conversion-script.sh.txt (997 Bytes)
After the discussion in Comparing filmic color science v5/v6 and GitHub issues/discussions like https://github.com/darktable-org/darktable/issues/11777, I’ve decided to check what
- darktable, applying the output profile
– using its own colour management
– via LCMS2
do when facing out-of-gamut colours (converting an image from a large colour space to a more limited one).
I’ve made a simple ‘colour ramp’, going from quite low saturation up to full, for each of red, yellow, green, cyan, blue.
color-ramp-rec2020-g10.tif (32.4 KB)
The image has Elle’s Rec2020 linear (‘Gamma = 1.0’) profile embedded.
I then opened it in darktable; set the monitor, soft-proofing and histogram profiles all to linear Rec709, and selected two areas: one, where the Rec709 values exceeded 255; another, when one of them went negative.
I exported the image from darktable with all rendering intents; I then repeated the process after telling darktable to use LCMS to apply the output profile.
On the command-line, I used
tificc (from LCMS) and
cctiff (from Argyll) to convert the Rec2020 ramp to ‘sRGB linear’ (sRGB uses the same primaries as Rec709). I used a script to iterate through the rendering intents for each; for LCMS, which supports V4 profiles, I repeated the process using Elle’s V4 profile for output. The conversion script is attached with a
Finally, I compared all output files using GraphicsMagick’s
gm compare tool.
For example, this command compares the results of the different intents in Argyll:
for i in p s r ; do gm compare -metric mse color-ramp-rec2020-g10.tif-argyll-sRGB-g10-ia.tif color-ramp-rec2020-g10.tif-argyll-sRGB-g10-i$i.tif ; done
So, the results (may be valid for this particular image only):
- it does not matter if LCMS or darktable’s own method was used for export, the output was identical;
- it does not matter which rendering intent was used in darktable, the output was identical;
- Argyll also produced the same output for each rendering intent;
- and so did LCMS, regardless of the V2 or V4 output profile.
The output from Argyll and darktable were the same (if I used Elle’s linear sRGB profile); there was a tiny mismatch (due to subtle differences in the profiles) if I used the built-in linear Rec709:
$ gm compare -metric mse argyll.tif dt.tif Image Difference (MeanSquaredError): Normalized Absolute ============ ========= Red: 0.0000000001 0.0 Green: 0.0000000000 0.0 Blue: 0.0000000001 0.0 Total: 0.0000000001 0.0
LCMS differed from the other two:
$ gm compare -metric mse argyll.tif lcms.tif Image Difference (MeanSquaredError): Normalized Absolute ============ ========== Red: 0.0000005761 0.0 Green: 0.0000011243 0.1 Blue: 0.0000002813 0.0 Total: 0.0000006605 0.0
Exaggerated differences in the Gimp:
One of the questions was what happens when we go out of gamut: do we get a reasonable compression/management, or are we simply clipping?
A red pixel that read > 100% in darktable’s darkroom (for easier comparison, I multiplied the RGB values by 256, as in the Gimp I got 16-bit readings). This is at Y=419 in the TIF, when opened in Gimp (note that I got the darkroom green simply by multiplying 2 by 256, so it’s not reliable; if I convert the Rec2020 value to linear sRGB in the Gimp, I get 0.006354, which corresponds to the reading of 416 that Argyll produced):
Rec2020: RGB: 46˙310, 5˙501, 5˙501; HSV: 346.10, 93.00, 105.00 ================================================================= Rec709 values: darkroom: RGB: 72˙960, 512, 4˙864; HSV: 356.42, 99.40, 111.65 dt export, & Argyll: RGB: 65˙535, 416, 4˙761; HSV: 345.40, 92.70, 100.00 LCMS: RGB: 65˙535, 733, 4˙761; HSV: 347.10, 89.30, 100.00
An even more saturated red, with a negative Rec709 green component, at line 463 in the original TIF (a conversion in Gimp yields a green value of -0.037803, or -2477; the darkroom value shown below comes from multiplying the darkrooms -10 reading by 256):
Rec2020: RGB: 52˙793, 3˙646, 3˙646; HSV: 333.40, 143.50, 112.20 ================================================================== Rec709 values: darkroom: RGB: 84˙736, -2˙560, 2˙816; HSV: 356.42, 102.87, 129.90 dt export, & Argyll & LCMS: RGB: 65˙535, 0, 2˙754; HSV: 346.40, 100.00, 100.00
100% Rec2020 red:
Rec2020: RGB: 65535, 0, 0; HSV: 331.10, 229.00, 124.80 ================================================================== darkroom: RGB: 108˙288, -8˙192, -1280; HSV: 356.42, 107.50, 166.05 dt export, & Argyll & LCMS: RGB: 65˙535, 0, 0; HSV: 346.40, 100.00, 100.00
So, with the negative values we simply get clipping. Most probably that is also what happens for values > 100%.
ps. The profiles are from Elle Stone. See Elle Stone's well-behaved ICC profiles and code or download from elles_icc_profiles/profiles at master · ellelstone/elles_icc_profiles · GitHub