HDR Merge: Kudos! Histogram: Question?

[afdojo]

KUDOS:

First and foremost, huge kudos to the Affinity Photo team for HDR Merge. Long story short but I recently scanned 1,200 35mm negatives using an $89 Jumbl scanner. As you can see in the first "Screen Shot" below, the scanner's default settings create a JPG file that is WAY TOO dark. I then scanned the negative three times using the EV={-1.0, 0.0, +1.0} options and opened the three files with HDR Merge. With minor tweaks to the Tone Map sliders, I got the second "Screen Shot" below. WAY BETTER! Thanks! You saved me a ton of money by not having to paying a scanning service!

QUESTION:

I exported HDR image to a JPG file and then opened it in Photo -- see third "Screen Shot" below. Although the images look identical, the histograms are WAY different. For example, the mean is like 68 in the HDR version but 129 in the exported version. So, the question is, why are the histograms so different?

 

Thanks,

Doug

[thomaso]

The different histogram appearance results from the comparison of the initial RGB/32 (HDR, linear sRGB) with the exported RGB/8 bit. Linear profiles use different gamma than non-linear profiles.

The 32-bit-Preview panel might interest, though it doesn't alter the image but previews only and its settings appear not to affect the histogram view.

https://affinity.help/photo/English.lproj/index.html?page=pages/Panels/32bitPanel.html?title=32-bit Preview

[NotMyFault]

Using HDR for scanned images is kind of total overkill. Film Rolls have a limited dynamic range. Gray/16 is enough with still plenty of headroom.

Using HDR and RGB/32 introduces the additional complexity you are just observing and reporting here.

You can save much of your time by using a single scan of correct brightness (normally adjustable within the scanning app).

A simple brightness/ contrast, levels or curves adjustment should be sufficient to get a perfect result.

 

[NotMyFault]

There are excellent (long) tutorials videos explaining the background of dynamic range and need for gamma encoding

https://en.wikipedia.org/wiki/Gamma_correction

 

 

[Old Bruce]

If all you can get out of the scanner is an 8 bit jpeg then I would definitely want at least a +/- 1 stop exposure for B&W negatives. Then I would probably convert them to 16 bits and do a Stack of the three or five images to get the full detail from the original negative.

[thomaso]

57 minutes ago, NotMyFault said:

There are excellent (long) tutorials videos explaining the background of dynamic range and need for gamma encoding

Nice video 😎. But sooo complicated 🧐. Aside all this tricky "real" eye, "real" camera, "real" science, "real" reality...
… if only it weren't for the inability of the human brain to imagine a logarithmic amount... 😷

[afdojo]

Thanks to all for your comments and suggestions.

A button allowing AF Photo users to switch histograms (between the Preview Panel and the HDR encoded image) would be useful. I have a little python program that plots frequency distribution for JPG files so I can export the image, run my python program, look at the histogram and then tweak the sliders (Levels, Curves, Contrast, etc). Lather, rinse, repeat. That works but it's pretty tedious. For my work, a histogram that does not correspond to the viewed image is pretty useless.

[Lisbon]

2 hours ago, afdojo said:

A button allowing AF Photo users to switch histograms (between the Preview Panel and the HDR encoded image) would be useful.

I agree. Really useful.

2 hours ago, afdojo said:

For my work, a histogram that does not correspond to the viewed image is pretty useless.

See if this helps.

Add a Live Procedural texture (Layer > New Live Filter Layer > Colours > Procedural Texture).

You can save the Procedural texture as a preset.

Open 32-bit Preview (View > Studio > 32-bit Preview) and set "Display Transform" to Unmanaged.

This should make the histogram match what you see.

Very important. When editing, the Live Procedural Texture should always be on top, above all other adjustments.
When finished editing, delete the procedural texture and change "Display Transform" to "ICC Display Tranform".

[afdojo]

Thanks @Lisbon -- very interesting but not quite what I'm looking for. Your steps changed the Preview image to match the histogram but now both are way darker than the exported image. What I need is shown in the third Screen Shot above where the histogram matches the image I'm seeing and that will be exported. Here's what the Procedural Texture and Unmanaged "Display Transform" produces.

[Lisbon]

3 hours ago, afdojo said:

Thanks @Lisbon -- very interesting but not quite what I'm looking for.

Sorry to hear that  @afdojo

3 hours ago, afdojo said:

...now both are way darker than the exported image...

Hmmmm... this is strange.
There must be some difference between the 32-bit version and the exported one. But the difference has to be almost imperceptible.

Let me show what i did.

1) I start with a 32-bit HDR
- Display transform: ICC Display Transform

2) I make the histogram match what i see. Image remains the same.
- Add a procedural texture
- Display transform: Unmanaged

3) I make my adjustments. Image becomes darker.

4) Go back to 32-bit default. Image remains the same. Histogram changes.
- Delete procedural texture
- Display transform: ICC Display Transform

5) Now i will export my image.
The image must be the same as in step 4 and the histogram as in step 3.

The histogram is not exactly the same as in step 3, but we have to take into account that we were in 32 bit and now in 8 bit.

Hope it helps.