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

[Jean-Michel Hautbois]

Hi Laurent,

As I am rebasing this series with the proper branch, I noticed a few 
comments I still not know how to address now) :

On 14/09/2021 04:54, 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.

Should I add a \todo ?

> 
>> + *
>> + * 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 :-)

This is Kieran's term, and I have changed it to:
s/facilitate the operation of /operate/

> 
>> + * 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.
> 
>> + * 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:
>