Hi David, On Fri, Aug 06, 2021 at 09:15:09AM +0100, David Plowman wrote: > Hi Sakari, Laurent > > Thanks for the various comments and discussion. It did prompt me to > have some second thoughts about some of the details here, as follows. > > These controls are aimed specifically at sensors that do some kind of > on-board "demosaic / remosaic" process, for instance to produce > standard Bayer patterns from non-standard ones. As such the principal > requirement is for the sensor to know what "grey" looks like, which we > tell it by sending it the red and blue gains from the white balance > algorithm. (This obviously enables it to reduce colour aliasing during > the processing that it does.) > > So perhaps the comparison here should be with the existing > V4L2_CID_RED/BLUE_BALANCE controls. I'm not sure that it really > matters exactly what the colours of the pixels on the sensor really > are, it's knowing what "grey" looks like that is important. Any new > controls could be quite cumbersome to use if you have to figure out > what the underlying pixel arrangement looks like, it certainly feels > much easier to refer simply to red/blue pixels, leaving the driver to > deal with its own internal idiosyncrasies. > > Having said that, the particular sensor I have exposes a gain for each > of the four Bayer channels, even though I find myself ignoring the > green ones!! > > Anyway, I certainly feel I could go either way on this one - do you > have any more thoughts on the matter? With an array control, we would have to decide (and document) which components are stored in the array, and in which order. For Bayer sensors, that would be B, Gb, Gr and R. As for the order, there are three options: - fixed order (e.g. always B, Gb, Gr, R) - matching the sensor's CFA native order (always the same for a given sensor, but varying between different sensors) - matching the currently configured format (the bayer pattern can change when moving the crop rectangle by one pixels or when mirroring the sensor readout) The last two options seem quite impractical to me. The first option, if we only consider Bayer sensors, is equivalent to the four controls your propose in the sense that the semantics is defined in the control specification and identical for all sensors, but with the advantage of bundling all four values together. It will also allow expanding this to other patterns if the need arise, which I think would be useful. If we were to redesign the red/blue gains, I'd go for a single array control today, with one value per CFA component. > On Thu, 5 Aug 2021 at 19:05, Sakari Ailus wrote: > > On Thu, Aug 05, 2021 at 06:49:33PM +0300, Laurent Pinchart wrote: > > > On Thu, Aug 05, 2021 at 06:40:42PM +0300, Sakari Ailus wrote: > > > > On Thu, Aug 05, 2021 at 06:22:32PM +0300, Laurent Pinchart wrote: > > > > > On Thu, Jul 22, 2021 at 01:12:48PM +0100, David Plowman wrote: > > > > > > We add new controls, one for each of the four usual Bayer channels: > > > > > > > > > > > > V4L2_CID_NOTIFY_GAIN_RED > > > > > > V4L2_CID_NOTIFY_GAIN_GREENR > > > > > > V4L2_CID_NOTIFY_GAIN_BLUE > > > > > > V4L2_CID_NOTIFY_GAIN_GREENB > > > > > > > > > > This will effectively limit the API to Bayer patterns. I wonder if we > > > > > should instead implement it as a single array control, with one element > > > > > per CFA component. > > > > > > > > There are other raw patterns, too. Supporting them would likely require one > > > > or a few more controls. > > > > > > > > That said, as the values change often it's more efficient to use a single > > > > control. But each colour combination (not each pattern) would require its > > > > own control in this case, eventually requiring more controls. > > > > > > I'm not sure to follow you. My idea is to define a single control, with > > > a number of elements that depends on the pattern being used, and the > > > order specified in the native sensor pattern. I don't think each colour > > > combination would then require its own control. > > > > Ah, I guess that would work, too. Then we'll need to define what kind of > > pixel orders are supported for that single control (and for which formats). -- Regards, Laurent Pinchart
