I’m looking into the Exposure tools, and don’t quite understand the meaning of the percentile slider in the automatic mode.
Documentation says “Define a location in the histogram to use for automatic exposure correction.”
The example helps a bit “A percentile of 50% denotes a position in the histogram where 50% of pixel values are above and 50% of pixel values are below that exposure”
With a 50% percentile, where is set the exposure, and how can I see that 50% is below and above that exposure ? Is the exposure the one defined in the EXIF as exposure biais ? or the target value we can set from the white point ?
If I move the percentile slider to 20% (for example), what does it really means? From the documentation example I would say that 20% of pixel are below and 20% are over the exposure, which fits also when the percentile is at 50%… I’m a bit confused I had the false impression the module was simple to use, I’m so wrong now.
Hi, I don’t know of anyone who uses the exposure module in automatic mode. Put it in manual and move the slider until the midtones look good. Sorry for the non-answer
The automatic mode is used to set the exposure in multiple images. It is mainly used to deflicker images in timelapse sequences. It was introduced in dt 2.0. Robert Hutton produced a nice video on this topic here https://youtu.be/VJbJ0btlui0?t=8m10s
I’m also using it manual, but to be honest, I never looked beyond the expsotion slider. It seems so obvious, but it has more under it’s belt, I’m curious to know what are the other options. Pass172 just did that
Deflicker percentile: What percentage of brightness should be used for the brightness matching.
Deflicker target : Desired exposure level for processed pics.
However, it seems from the forum, that the target has to be -4EV (for example) with no explicit reference from where. Where, in Darktable the target is relative to the white point. Don’t know if these are the same.
I haven’t verified the function of auto-exposure. I played with it only briefly out of curiosity in the past.
First, are we on the same page on the meaning of percentile?
Second
Don’t worry about how various apps define their highlights, mid-tones and shadows. The important thing is how the intensity levels are distributed, esp. the mid-tones (based on a middle statistic), which is the point of any auto-magical method.
I always thought you selected an area, and the percentile would be used to set ‘non clipping exposure’. Basically like rawtherapee auto.
If I set it to 99, then the exposure would be set so high that 1% of the brightest pixels would be clipped.
Guess I was completely wrong? I never read the explanation or a manual on this, I just assumed… .
Since exposure for me in darktable is pure an artistic tool, I never used the auto mode though.
In rawtherapee getting all your values in a 0.0 - 1.0 range (and then mapping those) is more important so I did use it there for quick starting points, but I guess the percentile is quite different there then…
In scene-referred workflow, exposure is much too early in the pipe to care about “clipping” since the pixel values are not constrained until the mapping to display-referred space in filmic (by setting the black/white points). We set exposure merely to define the location of mid-grey for the remainder of the pipe.
My guess about percentile is that it is coming from Magic Lantern, and may be for this app, it makes sense to have some clipping. It may be make less sense in DT because it’s too early in the pipe ? Just a wild guess.
I had the false impression the module was simple to use, I’m so wrong now.
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