  0,0 1,0 ... num-col,0
> +             (0,0)------------------------>
> +                                   y
> +
> +    And the pixel scan-out sequence would then proceed as follows::
> +
> +            (0,line-len)            (num-cols,line-len)
> +                 ^\    ^\    ^\    ^\    ^
> +                 ! \   ! \   ! \   ! \   !
> +                 !  \  !  \  !  \  !  \  !
> +                 !   \ !   \ !   \ !   \ !
> +                 !    \!    \!    \!    \!
> +               (0,0)  (1,0) ....      (num-cols,0)
> +
> +    Which when applied to the capture scene gives::
> +
> +            (0,line-len)            (num-cols,line-len)
> +                ^\    ^\    ^\    ^\    ^
> +                ! \   ! \   0 \   ! \   !
> +                !  \  !  \ -|- \  !  \  !
> +                !   \ !    / \  \ !   \ !
> +                !    \!    \!    \!    \!
> +              (0,0)  (1,0) ....      (num-cols,0)
> +
> +    Producing the following image once captured to memory and
> +    displayed to the user::
> +
> +             \ !
> +               --0
> +             / !
> +
> +    Which has a rotation of the same amount of degrees applied on the opposite
> +    rotation direction along the axis that goes from the observer to the
> +    displayed image.
> +
> +    In order to compensate the sensor mounting rotation, when expressed
> +    as counterclockwise rotation along the axis directed from the sensor to
> +    the captured scene, a rotation of the same amount of degrees in the
> +    same counterclockwise rotation direction but applied along the axis
> +    directed from the observer to the captured image, has to be applied.::

.:: -> :

> +
> +                -------   90 degree counterclockwise
> +                |   o  |  mounting rotation applied
> +                |  -|- |  along the axis directed
> +                |  / \ |  away from the sensor lens
> +                -------
> +                -------
> +                | \ !  |  Resulting captured
> +                |  --0 |  image when displayed
> +                | / !  |  on screen
> +                -------

Trying this with my webcam turning it 90 degrees counterclockwise, I
and up with my head to the left, not to the right.

> +                -------
> +                |   o  |  Rotation compensation
> +                |  -|- |  is 90 degrees counterclockwise
> +                |  / \ |  along the axis directed to the
> +                -------   displayed image
> +
> +
>  .. [#f1]
>     This control may be changed to a menu control in the future, if more
>     options are required.
> 

Regards,

	Hans