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I've made a photo in Entangle, and then opened it in RawTherapee. When comparing the histograms, I noticed that they are very different. See the screenshot:

screenshot comparing two histograms of the same image

Here we can see a deep dip in G and B channels but almost no lack of dark piixels in RawTherapee, with very mild dip in Entangle and a considerable lack of dark pixels.

What could be a reason for this? Does one (or both) of the programs show histogram of somehow post-processed image? How do I see the real histogram of the RAW image?

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    Is it possible that the lower one is showing a logarithmic scale for the Y-axis? – user1118321 May 18 '17 at 3:17
  • @user1118321 Seems you're right. I've extracted the JPEG image embedded in the CR2 file, opened with GIMP, and choosing logarithmic mode in Histogram dialog indeed reproduced the shape Entangle shows. – Ruslan May 18 '17 at 5:06
  • Interesting! I've never seen it done that way, but it makes sense, since the range of values in a histogram can be really huge (between 0 and the number of pixels in the image). – user1118321 May 18 '17 at 5:08
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Entangle shows the histogram of the JPEG image embedded in the RAW file, in logarithmic scale. RawTherapee, on the other hand,

  1. shows the histogram in linear scale,
  2. shows the histogram of its pre-processed form of the RAW file, not of the embedded image.

This explains the difference.

To see histogram of raw data just enable the Show/Hide raw histogram Show/Hide raw histogram button icon checkbutton in the RawTherapee histogram's panel.

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  • +1 for the linear vs log scale. I suspected that was the prime difference in the heights of the histograms. – scottbb May 18 '17 at 19:19
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A RAW image is not an image by itself, it is a set of data that must be processed is some way to get an image. So you can't have the histogram of the RAW image as it doesn't exists and yes, two different programs will give you two different histograms.

But, not that differents. So, what's about those two histograms? I don't have first hand experience of the two softwares, but my guess is that the second one is the right part of the first one, with all the differences mostly due to a different color balance. Maybe, maybe, that the first histogram is the full bit representation of the image, while the second one is the histogram of the 8 bit output.

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    even if you insist that RAW is not an image (which is weird but commonspread belief), displaying a histogram of data that is not an image is quite straightforward and useful. you don't have to "develop" RAW to display histograms of its red/green/blue channels – szulat May 25 '17 at 9:56

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