[libcamera-devel] [PATCH 1/1] libcamera: controls: Add DigitalGain control

[Laurent Pinchart]

Hi David,

On Tue, Nov 24, 2020 at 12:40:38PM +0000, David Plowman wrote:
> On Tue, 24 Nov 2020 at 11:28, Jacopo Mondi <jacopo at jmondi.org> wrote:
> > On Tue, Nov 24, 2020 at 10:15:25AM +0000, David Plowman wrote:
> > > Hi Jacopo
> > >
> > > You're right, there's a relationship there. ColourGains obviously
> > > gives you the red and blue gains determined by the AWB usually. You
> > > might get the values 1.6 and 2.0 (for red and blue)
> > >
> > > In our case, if we report a single "global gain" value you can kind of
> > > imagine it as the green gain, where the colour gains were normalised
> > > for a green gain of 1. So if the global gain was 1.25, then the actual
> > > RGB gains used in my example would be 1.25 * (1.6, 1.0, 2.0)  = (2.0,
> > > 1.25, 2.5).
> >
> > That's a really helpful explanation for me, thanks
> >
> > > In the per-channel case I guess you'd be reporting these 3 numbers
> > > directly. For me this duplicates information that's already in the
> > > ColourGains, and it seems to muddle things up a bit. Imagine you had a
> > > pipeline that lets you set a global gain - you'd have to query the
> > > current white balance and work out all three numbers and set them. But
> > > then you've set the white balance as well. Or maybe we do something
> > > special so that you haven't? You see why it confuses me! So on balance
> > > I'm with the single value approach, though I could live either way.
> >
> > I see. With the above explanation I really see why a single global
> > value is probably enough.
> >
> > My question is now: are the above notions (blue/red gains normalized
> > on a green value of 1, how global gain is used to obtain per-channel
> > gains etc) standard knowledge I'm lacking, or:
> >
> > 1) It's worth to describe them in the control documentation as they're
> > "standard" and won't change per-pipeline
> > 2) They might differe per-pipeline and they need to be
> > described in the pipeline model documentation ?
> > 3) It is assumed the reader knows she's dealing with
> 
> Difficult. I can't really see too many other ways of defining it, but
> you know, pipelines all have their nuances. You probably can't
> guarantee that what I described is completely "standard".

This is really what I'd like to address :-)

Have you seen the "libcamera: camera: Document the camera and pipeline
model" patch (which is now in the master branch) ? It's the very first
step in standardizing a logical pipeline model that pipeline handlers
need to confirm to.

In a nutshell, the idea is that libcamera needs to standardize the
behaviour of cameras as seen from applications in order to make
applications portable. To do so, we define a logical pipeline model that
describes the processing operations performed by the camera, from the
pixel array to the captured image.

Hardware platforms differ in the feature they support and how they
implement them, so most operations in the pipeline model are optional.
Furthermore, there's no requirement to have the hardware performing
those operations in the same order, or more generically in the same way,
as the logical model mandates, as long as the visible effect from an
application point of view is the same. Pipeline handlers are responsible
for exposing the standard logical model and mapping it to device
features.

I expect that not all devices will be able to expose the same pipeline
model, so we will need to support multiple options, with a way for the
pipeline handlers to report which options apply to the cameras they
handle. This can be reported through properties or through other means.
We already support this, albeit implicitly: pipeline handlers that don't
support some of the processing operations simply don't expose the
corresponding controls. I expect we will also need properties to report
the order of operations, when different orders lead to different results
and all need to be supported. This should however remain the exception
rather than the rule.

I would like to expand the pipeline model documentation to include
colour processing, and I think digital gain really fits here. We can
still apply your patch (or the next version of it) before finalizing
documentation of the colour processing in the pipeline model, but
controls will then likely change soon after, which would require
adapting your applications. It may end up causing more pain than gain.

As you have more experience than me in this area, I wonder if you could
help reviewing an initial proposal ? I envision the following
sensitivity and colour-related operations, in order.

- Total gain in the sensor, made of analog and/or digital gain. This
  applies to all images, RAW and processed, and to all channels. If
  auto-exposure is enabled, the control value is ignored. The actual
  value is always reported in metadata.

  Questions:

   - Should we mandate that pipeline handlers and IPAs prioritize analog
     gain before applying digital gain ?

   - Can there be use cases for per-channel sensor-side gains ? In most
     cases I expect the white balance to be applied in the ISP, but will
     that always be the case ? What if auto-white-balance is disabled,
     and the user sets manual per-channel gains, could there be a reason
     to apply them in the sensor ?

- Digital gain on the ISP side (a.k.a. "post-RAW sensitivity boost" in
  Android). This applies to processed images only, and to all channels.
  If auto-exposure is enabled, the control value is ignored. The actual
  value is always reported in metadata.

  Questions:

  - Should this be renamed ? "Digital gain" can be confused with the
    digital gain in the sensor. A name that captures the fact that the
    gain is applied on processed images only may be better (I don't like
    the Android name too much, but it handles this issue).

- Per-channel gains. This is currently handled as red and blue gains,
  with the green gains being hardcoded to 1.0.

  Questions:

  - Should we change this ? I see two potential issues:

    - Not all systems may use a Bayer colour filter array, so other
      gains may be needed (I'm thinking about RGBW patterns for
      instance, or any other from
      https://en.wikipedia.org/wiki/Color_filter_array).

    - Hardcoding the green gains to 1.0 means that we will have an
      "average" gain different than 1.0 in most cases. This will
      effectively change the sensitivity of the camera, and should be
      taken into account to compute the ISO value. Applications could do
      so, but I wonder if we shouldn't mandate that the "average" gain
      remains 1.0, with the "sensitivity boost" being handled by the
      digital gain control only. How to compute the "average" gain is an
      interesting problem here, as I expect it will need to take a full
      colour model into account.

- Demosaicing. This shouldn't apply any gain.

- RGB-to-RGB colour transformation matrix.

  Questions:

  - Similarly to the per-channel gains, should we require that this
    matrix applies no gain (i.e. the sum of elements in a line should be
    1.0) ?

- RGB-to-YUV colour encoding matrix.

  Questions:

  - Similarly to the RGB-to-RGB matrix, should we forbid gains ?

Is there any other colour processing operation I have missed ?

> A slightly more qualitative definition seems OK to me - the amount of
> linear gain applied by the pipeline to all pixels (in addition to the
> colour gains). Particular pipelines might feel they need to document
> it more carefully if there are any complications in how they deal with
> it.
>
> > > On Tue, 24 Nov 2020 at 08:43, Jacopo Mondi <jacopo at jmondi.org> wrote:
> > > > On Tue, Nov 24, 2020 at 08:28:09AM +0000, David Plowman wrote:
> > > > > Hi everyone
> > > > >
> > > > > Sounds like we're happy enough from the point of view of this thing
> > > > > being read-only (for Raspberry Pi at least). Would anyone want any
> > > > > changes to the wording? Perhaps the final sentence/paragraph might now
> > > > > be better as
> > > > >
> > > > > "This control is present in a request's ControlList only if the
> > > > > pipeline supports setting the value. Even when it cannot be set by an
> > > > > application, the pipeline may still report the actual value used in
> > > > > the metadata returned with completed requests."
> > > >
> > > > I don't think it it necessary. It is implied that if a pipeline
> > > > handler does not support changing the digital gain it should not
> > > > expose it a one of the Camera's controls.
> > > >
> > > > Likewise, if it is something that applications should be informed of,
> > > > it will be reported via metadata.
> > > >
> > > > I think we're good to go, except for the point that we've left
> > > > floating about having this a single value or a per-channel value.
> > > >
> > > > I'm trying to get a feeling how this would be reported by your ISP. I
> > > > see in example you have two per-channel values for the ColourGains
> > > > control. Is this anyway related ?
> > > >
> > > > > Any other thoughts?
> > > > >
> > > > > On Mon, 23 Nov 2020 at 09:17, Kieran Bingham wrote:
> > > > > > On 23/11/2020 08:58, Jacopo Mondi wrote:
> > > > > > > On Mon, Nov 16, 2020 at 10:40:25AM +0000, Kieran Bingham wrote:
> > > > > > >> On 27/10/2020 14:12, David Plowman wrote:
> > > > > > >>> This control reports the global digital gain applied by the pipeline
> > > > > > >>> as a whole, after capturing a raw image from the sensor.
> > > > > > >>>
> > > > > > >>> Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > > > > > >>> ---
> > > > > > >>>  src/libcamera/control_ids.yaml | 11 +++++++++++
> > > > > > >>>  1 file changed, 11 insertions(+)
> > > > > > >>>
> > > > > > >>> diff --git a/src/libcamera/control_ids.yaml b/src/libcamera/control_ids.yaml
> > > > > > >>> index c8874fa9..e6362c74 100644
> > > > > > >>> --- a/src/libcamera/control_ids.yaml
> > > > > > >>> +++ b/src/libcamera/control_ids.yaml
> > > > > > >>> @@ -530,4 +530,15 @@ controls:
> > > > > > >>>          This control is only present when the pipeline supports scaling. Its
> > > > > > >>>          maximum valid value is given by the properties::ScalerCropMaximum
> > > > > > >>>          property, and the two can be used to implement digital zoom.
> > > > > > >>> +
> > > > > > >>> +  - DigitalGain:
> > > > > > >>> +      type: float
> > > > > > >>> +      description: |
> > > > > > >>> +        Global digital gain value applied to the image during all the
> > > > > > >>> +        processing steps after capturing the image from the sensor. Any raw
> > > > > > >>> +        images, therefore, will not include this gain, but the final images
> > > > > > >>> +        output by the imaging pipeline as a whole will include it.
> > > > > > >>> +
> > > > > > >>> +        This control is intended to report the value used by the image
> > > > > > >>> +        processing pipeline.
> > > > > > >>
> > > > > > >>
> > > > > > >> If this is a per-stream thing anyway, I guess it will then be up to
> > > > > > >> pipeline handlers to set this to the appropriate value for each stream
> > > > > > >> when it completes. The fact that this value would not be applicable to a
> > > > > > >> RAW stream makes me think it certainly should be a per-stream metadata
> > > > > > >> style value.
> > > > > > >>
> > > > > > >>
> > > > > > >> I'd hope this could be handled by a common helper in that instance so it
> > > > > > >> doesn't get left out of some pipeline handlers, but included in some,
> > > > > > >> and become inconsistent. Not yet sure how we can handle that, but that
> > > > > > >> will be a core issue anyway.
> > > > > > >>
> > > > > > >>
> > > > > > >> I wonder if we should mark this somehow as read-only, at least until we
> > > > > > >> determine that someone needs to set it.
> > > > > > >>
> > > > > > >> We could introduce a control property between type: and description:
> > > > > > >>   read-only: true
> > > > > > >
> > > > > > > Isn't a read-only control just a metadata ?
> > > > > > >
> > > > > > > Wouldn't it be enough for a pipeline that does not support changing
> > > > > > > the control value from applications not reporting it in the list of
> > > > > > > supported Camera's controls, but only report it as part of a completed
> > > > > > > request's metadata ?
> > > > > >
> > > > > > Ah, yes of course - because if the control is not listed as supported it
> > > > > > won't be there to set in the first place! I forgot about that.
> > > > > >
> > > > > > So - indeed, no requirement to mark anything as read-only. That will be
> > > > > > implicit.
> > > > > >
> > > > > > >> Otherwise, I see no objections currently. I think we're just waiting on
> > > > > > >> top-level thoughts from Laurent. (And perhaps per-stream controls, but
> > > > > > >> that brings it's own questions )
> > > > > > >>
> > > > > > >> Reviewed-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> > > > > > >>
> > > > > > >>>  ...

-- 
Regards,

Laurent Pinchart