[libcamera-devel] [PATCH v3 0/6] Digital zoom

[Laurent Pinchart]

Hi David,

On Mon, Oct 12, 2020 at 07:49:31PM +0100, David Plowman wrote:
> Hi Laurent
> 
> Thanks for the comments. Some interesting points.

Sorry about chiming in so late and challenging some of the base
assumptions though. I'll now try not to introduce any further delay.

> On Sun, 11 Oct 2020 at 22:47, Laurent Pinchart wrote:
> 
> > Hi David,
> >
> > Thank you for the patches. And more than sorry for reviewing the series
> > so late.
> >
> > On Tue, Sep 29, 2020 at 05:39:54PM +0100, David Plowman wrote:
> > > Hi everyone
> > >
> > > Despite not originally intending to, I've actually made a version 3 of
> > > the digital zoom patches, just to take care of a few things that might
> > > be a bit annoying otherwise.
> > >
> > > 1. I've improved the description of the IspCrop control as was
> > > suggested.
> > >
> > > 2. I've improved the description of the zoom option in cam (if we
> > > decide to use this patch!), also as was proposed.
> > >
> > > 3. There was actually a problem with the "{}" syntax to denote zero
> > > Rectangles in include/libcamera/ipa/raspberrypi.h. They were failing a
> > > range type test in ControlInfoMap::generateIdmap() and so the control
> > > wasn't working. I've replaced "{}" by "Rectangle{}" which seems OK.
> > >
> > > 4. There's been a bit of churn in the RPi pipeline handler lately so
> > > rebasing gave some conflicts. I've fixed those up.
> > >
> > > Otherwise everything else remains the same.
> >
> > I've gone through the different versions of this series, and the first
> > thing I want to see is that it has improved very nicely over time. Nice
> > job !
> >
> > Configuring zoom through an absolute crop rectangle is I believe the way
> > to go, so the overall approach is good in my opinion. There are however
> > a few points I'd like to discuss, related to the SensorOutputSize
> > property and IspCrop control.
> >
> > Reading through the discussion, I become increasingly aware of a topic
> > that was present in the background without ever being named, and that's
> > the camera pipeline model. As mentioned yesterday in a few other replies
> > to selected patches in this series and its previous versions, the job of
> > libcamera is to expose to applications a camera model that abstract
> > hardware differences. This is not an easy job, as we have to strike the
> > right balance between a higher-level inflexible but simple to use model
> > that can't support many of the less common use cases, and a lower-level
> > powerful but complex to use model that exposes a very large range of
> > hardware features. We have implicitly defined the skeleton of such a
> > model through the API of the Camera and PipelineHandler classes, but
> > have never made it explicit.
> >
> > This needs to change. I see no reason to block the digital zoom feature
> > until we finish documenting the pipeline model, but I would like to
> > design the feature while thinking about the bigger picture. Here are the
> > assumptions I think the pipeline model should make for devices that
> > support digital zoom.
> >
> > - The camera pipeline starts with a camera  sensor, that may implement
> >   binning, skipping and/or cropping.
> >
> > - The subsequent blocks in the pipeline may perform additional cropping,
> >   either at the direct command of the pipeline handler (e.g. cropping at
> >   the input of the scaler), or automatically to support image processing
> >   steps (e.g. colour interpoloation often drops a few lines and columns
> >   on all edges of the image).
> >
> > - The pipeline ends with a scaler that can implement digital zoom
> >   through a combination of cropping followed by scaling.
> >
> > The exact order of the processing steps at the hardware level doesn't
> > need to match the pipeline model. For instance, cropping at the input
> > and output of the scaler are interchangeable (not taking into account
> > sub-pixel differences). It doesn't matter if the ISP scales before
> > cropping the output, the hardware scaler parameters and output crop
> > rectangle can be computed from an abstract input crop rectangle and
> > output size. This is crucial to consider for the definition of the
> > pipeline model: we need to design it in a way that ensures all features
> > can be mapped to how they are implemented in the different types of
> > hardware we want to support, but we're otherwise free to not map
> > controls and properties 1:1 with the hardware parameters. When multiple
> > options are possible, we should be guided by API design criteria such as
> > coherency, simplicity and flexibility.
> >
> > Coming back to digital zoom, this series exposes a new SensorOutputSize
> > property and a new IspCrop control. The SensorOutpuSize property is
> > introduced to support the IspCrop control, as the base rectangle in
> > which the scaler can crop. There are two issues here that bother me:
> >
> > - The property, despite being named SensorOutputSize, potentially takes
> >   into account the cropping added by the CSI-2 receiver and by the ISP
> >   for operations that consume lines and columns on the edges of the
> >   image. The naming can create some confusion, which can possibly be
> >   addressed by a combination of documentation (you're covering that
> >   already) and possibly a more explicit name for the property. However,
> >   as the property bundles crop operations perfomed in different stages
> >   of the pipeline, I'm worried that it will turn out to be a bit
> >   ill-defined regardless of how well we document it, with slightly
> >   different behaviours in different implementations.
> >
> > - Ignoring the additional cropping performed in the CSI-2 receiver and
> >   ISP (if any), the property exposes the sensor binning, skipping and
> >   cropping. It bundles those three operations together, and thus doesn't
> >   convey how the cropping affects the field of view as a given output
> >   size can be achieved with different combinations of skipping/binning
> >   and cropping.
> >
> > For these reasons, I'm concerned that the SensorOutputCrop property is a
> > ad-hoc solution to provide a reference for the IspCrop property, and
> > will not fit clearly in a pipeline model that should be based on simple,
> > base building blocks. I would thus like to propose an alternative
> > option.
> >
> > Instead of expressing the IspCrop controls (which I think we should
> > rename to ScalerCrop) relatively to the SensorOutputSize property, could
> > we express it relatively to the existing PixelArrayActiveAreas property
> > ? This would have the advantage, in my opinion, of abstracting binning
> > and skipping from applications. The pipeline handlers would need to
> > perform a bit more work to compute the crop rectangle actually applied
> > to the scaler, in order to take sensor binning/skipping and all
> > additional cropping in the pipeline into account. The upside is that the
> > ScalerCrop will directly define how the field of view is affected. It
> > would also simplify the API, as no intermediate property between
> > PixelArrayActiveAreas and ScalerCrop would need to be defined, and the
> > ScalerCrop coordinates wouldn't depend on the active camera
> > configuration. I think this would be easier to clearly document as part
> > of a camera pipeline model.
> 
> Renaming IspCrop to ScalerCrop sounds fine to me. It has had so many
> different names!
> 
> I'm less sure about trying to derive the SensorOutputSize (or
> ScalerInputSize or whatever else we want to call it!) from the
> PixelArrayActiveAreas property. Let me try and take a step back.
> 
> So first, I think knowing the PixelArrayActiveArea isn't enough. How would
> you know if the pipeline handler was doing some extra cropping that wasn't
> "strictly necessary", perhaps to reduce memory traffic, or for a faster
> framerate. How would the application know not to try and zoom there? It
> seems to me that this really is a decision for the pipeline handler based
> on the sensor driver, it isn't available from the properties of the sensor
> itself.
> 
> Actually I'd quite like to leave the discussion there for the moment and
> see if that much is controversial or not. Of course we then have to move on
> but maybe let's see what we think about that first...
> 
> Thoughts??

I fully agree with you that the pipeline handler has the last word when
it comes to the (combined) internal cropping. It may not have any choice
(if the CFA interpolation eats two or four lines and columns on each
side of the image due to the hardware implementation, there's nothing
that can be done against it), or may just want to decide on policies for
whatever reason it sees fit (while trying not to over-police though, to
leave options to applications).

My proposal doesn't deny this from the pipeline handler, but tries to
express the crop rectangle in a way that maps directly to the field of
view. To do so, binning/skipping would be hidden from applications by
using a reference for the ScalerCrop property that has no
binning/skipping applied.

The pipeline handler would restrict the requested ScalerCrop control
based on the internal cropping that is always applied (combining sensor,
CSI-2 receiver and ISP), and report the result through the request
metadata. The application would thus know the exact crop rectangle that
has been used, in unscaled (binning/skipping) sensor coordinates.

While in some use cases the actual crop rectangle reported through
metadata would be enough (for instance, in a GUI that starts with an
unzoomed view, the application would get the maximum possible crop
rectangle in the metadata of the first frame), I can imagine we would
have use cases that need this information before capturing the first
frame. I initially thought we could report it through the max value of
the ControlInfo instance for the ScalerCrop control, but this is
supposed to be static information. As this would be (in my opinion) a
neat solution, I'd like to investigate the possibility of making
ControlInfo dynamic, but maybe we will need a different solution. For
foresee the addition of an API that will create request templates,
filling them with default control values based on a requested use case,
and the maximum ScalerCrop could possibly be reported through that
mechanism. I'm also fine reporting it through a temporary mechanism (for
instance adding it to CameraConfiguration, or creating a dynamic
property) for the time being, if you already have pressing use cases for
knowing the maximum value before capturing the first frame.

Thoughts ? :-)

> > Two additional points I'd like to consider (and which are orthogonal to
> > the previous one) are:
> >
> > - Should we automatically adjust the ScalerCrop rectangle to always
> >   output square pixels, or should we allow modifying the aspect ratio
> >   when scaling ? Most use cases call for square pixels, but I don't
> >   think we necessarily want to create an artificial limitation here (as
> >   long as we make it easy for applications to compute the scaling
> >   parameters that will give them square pixels)/
> 
> Personally I see no reason to restrict what an application can request. We
> need to make it easy to request the right aspect ratio (hence those
> geometry helpers), but if people actually have a use-case for something
> strange...

Fine with me.

> > - Should we allow a ScalerCrop rectangle smaller than the stream output
> >   size, or should we restrict scaling to down-scaling only ?
> 
> I think up-scaling is probably the most common use-case for us (though
> downscaling will happen too). Think of all those (rubbish) 30x zoom
> pictures that some phones like to produce...!

Rubbish is exactly the work I would have used :-) I'm tempted to try and
teach users that they shouldn't do this by disabling this feature, but
that would be pretentious. I suppose there's no good reason to forbid
this. Should we put a limit on the upscaling factor though ?

> > > David Plowman (6):
> > >   libcamera: Add SensorOutputSize property
> > >   libcamera: Initialise the SensorOutputSize property
> > >   libcamera: Add IspCrop control
> > >   libcamera: Add geometry helper functions
> > >   libcamera: pipeline: raspberrypi: Implementation of digital zoom
> > >   cam: Add command line option to test IspCrop control
> > >
> > >  include/libcamera/geometry.h                  |  20 +++
> > >  include/libcamera/ipa/raspberrypi.h           |   1 +
> > >  src/cam/capture.cpp                           |  25 +++-
> > >  src/cam/capture.h                             |   2 +-
> > >  src/cam/main.cpp                              |   3 +
> > >  src/cam/main.h                                |   1 +
> > >  src/ipa/raspberrypi/raspberrypi.cpp           |   7 +
> > >  src/libcamera/camera_sensor.cpp               |   6 +
> > >  src/libcamera/control_ids.yaml                |  12 ++
> > >  src/libcamera/geometry.cpp                    | 129 ++++++++++++++++++
> > >  .../pipeline/raspberrypi/raspberrypi.cpp      |  47 +++++++
> > >  src/libcamera/property_ids.yaml               |  19 +++
> > >  12 files changed, 269 insertions(+), 3 deletions(-)

-- 
Regards,

Laurent Pinchart