Re: [v4l-dvb-maintainer] [RFC] Zoom controls in V4L2

["Hans Verkuil" <hverkuil@xxxxxxxxx>]

Hi Laurent,

> Hi everybody,
>
> I haven't received any answer so far. Is everybody busy or just not
> interested
> in zoom support ? Please don't let the length of the mail scare you :-)

Ah yes, I read it at the time, but forgot to reply. Not that there is much
to say, since I pretty much agree with this RFC. Just go for it!

Regards,

         Hans

> On Tuesday 04 November 2008, Laurent Pinchart wrote:
>> Hi everybody,
>>
>> USB cameras with integrated optical zoom support are hitting the market.
>> V4L2 currently lacks the necessary controls to support zoom. This RFC
>> tries
>> to define zoom-related controls.
>>
>> As few camera models currently support optical zoom, only a subset of
>> zoom
>> functions are implemented in existing products, making it a bit harder
>> to
>> define a future proof zoom API in V4L2. To gather more usecases I've
>> taken
>> all zoom controls defined in the USB Video Class specification into
>> account, even if they are not all implemented in existing products.
>>
>> Zoom in digital cameras is implemented as optical zoom, digital zoom or
>> a
>> combination of both. V4L2 supports digital zoom through cropping and
>> scaling (section 1.11). Digital cameras often implement digital zoom
>> through a single linear control, providing a subset of the scaling
>> capabilities of V4L2 with no easy way to map between both. Still,
>> defining
>> a new digital zoom API in addition to the V4L2 cropping and scaling
>> mechanism would confuse developers and users and should be avoided. We
>> should instead concentrate on defining a clear mapping between linear
>> digital zoom and crop/scale.
>>
>> As I don't own any UVC device with digital zoom support, and as I'm not
>> knowledgeable about digital zoom support in non-UVC webcams, ideas for a
>> mapping between linear digital zoom and crop/scale are welcome. In case
>> of
>> lack of feedback on the subject, I propose to concentrate on optical
>> zoom
>> only, except when digital zoom interacts with optical zoom.
>>
>> The UVC specification approximates the optical magnification factor as
>> the
>> ratio between the ocular lens focal length and the objective lens focal
>> length. Although lens groups can be much more sophisticated than that,
>> the
>> model can approximate most lens groups that are likely to be encountered
>> in
>> practice. Zoom can then be expressed either as the magnification factor
>> or
>> as the objective lens focal lens. To support both representations V4L2
>> should let the device set its minimum and maximum zoom values. In both
>> cases the zoom is either an unsigned integer or an unsigned rational
>> number
>> that can be expressed with a fixed-point representation.
>>
>> Optical zoom can be controlled in an absolute or relative fashion.
>> Absolute
>> zoom can easily be handled with a single unsigned integer control
>> mapping
>> to the magnification factor or objective lens focal length as described
>> above. The absolute zoom control should not interact with any digital
>> zoom
>> function implemented in the device, if any. I suggest naming the control
>> V4L2_CID_ZOOM_ABSOLUTE.
>>
>> Relative zoom is a tad more complex. To begin with, there are two
>> relative
>> zoom implementations I can think of: incremental or continuous.
>> Incremental
>> relative zoom moves the optical zoom level by a fixed amount. This is
>> how
>> the relative pan, tilt and focus controls are specified in V4L2.
>> However,
>> this is not how relative zoom is specified in UVC.
>>
>> UVC specifies relative zoom as a control that starts a zoom focal length
>> modification at a given speed in the given direction until interrupted
>> by
>> the user (through the relative zoom control) or by a limit in the range
>> of
>> motion. This behaviour is closer to what a user would expect when
>> controlling the zoom relatively : pressing a button would start zooming
>> in
>> or out, and releasing the button would stop zooming. A single V4L2
>> control
>> encoded as a signed integer can set the speed and direction. The speed
>> range should be device dependant.
>>
>> We are thus facing a situation where three types of zoom controls can be
>> implemented, among which two are of the relative type. The UVC
>> specification specifies a continuous relative zoom only, but V4L2
>> already
>> uses the _RELATIVE suffix for incremental relative pan, tilt and focus
>> controls. Using V4L2_CID_ZOOM_RELATIVE for continuous relative zoom
>> would
>> not be consistent with the V4L2 pan, tilt and focus controls, while
>> using
>> V4L2_CID_ZOOM_RELATIVE for incremental relative zoom would not be
>> consistent with the UVC specification. As the V4L2 specification is
>> already
>> not consistent with the UVC specification when it comes to relative pan,
>> tilt and focus, I propose to call the incremental relative zoom control
>> V4L2_CID_ZOOM_RELATIVE and use a different name for the continuous
>> control.
>> Comments are welcome, as well as suggestions for the control name.
>>
>> Continuous relative zoom also suffers from another issue. While absolute
>> and incremental relative zoom do not interact with digital zoom (when
>> implemented by the device), it might be interesting to let the
>> continuous
>> relative zoom use digital zoom as an option when reaching the end of the
>> optical zoom capabilities. This would give the user a large zoom range
>> combining optical and digital zoom that can be navigated using a single
>> control. This is how the UVC specification defines the relative zoom
>> control. We would then need an additional control to enable or disable
>> digital zoom when using the continuous relative zoom. Both the digital
>> zoom
>> enable and continuous relative zoom (sign + speed) values should then be
>> set in a single operation through the extended controls API. Comments on
>> this subject are welcome as well.
>
> Best regards,
>
> Laurent Pinchart
>
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