[digikam] [Bug 508104] Add Fujifilm focus-point extractor support to digiKam

[bugzilla_noreply at kde.org]

https://bugs.kde.org/show_bug.cgi?id=508104

--- Comment #6 from focus-point at mailinator.com ---
Thank you Maik for quick implementation!

Tomorrow, I will provide set of photos (raw + jpeg + heic) covering many
combination of focus settings (AF-C, AF-S, M VS Point, Zone, Full, Face,
Subject). However, I will drop some combinations, if they all likely produce
same focus metadata as AF-S single point (returning single pixel coordinate for
focus area)

To make it easier to test if we draw the focus frame in right place, I am
planning to focus a distinct object with the same size as the focus point on
the camera. 



About your request for understanding how large the frame around focus point is
displayed, I will send you screenshots from camera. However, I did a small
research for you to figure out what is best relative width x height for the
frame around focus point.

TLDR: If you take length of short side of the image and divide by 9, this will
be equal to width=height of the square focus frame we need to draw around focus
point (center of square is focus coordinates in the metadata).

Basics:
- Other than my own camera, I checked relevant specs and manuals of 50 fujifilm
cameras released since 2010.
- Each focus point is displayed as a square frame around focus point, not
rectangle.
- These squares makes a grid that covers photos almost entirely (with some
exceptions that I will explain but they don't effect our calculations).
- So, for example, if we take height of the image, and divide by how many
squares are there vertically on the grid, we can find size of of each square
relatively to height of the image!

Aspect ratio -> width x height of grid of squares:
3:2 -> 13 square x 9 square
16:9 -> 13 square x 7 square
1:1 -> 9 square x 9 square
4:3 -> 11 square x 9 square
5:4 -> 11 square x 9 square

Conclusion:
- As you can see, ratio of grid dimension and aspect ratio of image almost
matches. So squares almost covers the image.
- Then for simplification, we can assume squares covers the image entirely.
Then dividing size of the image by number of squares, we can find size of a
square.
- The for simplification of this, we can assume short side of the image is
always equal to 9 squares (it's true with the exception of 13x7 but the smaller
1/9 is still ok there anyway)
- So then size of square should be best estimated as following:

width or height of square = min(image width, image height)/9


If you think this makes sense, let's take this as starting point and improve as
we go through tests.

Exceptions:
- There is a setting to choose finer grids with half the size squares. For
example 3:2 aspect ratio has 25x17 grid option in settings. However, then
square focus frame we draw will be very small and not very meaningful to check
focus areas by eye.
- For first generation older cameras, grid doesn't cover image fully sometimes.
However, it doesn't matter because square size is still same! For, example,
they use 13x7 grid instead of 13x9 grid for 3:2 aspect ratio BUT each square is
still approximately sized as 1/9 of shortest side of image. 
- I am sure there are other exceptions but this is how much I figured out by my
research.

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