[PATCH 1/2] utils: raspberrypi: ctt: Fix NaNs in lens shading tables

[Kieran Bingham]

Quoting David Plowman (2025-04-28 14:53:28)
> Hi Kieran
> 
> On Mon, 28 Apr 2025 at 14:47, Kieran Bingham
> <kieran.bingham at ideasonboard.com> wrote:
> >
> > Quoting Naushir Patuck (2025-04-28 14:20:26)
> > > Hi David,
> > >
> > > On Mon, 28 Apr 2025 at 11:36, David Plowman
> > > <david.plowman at raspberrypi.com> wrote:
> > > >
> > > > The problem occurs when the calculation could lead to a final row (or
> > > > column) of grid squares with no pixels in them (and hence, NaNs).
> > > >
> > > > One specific case is a Pi 5 with an image width (or height) of 1364,
> > > > so that's 682 Bayer quads. To give 32 grid squares it was calculating
> > > > 22 quads per cell. However, 31 * 22 = 682 leaving nothing in the final
> > > > column.
> > > >
> > > > The fix is to do a rounding-down division by the number of cells minus
> > > > one, rather than a rounding-up division by the number of cells. This
> > > > turns the corner case from one where the final row/column has no
> > > > pixels to one where we don't quite cover the full image, which is how
> > > > we have to handle these cases.
> > > >
> > > > Signed-off-by: David Plowman <david.plowman at raspberrypi.com>
> > >
> > > Reviewed-by: Naushir Patuck <naush at raspberrypi.com>
> >
> >
> > Acked-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
> >
> > Will merge these two.
> >
> > Would you like to add any Bug/Fixes reference so it appears in the
> > libcamera release notes ?
> 
> Well, the bug would be this one:
> https://github.com/raspberrypi/libcamera/issues/254 (ditto for the
> other patch).
> Are we OK to reference a URL?

Sure: Plenty of precedence, and I think it helps to be able to tie the
fix to the original requests for more information.

git log | grep Bug:
libcamera$ git log | grep Bug:

(Snipped examples:)
      - Bug: https://github.com/raspberrypi/libcamera/issues/138
      - Bug: https://bugs.libcamera.org/show_bug.cgi?id=201

I'll add:

Bug: https://github.com/raspberrypi/libcamera/issues/254

--
Kieran

> 
> Thanks
> David
> 
> >
> >
> > --
> > Kieran
> >
> >
> >
> > >
> > > > ---
> > > >  utils/raspberrypi/ctt/ctt_alsc.py | 7 ++++---
> > > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/utils/raspberrypi/ctt/ctt_alsc.py b/utils/raspberrypi/ctt/ctt_alsc.py
> > > > index 1d94dfa5..f4fd09e3 100644
> > > > --- a/utils/raspberrypi/ctt/ctt_alsc.py
> > > > +++ b/utils/raspberrypi/ctt/ctt_alsc.py
> > > > @@ -127,11 +127,12 @@ def alsc(Cam, Img, do_alsc_colour, plot=False, grid_size=(16, 12), max_gain=8.0)
> > > >      channels = [Img.channels[i] for i in Img.order]
> > > >      """
> > > >      calculate size of single rectangle.
> > > > -    -(-(w-1)//32) is a ceiling division. w-1 is to deal robustly with the case
> > > > -    where w is a multiple of 32.
> > > > +    The divisions here must ensure the final row/column of cells has a non-zero number of
> > > > +    pixels.
> > > >      """
> > > >      w, h = Img.w/2, Img.h/2
> > > > -    dx, dy = int(-(-(w-1)//grid_w)), int(-(-(h-1)//grid_h))
> > > > +    dx, dy = (w - 1) // (grid_w - 1), (h - 1) // (grid_h - 1)
> > > > +
> > > >      """
> > > >      average the green channels into one
> > > >      """
> > > > --
> > > > 2.34.1
> > > >