libcamera: controls: Add focus Figure of Merit (FoM) control

Provide a control to allow the IPA to return a FoM to indicate how
in-focus a scene is. Note, this is not to be used as a means to
implement a focus algorithm by the application, rather an indication of
how in-focus a scene is.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/control_ids.yaml

@@ -251,4 +251,15 @@ controls:
         higher than anyone could reasonably want. Negative values are
         not allowed. Note also that sharpening is not applied to raw
         streams.
+
+  - FocusFoM:
+      type: int32_t
+      description: |
+        Reports a Figure of Merit (FoM) to indicate how in-focus the frame is.
+        A larger FocusFoM value indicates a more in-focus frame. This control
+        depends on the IPA to gather ISP statistics from the defined focus
+        region, and combine them in a suitable way to generate a FocusFoM value.
+        In this respect, it is not necessarily aimed at providing a way to
+        implement a focus algorithm by the application, rather an indication of
+        how in-focus a frame is.
 ...