Hi David, On 17/05/2021 12:02, David Plowman wrote: > Hi > > I'd like to propose some new V4L2 controls as defined in the attached > patches. The controls are: > > V4L2_CID_NOTIFY_GAIN_RED > V4L2_CID_NOTIFY_GAIN_GREENR > V4L2_CID_NOTIFY_GAIN_BLUE > V4L2_CID_NOTIFY_GAIN_GREENB > > The purpose of these controls is to be able to notify a raw sensor > what colour gains will be applied by subsequent processing (such as by > an ISP). Equivalently we can think of them as telling the sensor what > the white point is. Note that the sensor is told these gains but does > not apply them - which the choice of name is trying to convey. > > Some sensors need to know these numbers for the processing that they > perform. Here I'm thinking in particular of so-called "quad Bayer" > (also sometimes "tetracell") sensors that have a special read-out mode > that converts the non-standard Bayer pattern into a standard one, also > at full resolution. > > Sensors of this type are becoming quite common on cell phones where, > for example, a 48MP sensor may be able to deliver multiple exposures > at 12MP (for HDR processing perhaps) but they may also have a mode as > described above where they can generate a standard Bayer output at > 48MP. This processing works better - we might expect less colour > aliasing? - when the sensor knows what values correspond to "white". > > One question in my mind is whether it's worth having a control for > each green channel. The sensor I'm currently looking at only wants a > single green gain, but perhaps it's one of those instances where it > would be annoying to put in a single gain and discover, sometime > later, a sensor that wants both. Opinions on the matter always > appreciated! I think I would like to see this series in combination with an actual sensor driver + ISP that uses them. I am a bit concerned about the fact that the units are sensor-specific, so how would an ISP driver be able to set them in a sensor-independent manner? It seems that this would require knowledge about the sensor internals, which is something you want to avoid in a ISP driver. Perhaps this should be a multiplication factor instead? I.e. if the value is X, then the ISP calculated red as the original red value times (X / 100). So if X = 212, then the red gain factor is 2.12. It's probably best to have two controls for green IMHO. Regards, Hans > > Thanks very much and best regards > > David > > David Plowman (2): > media: v4l2-ctrls: Add V4L2_CID_NOTIFY_GAIN_XXX controls > media: v4l2-ctrls: Document V4L2_CID_NOTIFY_GAIN_XXX controls > > .../media/v4l/ext-ctrls-image-source.rst | 28 +++++++++++++++++++ > drivers/media/v4l2-core/v4l2-ctrls.c | 4 +++ > include/uapi/linux/v4l2-controls.h | 4 +++ > 3 files changed, 36 insertions(+) >
