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Old 01-25-2014, 06:38 PM   #1
theKbStockpiler
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Gimp levels enlightenment needed


I have read every guide on levels that I think exist and the levels adjustment slides seem to work the opposite of how the guides explain. If I'm filtering out the darkest pixel levels (as in moving the dark indicator towards the right),why is it that the image gets darker instead of lighter?

Thanks in advance!
 
Old 01-25-2014, 07:20 PM   #2
astrogeek
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Quote:
Originally Posted by theKbStockpiler View Post
I have read every guide on levels that I think exist and the levels adjustment slides seem to work the opposite of how the guides explain. If I'm filtering out the darkest pixel levels (as in moving the dark indicator towards the right),why is it that the image gets darker instead of lighter?

Thanks in advance!
First, I am not a Gimp expert, but use it often (never really used levels).

But from here:

Quote:
Level ranges can be modified in three ways:

Three triangles as sliders: one black for dark tones (Shadows), one grey for midtones (Gamma), one white for light (Highlights) tones.

The black slider determines the black point : all pixels with this value or less will be black (no color with a color channel selected / transparent with the Alpha channel selected).

The white slider determines the white point : all pixels with this value or higher, will be white (fully colored with a color channel selected / fully opaque with the Alpha channel selected).

The gray slider determines the mid point. Going to the left, to the black, makes the image lighter (more colored / more opaque) . Going to the right, to the white, makes the image darker (less colored / more transparent).

Two eye-droppers: when you click them, the mouse pointer becomes an eye-dropper. Then clicking on the image determines the black or the white point according to the chosen eye-dropper. Use the left, dark one to determine the black-point; use the right, white one to determine the white point.

Three numeric text boxes to enter values directly.
...From which I think that your perspective that it is "filtering" the dark pixels is not correct. It is shifting the point below which all values are dark (and above which all values are lighter). Or similarly, the white end sets the point above which all values are white and below which all values are darker.

So don't think of it as a filter, it is a set-point. As you have found, that works the opposite of a filter.

Last edited by astrogeek; 01-25-2014 at 07:27 PM.
 
Old 01-26-2014, 09:21 PM   #3
frankbell
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See this episode of Meetthegimp: http://blog.meetthegimp.org/episode-172-chasing-ducks/
 
Old 01-27-2014, 03:14 AM   #4
John VV
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because you are MOVING the BLACK point to a lighter value
so all values will get DARKER

basic color and tone theory

and so would lowering the GAMA the "grey point" will be moved to LIGHTER values
 
Old 01-27-2014, 01:24 PM   #5
theKbStockpiler
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Thanks for the help

Right now I'm trying to get a handle on Illuminance. I don't have the technical knowledge to understand what it does to move a slider in GIMP and the GIMP docs don't bridge that! With the Value Channel the issue is (ambient lighting) obviously.
 
Old 01-27-2014, 08:32 PM   #6
John VV
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theKbStockpiler that skill will come in time
i worked in professional Photographic darkrooms for over 12 years
before moving into digital in the end of 1990's

also the current gimp is only 8 bit
it dose not support HDR 16 bit per layer nor 32 bit per layer rgb images

for that i use Nip2/vips
http://www.vips.ecs.soton.ac.uk/index.php?title=VIPS

and the command line version of "Gmic"
http://gmic.sourceforge.net/

and convert raw camera images to 16bit png's or 32bit multilayer tiff's

there is a gimp plugin but that is 8 bit ONLY


also you will find that the "level" tool is mostly useless ( except for a "quick" GAMA change )
the "curves" tool under it in the menu is a LOT more useful
by moving( click and drag) the 45 degree line and changing "the curve"

basically it represents the "expose " curve

Last edited by John VV; 01-27-2014 at 08:34 PM.
 
Old 01-28-2014, 08:40 AM   #7
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If you're wanting to lighten dark areas, curves might be better suited for that task. Levels is way to sensitive for my tastes. But I guess it depends on if you're doing it in gimp or scripting it fu style.
 
Old 01-28-2014, 07:13 PM   #8
theKbStockpiler
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Well I have read a lot of web pages and have this to say.

It looks like the Values Channel treats all the colors in the image like they are a grayscale and the triangles narrow the spectrum of shades represented.Also, the colors that should not be effected get darker. If you move the black triangle to the midway point ,whatever was at the midway point is now black along with every shade the triangle passed over. Having the center triangle move automatically keeps the contrast from drastically changing but it still changes. If you use the brightness adjustment the contrast does not change.


Can anyone explain what happens to the actual histogram dialog box when the triangles are moved? It does not match what is indicated in the levels dialog box.
 
Old 01-28-2014, 07:39 PM   #9
John VV
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you do have the "input" and "output" slider for levels
-- screenshot
http://imgbox.com/l4ct7r1L

it is a good idea to go through tutorials
BUT
for the basics
spend a few days and PLAY
play around with things , you will start to "get the hang" of it and remember the tools


as to the histogram it is compressed or expanded or the "gray point" is moved

Last edited by John VV; 01-28-2014 at 07:42 PM.
 
Old 01-29-2014, 08:19 PM   #10
frankbell
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I have a book on GIMP (The Book of GIMP from No Starch Press) and it recommends using the "input" slider for adjustments. To lighten a picture, it says to grab the right handle and slide it to the left towards the first bump in the curve.

It seems to work for me, though I sometimes have to also tinker with brightness and contrast to fine-tune the result. I'm just a snapshot photographer, though.
 
Old 01-30-2014, 02:55 PM   #11
theKbStockpiler
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Thanks for the input.

Are there any theories on the R G or Blue channels on why you can have either yellow or blue for the blue channel? I'm guessing they are colors that don't mix to create another color. I would say it just looks like controlled color casting. :
 
Old 01-30-2014, 04:18 PM   #12
astrogeek
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Quote:
Originally Posted by theKbStockpiler View Post
Are there any theories on the R G or Blue channels on why you can have either yellow or blue for the blue channel? I'm guessing they are colors that don't mix to create another color. I would say it just looks like controlled color casting. :
From the page I originally linked...

Quote:
Red, Green and Blue work on a particular color channel: the image gets more or less color. Remember that adding or removing a color result in removing or adding the complementary color.
Yellow is the complementary color to blue, hence removing blue adds yellow, adding blue removes yellow...
 
Old 01-30-2014, 04:45 PM   #13
John VV
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additive /subtractive color theory
Red,Green,Blue / Cyan,Magenta,Yellow
http://www.novaprinting.ca/images/CMYK.jpg

Cyan + Magenta = blue
Magenta + Yellow = red

you can change any 3 colors by changing ONLY Magenta and Yellow

darkroom background , using hand enlargers for too many years

A link
http://cs.nyu.edu/courses/fall02/V22...lor_theory.htm

also Google " color theory"
 
Old 01-30-2014, 05:16 PM   #14
theKbStockpiler
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Thanks! I'll look into that!
 
  


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