How to know what Auto Input Level does?

[ajax]

Maybe I posted this question in the wrong forum.

My assumption is that "Auto" means to algorithmically do what the user could have done by manually exercising the various controls.  If so it would be most helpful to display the results in the dialogue window in the various display areas that get updated when the controls are exercised individually be the user.  Might it make sense to be able to save a good result as a preset?

Also, while good, sometimes the algorithm "over does it" just a bit and the user ought to be able to tweak it just a bit.  The way I'm doing it produces no clue about what was done that may have been a pretty darn good place to start.

[Ofnuts]

(05-25-2020, 03:53 PM)ajax Wrote: Maybe I posted this question in the wrong forum.

My assumption is that "Auto" means to algorithmically do what the user could have done by manually exercising the various controls.  If so it would be most helpful to display the results in the dialogue window in the various display areas that get updated when the controls are exercised individually be the user. 

This is exactly what you see if you hit the "Auto input levels" followed by "Edit these settings as curves". AFAIK it stretches the inpiut values so that in each channel the top X% lighter pixels are 255 and the top X% darker are 0, in other words it drags the left and right input handles until they touch the histogram. The problem is that this can induce a color shift. In a picture with a blue tint, the highest "red" may be around 128, so all light pixels are blue/cyan, and after auto-levels the highest red will be 255, as will be the green and blue, so the lighter pixels are now white.

(05-25-2020, 03:53 PM)ajax Wrote: Might it make sense to be able to save a good result as a preset?

No... because that's the point of "Auto". It is computed for the image at hand. What is good for one image would not be good for the next. If you really want to save settings, you can go to the Curves dialog as indicated above and save these settings

(05-25-2020, 03:53 PM)ajax Wrote: Also, while good, sometimes the algorithm "over does it" just a bit and the user ought to be able to tweak it just a bit.  The way I'm doing it produces no clue about what was done that may have been a pretty darn good place to start.

In he Curves dialog you can fix this.

[tmanni]

Observe this:
when you run Colors > Levels... and change the Channel option to Red, Green or Blue (initially set to Value), you can see the 3 small triangles under the Input Levels histogram.
The black triangle is in the left, the gray triangle is in the middle and the white triangle is in the right.
Now, click the "Auto Input Levels" and observe again the position of the 3 triangles for the 3 RGB channels, they have moved... (or maybe not, depending of the image).

The algorithm implied here is a "contrast strechting" applied independently on each of the RGB channels.

[Ofnuts]

(05-25-2020, 08:07 PM)tmanni Wrote: Now, click the "Auto Input Levels" and observe again the position of the 3 triangles for the 3 RGB channels, they have moved... (or maybe not, depending of the image).

You have to look at the color channels.... if you just look at "value" they don't move.

[ajax]

(05-25-2020, 08:07 PM)tmanni Wrote: ...
Now, click the "Auto Input Levels" and observe again the position of the 3 triangles for the 3 RGB channels, they have moved... (or maybe not, depending of the image).
...

That is exactly what I was thinking ought to happen.  So now my question is as simple as "why NOT also do that when using Value is selected instead of Red, Green, or Blue?".

[Ofnuts]

(05-26-2020, 03:56 PM)ajax Wrote:

(05-25-2020, 08:07 PM)tmanni Wrote: ...
Now, click the "Auto Input Levels" and observe again the position of the 3 triangles for the 3 RGB channels, they have moved... (or maybe not, depending of the image).
...

That is exactly what I was thinking ought to happen.  So now my question is as simple as "why NOT also do that when using Value is selected instead of Red, Green, or Blue?".

Because the "auto" didn't apply to the "value" but to each channel, each with specific settings. In the general case, there is no "Value" handle position that can show what happened.

[ajax]

OK!  I think I now understand.

As a bit of a novice I haven't tried messing with the color channels separately and in fact that seemed to be one of the things I liked about the Levels tool.  My big surprise is finding out that Auto Input Levels doesn't do anything to Value.  I think I've also discovered that changing Value doesn't affect any of the color channels (Red, Green, or Blue).  Bottom line is that I apparently don't have a proper understanding of what Value is all about.  Might there be some reference material (i.e., tutorial type stuff) that would help correct this problem?

Thanks for the help which I clearly need!

[Ofnuts]

Value is max(R,G,B). So (0,255,0) (quite bright), (255,0,0) (moderately bright), (0,0,255) (not so bright), and (255,255,255) all have the same "value": 255.

When you use Levels on "Value", the range of the R/G/B channels are stretched/compressed appropriately. Ideally, Levels would use HSV, HSL or better, Lab models...

[ajax]

(05-26-2020, 05:28 PM)Ofnuts Wrote: Value is max(R,G,B). So (0,255,0) (quite bright),  (255,0,0) (moderately bright), (0,0,255) (not so bright), and (255,255,255) all have the same "value": 255.

When you use Levels on "Value", the range of the R/G/B channels are stretched/compressed appropriately. Ideally, Levels would use HSV, HSL or better, Lab models...

I see, might this be an answer to my prior question about reference information?  If so it looks like it might take me a while to try and absorb that.

When trying to learn what "Levels" are about I found this article about how it works in Photoshop (which I do NOT use) to be quite helpful from an instructional point of view.  However, I now notice that it refers to RGB as a channel selection and NOT Value.  I've assumed this  to mean the 3 color channels combined which I'm guilty of conflating with Value which also seemed to imply the same thing.  It now sounds like RGB and Value are different even if they do something similar.  Yeh? Ney?

I also work with Rawtherapee (RT) and notice that it does NOT seem to have a "Levels" tool which I've asked about here.  That one didn't trigger anything like the help provided herein.  Therefore, I thought it worth mentioning here.  Having now spent a little more time trying to figure this out I'm now thinking that the answer is that  RT is content with expecting curves to be used for such purposes.  I've only yet worked with what are called Tone Curves on the Exposure tab in RT.  Interestingly Rawpedia says "the histogram displayed as the curve's background shows you the levels of the data".  Might that mean Value in GIMP terms?

There are also curves for Lab Adjustments on the Exposure tab.  Then on the Color tab there are RBG Curves and, interestingly, something called an HSV Equalizer that might be called curves but nothing like the others.  Possibly this is an even more sophisticated way to deal with Value.  Yeh? Ney?

[Ofnuts]

(05-27-2020, 05:52 PM)ajax Wrote: I see, might this be an answer to my prior question about reference information?  If so it looks like it might take me a while to try and absorb that.

Yes, and also this

(05-27-2020, 05:52 PM)ajax Wrote: When trying to learn what "Levels" are about I found this article about how it works in Photoshop (which I do NOT use) to be quite helpful from an instructional point of view.  However, I now notice that it refers to RGB as a channel selection and NOT Value.  I've assumed this  to mean the 3 color channels combined which I'm guilty of conflating with Value which also seemed to imply the same thing.  It now sounds like RGB and Value are different even if they do something similar.  Yeh? Ney?

Looks like their "RGB" is the same as Gimp's "Value". A nice property of Value (ie, Max(R,G,B)) in Levels is that you cannot push it beyond what the strongest channel allows, so you are not likely to produce color shifts

(05-27-2020, 05:52 PM)ajax Wrote: I also work with Rawtherapee (RT) and notice that it does NOT seem to have a "Levels" tool which I've asked about here.  That one didn't trigger anything like the help provided herein.  Therefore, I thought it worth mentioning here.  Having now spent a little more time trying to figure this out I'm now thinking that the answer is that  RT is content with expecting curves to be used for such purposes.  I've only yet worked with what are called Tone Curves on the Exposure tab in RT.  Interestingly Rawpedia says "the histogram displayed as the curve's background shows you the levels of the data".  Might that mean Value in GIMP terms?

RT is very photo-oriented. Most of what you do with Levels is in RT's Exposure Compensation slider. But this slider also moves the Gama/midtones slider so that the picture remains as your camera would have produced it.

(05-27-2020, 05:52 PM)ajax Wrote: There are also curves for Lab Adjustments on the Exposure tab.  Then on the Color tab there are RBG Curves and, interestingly, something called an HSV Equalizer that might be called curves but nothing like the others.  Possibly this is an even more sophisticated way to deal with Value.  Yeh? Ney?

Clever widget, it allows you to change S/V for specific H bands, but it is closerr to the Hue/Saturation tool in Gimp.