dtourolle
1fdfb5990c
dr-film's README still described one 32³ lookup that took a negative
through the print and the paper, with the sliders' values baked into
it. Since 6b99f67 nothing a slider moves is baked: the curves are one
row per development time the datasheet measures and push interpolates
between them, a print is two lookups split at the paper's log exposure
with the enlarger's exposure added between them, and exposure, push,
print exposure and format reach the shader as uniforms. That is what
lets a mask layer hold film settings of its own.
The section now says so, and that the film's Exposure on the whole
photograph is the one setting that rebakes, because the enlarger's
filtration is solved against it.
4.6 KiB
Film stocks
One file per stock in profiles/. Adding a stock is adding a
file — no code change, no shader, no new operation — for the same reason
dr-decode's base curves work that way: under the GPLv3 a stock should be
contributable without a release.
What a profile is
Three measured tables, all of them published in the manufacturer's datasheet:
| Field | What it decides |
|---|---|
log_sensitivity |
what each emulsion layer sees, per wavelength |
density_curves |
contrast, latitude, and where the stock clips |
dye_density |
what the developed stock looks like, per wavelength |
base_density |
the support: film base, and a colour negative's orange mask |
Plus kind (negative or positive), support (film or paper), and the two
illuminants the data is referenced to. A print paper is a stock like any
other; support exists so an interface can offer papers separately, not
because the renderer treats them differently.
Why it is not a LUT
Because the parameters stay physical. Opening up a stop moves the picture along the film's own characteristic curve — toe, shoulder and all — instead of scaling a number somebody baked at one exposure. A scanned negative comes out orange and inverted because that is what a negative is, and it becomes a photograph when a paper profile prints it, exactly as it would in a darkroom.
The data cost runs the other way from a LUT collection too: a stock is about 17 kB of measurements, where one HaldCLUT is roughly 800 kB of one person's grade.
How it runs
The spectral chain reduces to three tables, and the reduction is exact where it
matters — see src/bake.rs for the argument:
- A 3×3 matrix, linear sRGB to the three layers' exposure. Exact, not an approximation: the reconstructed scene spectrum is linear in the sRGB triple, so the integral collapses into nine numbers.
- Three 1D curves, log exposure to density, sampled at 256 points — one row per development time the datasheet measures. Push picks between the rows, and interpolating them is exact, because density is linear in push between two measured processes.
- One 32³ lookup, density to linear sRGB — dye absorption, the viewing illuminant and the chromatic adaptation, all of which take exactly three numbers in. A printed negative is two: the film's cube ends at the paper's log exposure through the negative, the enlarger's exposure is added there, and the paper's own curve row and cube take it to linear sRGB.
Per pixel that is a matrix multiply, a handful of curve taps and one texture fetch — two for a print. Splitting 2 from 3, rather than baking one LUT over exposure, is measured rather than assumed: the curve carries all the sharp shape and the dye mixing is smooth, so folding the curve into the 3D lookup would need it three times larger for the same error. At 32³ the worst interpolation error is about 0.003 in linear sRGB, below one 8-bit code value, and there is a test that says so.
No slider is baked. Camera exposure is a gain before the matrix, push chooses between curve rows, print exposure is the addition between the two cubes, and format sets the grain; each reaches the shader as a uniform that is linear in what it does. That is what lets a mask layer hold its own film settings, and a pixel under several layers take the weighted average of them. Only the enlarger's filtration is solved at bake time, against the photograph's exposure — an enlarger has one filtration for the whole print — so the film's Exposure, set on the whole photograph, is the one slider that rebakes. The stock and its paper are the photograph's; a layer has no picker.
Adding a stock
If spektrafilm has it, add its name to STOCKS in
tools/film-profiles/convert.py and
re-run it. Otherwise write the YAML by hand from the datasheet; the loader
validates the table lengths and says which file and field is wrong.
Either way, list it in BUILT_IN in src/lib.rs to compile it
in — or drop it in the profile directory at runtime, which is the path meant
for stocks that ship separately from the binary.
Provenance
The shipped profiles are converted from
spektrafilm by Andrea Volpato,
licensed CC BY-SA 4.0. See profiles/LICENSE-PROFILES.txt
for the licence and profiles/CHANGELOG.txt for what
the conversion changed and what it deliberately did not.
The sRGB reflectance basis is Mallett & Yuksel (2019); the observer is the CIE 1931 2°.