[libcamera-devel] [PATCH 01/11] ipa: ipu3: Document IPAIPU3 class interface

[Laurent Pinchart]

On Tue, Sep 14, 2021 at 05:54:01AM +0300, Laurent Pinchart wrote:
> Hi Jean-Michel,
> 
> Thank you for the patch.
> 
> On Mon, Sep 13, 2021 at 04:58:00PM +0200, Jean-Michel Hautbois wrote:
> > The IPU3 IPA is maturing to a modular and extensible system capable of
> > handling specific algorithms for the accelleration clusters on the ImgU.
> 
> s/accelleration/acceleration/
> 
> Where does the term "acceleration cluster" come from ? It sounds weird
> to me, it's the first time I come across it for ISPs. Have I missed
> something ? Or should we say "processing blocks", "processing
> operations" or something similar ?
> 
> > Provide a top-level class documentation to provide an overview of the
> > IPA, detailing what events are used and what algorithms are currently
> > supported, as well as the limitations currently imposed.
> > 
> > Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois at ideasonboard.com>
> > Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> > ---
> >  src/ipa/ipu3/ipu3.cpp | 64 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 64 insertions(+)
> > 
> > diff --git a/src/ipa/ipu3/ipu3.cpp b/src/ipa/ipu3/ipu3.cpp
> > index c925cf64..90053069 100644
> > --- a/src/ipa/ipu3/ipu3.cpp
> > +++ b/src/ipa/ipu3/ipu3.cpp
> > @@ -145,6 +145,70 @@ LOG_DEFINE_CATEGORY(IPAIPU3)
> >  
> >  namespace ipa::ipu3 {
> >  
> > +/**
> > + * \brief The IPU3 IPA implementation
> > + *
> > + * The IPU3 Pipeline defines an IPU3 specific interface for communication
> 
> s/IPU3 specific/IPU3-specific/
> 
> As far as I know, a hyphen is used for compound adjectives (an
> IPU3-specific interface) but not for predicates (the interface is IPU3
> specific).
> 
> There are three other instances below.
> 
> > + * between the PipelineHandler, and the IPA module.
> 
> s/PipelineHandler,/PipelineHandler/
> 
> > + *
> > + * We extend the IPAIPU3Interface to implement our algorithms and handle events
> > + * from the IPU3 PipelineHandler to satisfy requests from the application.
> > + *
> > + * At initialisation time, a CameraSensorHelper is instantiated to support
> > + * camera specific calculations, while the default controls are computed, and
> > + * the algorithms are constructed and placed in an ordered list.
> > + *
> > + * The IPU3 ImgU operates with a grid layout to divide the overall frame into
> > + * rectangular cells of pixels. When the IPA is configured, we determine the
> > + * best grid for the statistics based on the pipeline handler Bayer Down Scaler
> > + * output size.
> 
> On a side note, I wonder if the IPA should be involved to compute the
> ImgU configuration. It's a topic for future research.
> 
> > + *
> > + * Two main events are then handled to facilitate the operation of the IPU3 ImgU
> 
> I think we do a bit more than "facilitating" ImgU operation :-)
> 
> > + * by populating its parameter buffer, and adapting the settings of the sensor
> > + * attached to the IPU3 CIO2 through sensor specific V4L2 controls.
> > + *
> > + * When the event \a EventFillParams occurs we populate the ImgU parameter
> > + * buffer with settings to configure the device in preparation for handling the
> > + * frame queued in the Request.
> > + *
> > + * When the frame has completed processing, the ImgU will generate a statistics
> > + * buffer which is given to the IPA as part of the \a EventStatReady event. At
> > + * this event we run the algorithms to parse the statistics and cache any
> > + * results for the next \a EventFillParams event.
> > + *
> > + * The individual algorithms are split into modular components that are called
> > + * iteratively to allow them to process statistics from the ImgU in a defined
> > + * order.
> > + *
> > + * The current implementation supports three core algorithms:
> > + * - Automatic white balance (AWB)
> > + * - Automatic gain and exposure control (AGC)
> > + * - Tonemapping (Gamma)
> 
> s/Tonemapping/Tone mapping/
> 
> Same below.
> 
> > + *
> > + * AWB is implemented using a Greyworld algorithm, and calculates the red and
> > + * blue gains to apply to generate a neutral grey frame overall.
> > + *
> > + * AGC is handled by calculating a histogram of the green channel to estimate an
> > + * analogue gain and shutter time which will provide a well exposed frame. An
> > + * IIR filter is used to smooth the changes to the sensor to reduce perceivable
> 
> I'd say "low-pass filter" (of possibly "low-pass IIR filter") instead of
> "IIR filter" here, as the infinite impulse response isn't what really
> matters.

By the way, are you aware that the output of the filter is actually
unused ?

> > + * steps.
> > + *
> > + * The Tonemapping algorithm provides a gamma correction table to improve the
> > + * contrast of the scene.
> > + *
> > + * The IPU3 ImgU has further accellerator clusters to support image quality
> 
> s/accellerator/accelerator/
> 
> Same comment as for the commit message.
> 
> > + * improvements through bayer and temporal noise reductions, however those are
> > + * not supported in the current implementation, and will use default settings as
> > + * provided by the kernel driver.
> > + *
> > + * Demosaicing is operating on the default values and could be further optimised
> 
> I'm not totally sure what "values" mean here, did you mean "operating
> with the default parameters" ?
> 
> > + * to provide improved sharpening coefficients, checker artifact removal, and
> > + * false color correction.
> > + *
> > + * Additional image enhancements can be made by providing lens and sensor
> > + * specific tuning to adapt for Black Level compensation (BLC), Lens shading
> > + * correction (SHD) and Color correction (CCM)
> 
> s/$/./
> 
> > + */
> >  class IPAIPU3 : public IPAIPU3Interface
> >  {
> >  public:

-- 
Regards,

Laurent Pinchart