[libcamera-devel] libcamera on Librem 5, phase detection auto focus

[Laurent Pinchart]

Hi Pavel,

(CC'ing Dorota)

On Mon, Nov 21, 2022 at 12:32:30PM +0100, Pavel Machek via libcamera-devel wrote:
> Hi!
> 
> I'm playing with camera on Librem 5. So far I added af/ae/awb support
> to millipixels done completely in software, but I guess libcamera
> would be more suitable place for it. If you have some hints/example
> where to look at, it would be nice.

The target for the Librem 5 is to use the GPU to do the heavy-lifting,
and in particular to perform CFA interpolation (a.k.a. debayering) and
to apply the colour gains. Dorota is working on this. The, for the AE
and AWB statistics, we should make use of the CSI bridge stats engine if
possible. This will require some work on the driver side (this feature
isn't implemented at the moment), and integrate that in libcamera.

We could also do all this purely in software, but I'm not sure it's
worth it here. libcamera doesn't have the necessary infrastructure at
the moment, as all the platforms we've worked on have an ISP or use YUV
sensors. It could be done, but I think the effort would be better
invested, on the Librem 5, in making use of the hardware features.

> So far I'm working with contrast detection, but samsung chip should
> support phase detection auto focus, too. So far I hacked
> drivers/media/i2c/s5k3l6xx.ko not to filter out those bits and took
> photos (and looked at them in darktable), but I don't see any obvious
> "defects" indicating presense of focus data. Do you have any pointers
> to more information by chance?
> 
> I took deliberately defocused photo to make "defects" easier to see. I
> guess I should take another one with less gain to keep other noise
> down. Any ideas how to locate those bits?

The PDAF pixels will be located at fixed positions in the pixel array,
documented by the sensor vendor. Some sensors will process the PDAF
pixels and provide the processed information in the CSI-2 embedded data.
I don't have access to the sensor datasheet, so I can't help much I'm
afraid :-(

-- 
Regards,

Laurent Pinchart