Changes made to the spektrafilm profiles shipped in this directory
=================================================================

The profiles here are derived from spektrafilm by Andrea Volpato
(https://github.com/andreavolpato/spektrafilm), licensed CC BY-SA 4.0. The
full licence is in LICENSE-PROFILES.txt and is reproduced unchanged.

CC BY-SA 4.0 section 3(a)(1)(B) requires that a modified copy say it was
modified. It was. This file says how, and tools/film-profiles/convert.py
performs the modification, so it can be re-run against upstream and the result
compared rather than taken on trust.


What was changed
----------------

1. Format. Upstream ships JSON; these are YAML, so that adding or correcting a
   stock is editing a legible file rather than a minified one. No value is
   altered by the reformat.

2. Trimmed to the fields this renderer reads:

     kept     info.*, data.wavelengths (implicitly, as the fixed grid),
              data.log_sensitivity, data.channel_density (renamed
              dye_density), data.base_density, data.log_exposure (kept as its
              two endpoints, since it is uniformly sampled),
              data.density_curves

     dropped  data.density_curves_model      - a 3-CDF fit of the curves; the
                                               sampled curves are shipped
                                               instead, and reproduce it to
                                               0.004 density
              data.density_curves_layers     - per-sublayer curves, used for
                                               grain, which is not implemented
                                               yet. Worth restoring when it is:
                                               real grain is per sublayer.
              data.hanatos2025_adaptation_*  - parameters for a spectral
                                               upsampling method this renderer
                                               does not use; see below
              data.midscale_neutral_density  - null in every profile shipped

   Dropping fields loses nothing for the stocks shipped, but it does mean a
   re-run of the converter is needed to pick up an upstream field later.

3. Numbers are written at 6 significant figures (5 for the density curves).
   The inputs are digitised datasheet curves, so this is well inside their
   measurement error; it is what takes a profile from 207 kB to 17 kB.

4. Nulls made explicit. Upstream uses null where a datasheet has no reading.
   In log_sensitivity that means the layer is blind there, written here as the
   sentinel -9; in the density tables it means no absorption, written as 0.


What was NOT changed
--------------------

No measured value has been rescaled, shifted, smoothed or refitted. The
renderer's own calibration conventions - mid-grey at 0.184, exposure
normalised on the green layer - are taken from spektrafilm's reference
implementation rather than invented, because the profile data is calibrated
against them.


Known deviation from upstream's rendering
-----------------------------------------

Upstream reconstructs a spectrum from an RGB triple with Hanatos (2025), which
needs a 4 MB coefficient table. This renderer uses the Mallett & Yuksel (2019)
sRGB basis instead, which is three curves and about 1 kB, at some cost in how
faithfully very saturated and out-of-gamut colours are handled. The
hanatos2025_adaptation_* parameters in the upstream profiles are therefore
unused here. This is a deliberate trade of accuracy at the gamut edge against
shipping four megabytes, and it is the first thing to revisit if saturated
colours look wrong.