Linux - DesktopThis forum is for the discussion of all Linux Software used in a desktop context.
Notices
Welcome to LinuxQuestions.org, a friendly and active Linux Community.
You are currently viewing LQ as a guest. By joining our community you will have the ability to post topics, receive our newsletter, use the advanced search, subscribe to threads and access many other special features. Registration is quick, simple and absolutely free. Join our community today!
Note that registered members see fewer ads, and ContentLink is completely disabled once you log in.
If you have any problems with the registration process or your account login, please contact us. If you need to reset your password, click here.
Having a problem logging in? Please visit this page to clear all LQ-related cookies.
Get a virtual cloud desktop with the Linux distro that you want in less than five minutes with Shells! With over 10 pre-installed distros to choose from, the worry-free installation life is here! Whether you are a digital nomad or just looking for flexibility, Shells can put your Linux machine on the device that you want to use.
Exclusive for LQ members, get up to 45% off per month. Click here for more info.
Distribution: RPM Distros,Mostly Mandrake Forks;Drake Tools/Utilities all the way!GO MAGEIA!!!
Posts: 986
Rep:
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?
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).
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.
Distribution: RPM Distros,Mostly Mandrake Forks;Drake Tools/Utilities all the way!GO MAGEIA!!!
Posts: 986
Original Poster
Rep:
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.
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
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"
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.
Distribution: RPM Distros,Mostly Mandrake Forks;Drake Tools/Utilities all the way!GO MAGEIA!!!
Posts: 986
Original Poster
Rep:
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.
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
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.
Distribution: RPM Distros,Mostly Mandrake Forks;Drake Tools/Utilities all the way!GO MAGEIA!!!
Posts: 986
Original Poster
Rep:
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. :
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...
LinuxQuestions.org is looking for people interested in writing
Editorials, Articles, Reviews, and more. If you'd like to contribute
content, let us know.