Episode: 4350
Title: HPR4350: GIMP: More Photo Fixes
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4350/hpr4350.mp3
Transcribed: 2025-10-25 23:29:10

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This is Hacker Public Radio episode 4,350 for Friday the 4th of April 2025.
Today's show is entitled, Gimp, More Photo Fixes.
It is part of a series Gimp.
It is hosted by Ahukah and is about 14 minutes long.
It carries a clean flag.
The summary is, a few more fixes to some of the common problems in photos.
Hello, this is Ahukah, welcoming you to Hacker Public Radio and another exciting episode.
And I'm going to, at this point, conclude for the time being, and perhaps forever, my
series on Gimp.
Now, previously, I did a few changes to photos, how to fix some of the problems.
And I'm going to do a little bit more in this episode.
And you're going to see some similarities here.
But I'm going to start with dark photos, okay?
Because, you know, sometimes photos just aren't exposed the way you want them to be.
It could happen because you took a photo at night, or you're in an area with low light
levels.
I've got some photos from the Mirror Woods in California.
They were taken in a daytime, but it looks like midnight, just because there was so much
little light there.
Now, can things like that be fixed?
Yes, to some degree.
What you have to bear in mind is you cannot create information if it was not captured
in the first place.
Now, I have a few photos that I took in Heidelberg, Germany, that had this problem on a sunny
day, because the sky was so bright that the camera adjusted to the sky, thus making the
city scene too dark.
So how do I handle this problem?
Well, first, open the photo and Gimp.
All right?
Almost goes without saying.
Go to the colors menu and select levels.
Now this will open a pop-up window to adjust color levels.
This becomes a trial and error process, but you may be amazed at what you can do with
just a few simple tweaks.
In fact, this whole episode is going to be about trial and error fixes.
It's not just push a button and you're done.
So with the adjust color levels open, you should see that the setting for channel will
be value.
Leave it there for the moment.
That means you will be adjusting all three color channels equally, which means it is
just the overall darkness and brightness that you're dealing with.
Now underneath that is the input levels setting, and there is a histogram.
And underneath it is a long box with triangles.
These are sliders, and there are three of them.
Though one may be half hidden if it is all the way to one side.
Therefore, black, gray, and white, or instead of gray, you could say mid-tone, those are
two valid ways of looking at it.
Now note that there is a check box in the lower right of the pop-up for split view.
This will split the screen down the middle, so you can see the original side by side with
your modified version.
I recommend using this.
You will see it on the other pop-ups we use as well.
Start with the middle slider and move it to the left.
You should see the picture start to brighten.
Depending on the original photo, this may be all you need to do.
As I said, it is trial and error, and what matters most is if you like the picture you
are getting.
But you can try to modify it a bit more with the other two sliders.
Then if you wish, you can go back to the value setting and select each of the primary
colors of red, green, and blue, and adjust them as well.
Click OK if you like what you have, which will close the adjust color levels pop-up.
Let's go to the colors menu and select exposure.
This will open another pop-up window.
In this pop-up window, you can adjust the exposure and the black level.
Make any adjustments you need to make here.
Remember to use split view.
When you like what we have, what you have, click OK, closing the pop-up.
For more tweaking, you can go to the color menu and select color temperature.
This term is something that comes from physics and is somewhat technical and rather counterintuitive,
at least the way I think about it.
We tend to think of reds and oranges as warm colors and blues and whites as cool colors,
but in color temperature, it's just the opposite.
You don't really need to get into all of this if you just want to think of this as another
possible tweak to your photo.
Adjust the sliders for original temperature and intended temperature until you like the
result and click OK to accept this and close the pop-up window.
One last adjustment can be made by going to the color menu and selecting curves.
This pop-up has a graph with a diagonal line running up from left to right, but you can
click on the line and drag it to create a smooth curve.
Usually you want to create something like the letter S, by dragging the right side up
a little and the left side down a little.
But again, I would use the split view and don't overdo it.
Very small changes can be just fine here.
If you finally have something that you're comfortable with, export the photo.
When I do this, I append dash fixed to the original file name and keep the original file
when I do this.
Now this may sound very daunting, it doesn't have to be.
If you're a graphic designer or some other type of visual artist, you should probably take
the time to master all of these tools as part of your craft.
But for a hobbyist who only takes holiday snapshots like me, maybe only one or two of these
tweaks will do what is needed.
Or one of my photos from Heidelberg, all I really needed to do is make the first adjustment
in adjust color levels than I was fine with the result.
With some of the other photos, I needed to go one or two steps further.
The only thing you should care about is if you have photos you like.
Now if you want a little more information, there's a video from Digitivity that I've
put a link to in the show notes and you can check that out.
Give you a little more information.
Now the other thing is my wife has some pictures that are 50 years old that have not aged well.
This is an example of a color problem because they have a strong green tint to them.
Now that's probably due to degradation of the magenta dye in the film.
The film colors are cyan magenta and yellow and magenta if it degrades just leave cyan
and yellow and those of course combine to form green.
Now GIMP can fix this to some degree and I'm going to give you a couple of ways to go
about this.
The easy one I'll start with is color balance.
So again open your photo and GIMP.
Go to the color menu and select color balance.
This will open a pop up window.
In this window you can adjust the colors for three different ranges, your shadows, midtones
and highlights.
You will see the biggest effect with midtones in most pictures but do work all three.
Remember to use split view.
Now the sliders underneath let you adjust individual color ranges and work by sliding
toward one color and away from the complement.
So if you slide towards cyan you will add more cyan to the picture and you will get less
red.
More magenta means less green, more yellow means less blue.
Now this again it's trial and error.
You won't get a perfect result right away but get it as close as you can and click OK
to accept the changes and close the pop up window.
For relatively minor adjustments this is a fairly easy approach.
But not as flexible and powerful as the next method.
And the next method goes back to what we were looking at before for dark photos and that
is the adjust color levels.
So you go to the color menu and select adjust color levels.
Again it's a pretty similar process only now the color channels become even more important.
So you want to use the sliders to adjust the red, green and blue levels is needed.
Also make sure to check the output levels to fine tune the result.
And when you get the best result click OK to close the pop up window and accept the picture
and export the final product.
Now if you have serious degradation this result is not going to be perfect.
Gimp can only work with the information it has but I was able to get significantly better
results on those old photos.
Now I'm going to explain a little something and you can skip this if you're not interested
in the details but you know all about histograms and those sliders what are they actually doing
here.
A histogram is a graph statisticians used to look at a frequency distribution which means
how many observations fall into a certain range or category.
In Gimp the histograms are telling you about the number of pixels with a certain value.
So at the far left you have pure black or pure red or pure green or pure blue depending
on what you've selected as your channel and on the right is pure white pure red etc.
Now in the adjust color levels pop up when you leave it on value it is looking at all three
channels together but that's the same as sort of a black and white because you're just
looking at the overall darkness and lightness and not changing the balance of colors in
any way.
So a very dark picture will have a very high curve on the left because there's lots
of dark pixels and a low curve on the right because there were fewer light pixels.
The three triangle sliders help adjust the balance.
If you pull the left most triangle to the right in other words towards the center you're
telling Gimp to treat every pixel up to it including that value as pure black and that
will darken the picture because you might have had some pixels that were sort of black
now they're going to be totally black.
Now if you pull the right most triangle to the left in other words towards the center
you're telling Gimp to treat every pixel from that value upwards as if it was pure white.
So it's going to lighten it.
On the middle triangles for setting your midpoint when you move it to the left you are shrinking
the area below the midpoint thus reducing the amount of darkness and so the image lightens.
And of course moving it to the right increases the area below the midpoint making the image
darker.
Now something similar happens when you move to the color channels but it's not exactly
intuitive here instead of the black and white balance you're working with the amount
of color for example if you're on the red channel and you move the left most triangle to
the right in other words towards the center which are actually telling Gimp is that all
pixels to the left of that triangle should now be cyan the complement to red and this
is counterintuitive because it wasn't that way when we were looking at the overall black
white balance.
So that will make the picture more blue and if you move the right most triangle to the
left towards the center it tells the Gimp to make all pixels to the right of that red
resulting in a redder picture.
Now this is all done by mathematics of course since the essence of digital photography
is to include the encode the level of colors as numbers.
Gimp is an 8-bit program so the numbers range from 0 to 255 and you can see that under
the sliders where there are numbers these show the position of the far left and far right
sliders and you can fine tune them using the up and down arrows next to the number.
Do you need to know this to use the tool?
Perhaps not but it might help you to use it more effectively and again I'm going to give you
a video to check this out because sometimes just listening to audio when we're talking about
a visual art is not ideal so I've got to link in the show notes to something that you can
check this out and with this I am concluding my Gimp series at least for now maybe something
will come up that I want to add to it in the future but I have now done what I set out to do
when I started the Gimp series was what I want to do is explain how you could use Gimp to
fix problems with pictures and spend a lot of time talking about Gimp in general but I have now
finally redeemed myself by doing what I said I would do all along.
So with that this is Ahuka for Hacker Public Radio signing off and as always encouraging
you to support free software bye bye
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