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

[David Plowman]

Brilliant, thanks Kieran! Will make a mental note for next time...

David

On Mon, 28 Apr 2025 at 16:09, Kieran Bingham
<kieran.bingham at ideasonboard.com> wrote:
>
> 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
> > > > >