[RFQ] Multiple ISP outputs and scaler crop

[Naushir Patuck]

Hi all,

We have come across some new use cases where users will need different
scaler crops applied to each of the 2 ISP outputs configured by the
application.  This is not possible on the VC4 hardware, but we can
apply different crop configurations on Pi5/PISP.

Right now, libcamera has a single ScalerCrop control that we apply to
both outputs.  This means that the input FoV is identical for both
outputs, and can distort the pixel aspect ratio on the "low res"
output if the output aspect ratio on both ISP outputs are not the
same.

I would like to propose a couple of solutions to this:

1) Change the ScalerCrop control to be a span, so that users can
specify crop for multiple output branches through the single control.
This will incur an API breakage though, so may not be desirable.

2) Add a new control called ScalerCrops (with an s), that behaves
identically to ScalerCrop, but works with a span of rectangles.  The
value is handled in exactly the same way as  ScalerCrop, but allows
the user to set muitple crops, one per output.  Of course the wording
of this control needs to be explicit about the behavior, e.g. index 0
corresponds to configured stream 0, and what if we only want to change
one stream's scaler crop, etc.

Thoughts?

Regards,
Naush