Do Exposure and Zone system use the same algorithm?

mosaster · May 12, 2019, 8:57am

Hi folks,
Do Exposure and Zone system use the same value manipulation algorithm? I was convinced they are, but…
I’m observing different results applying them to the same region.

anon41087856 · May 23, 2019, 1:12pm

Exposure works by multiplying RGB input by a constant equal to 2^{EV}, so for example, adding 2 EV results in a multiplication by 4, subtracting 2 EV results in a multiplication by 0.25.

Zone system works in Lab, scales the L channel logarithmically then split it in zones, which really does not make sense at all (EV readings from Lab are not EV). The next drawback of this module is you can’t push a zone without pulling another. The last but not least is, as usual, doing magic on the L channel messes up colors in a strange way.

I’m working on a tone equalizer that uses the same algorithm as the exposure but allows to define a different exposure compensation for each zone, then blends them using a gaussian series. It’s WIP, see there :

github.com/darktable-org/darktable

tone equalizer

darktable-org:master ← aurelienpierreeng:toneequalizer

opened 07:08AM - 16 Dec 18 UTC

aurelienpierre

+4694 -19

introduce a new "tone equalizer " module, intended to allow selective exposure a…djustements in linear RGB (supposed to be color-safe). This is very similar to Lightroom, with blacks/shadows/midtones/highlights/whites cursors. The luminance channels are blended with gaussian masks and the modifications are applied on a gaussian pyramid to preserve the local contrast (luminance modified in low frequency layers). For now, nothing is optimized (naive C implementation) and the module is expected to be slow. This is just a proof of concept. I will make it faster once the core algorithm is validated.

mosaster · May 23, 2019, 4:27pm

Thank you @anon41087856 I was hoping someone would answer my doubts. There is definitely a difference between those two.
Sadly there is no blend mode that can run Zone System in RGB scale. Do you happen to know is Zone System using gaussian algorithm to break down an image?

anon41087856 · May 23, 2019, 4:52pm

No, it looks like a sort of sigmoid function:

github.com

darktable-org/darktable/blob/556887d7a973bdf0611fcc57231e107b503d9949/src/iop/zonesystem.c#L145-L170


      
          static inline void _iop_zonesystem_calculate_zonemap(struct dt_iop_zonesystem_params_t *p, float *zonemap)
          {
            int steps = 0;
            int pk = 0;
          
            for(int k = 0; k < p->size; k++)
            {
              if((k > 0 && k < p->size - 1) && p->zone[k] == -1)
                steps++;
              else
              {
                /* set 0 and 1.0 for first and last element in zonesystem size, or the actually parameter value */
                zonemap[k] = k == 0 ? 0.0 : k == (p->size - 1) ? 1.0 : p->zone[k];
          
                /* for each step from pk to k, calculate values
                    for now this is linear distributed
                */
                for(int l = 1; l <= steps; l++)
                  zonemap[pk + l] = zonemap[pk] + (((zonemap[k] - zonemap[pk]) / (steps + 1)) * l);

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mosaster · May 24, 2019, 7:35am

Thank you @anon41087856 for your contribution. Your inventions keep pushing DT forward. I wish I was programmer and could help you with creation of Tone Eq, but I’m not. I need tone EQ very much. Let me know if any help is needed in non-programming tasks.