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

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

Hi Jacopo,

Sorry for the late reply, it fell through the cracks...

On 9/13/19 8:49 PM, Jacopo Mondi wrote:
> Hi Hans,
> 
> On Fri, Sep 13, 2019 at 04:02:45PM +0200, Hans Verkuil wrote:
>> 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?
>>
> 
> It really depend along which axis direction you are applying the mounting
> rotation and the compensation rotation... See below...
> 
>>> +
>>> +    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?
> 
> Yes indeed sorry
> 
>> 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.
>>
> 
> num-col is totally wrong, that should have been num-lines
> 
> In general
> s/line-len/num-col
> s/num-col/num-lines
> 
>>> +              ! 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.::
>>
>> .:: -> :
>>
> 
> Don't I need the :: to mark the following block of text as verbatim ?

Ah, sorry, this is for the markup. I missed that.

> 
>>> +
>>> +                -------   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.
>>
> 
> Along which axis direction are you rotating the camera counterclockwise ?
> 
> If you see your face, and you rotate the camera counterclockwise while
> looking at it, you're actually rotating along the axis directed -towards-
> the sensor.
> 
> The rotation here in the example and in the 'rotation' property
> description has to be applied along the axis pointing aways from the
> sensor, so what you're actually doing is rotating clockwise along that
> direction (I guess)... So yes, to compensate that, you need to rotate
> clockwise when you look at the image on the screen... Confusing,
> right?

I think you are right, but let me take another look at this when v4 is
posted. The line-len/num-col confusion didn't help :-)

Regards,

	Hans

> 
>>> +                -------
>>> +                |   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