hpr-knowledge-base/hpr_transcripts/hpr3170.txt

Episode: 3170
Title: HPR3170: GIMP: Color Tools
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3170/hpr3170.mp3
Transcribed: 2025-10-24 18:10:18

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This is Hacker Public Radio Episode 3174 Friday, 25 September 2020. Today's show is entitled,
Gimp, Color Tools,
and is part of the series Gimp. It is the 190th anniversary show of Ahuka,
and is about 20 minutes long, and carries a clean flag. The summary is
with color tools you can adjust the colors and brightness of an image.
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Hello, this is Ahuka, welcoming you to Hacker Public Radio in another exciting episode,
and I am continuing my series on Gimp,
and what I want to do today is I want to take a look at color tools.
Now, obviously, color is very important in a graphics program,
and tools to add, modify, or duplicate colors are thus very important.
Gimp has ten of these tools, and nine of them are found in the colors menu,
and also in the tools menu.
It's the same nine tools in either place,
but going to the colors menu is one less mouse click.
Interestingly, none of these nine tools is on the tools sidebar in the toolbox.
Now, if you want to have any of them there for quick use,
you could go to Edit, Preferences, Toolbox,
and click on the one you want to add,
and when you do that, you should then see an eyeball icon next to it,
which means it's visible.
Generally, that eyeball icon in Gimp, anywhere you see it,
is you see the eyeball, whatever that is, is visible.
So it could be a layer, it could be a path, whatever.
Now, you can also remove tools as well by clicking on one,
and then the eyeball disappears, and then it's not going to be visible there in the toolbox.
So, I said nine of the ten are on the colors menu, but not the toolbox.
Now, there is one color tool that is in the toolbox,
but not on the colors menu, and that is the color picker.
Why this is the case I could not say, as it seems to me,
the color picker is a color tool, but no one asked my opinion on the matter.
In any case, the ten color tools, as I will call them,
are the color picker, which has a shortcut key of O,
then there's color balance, hue saturation, colorize, brightness dash contrast,
threshold, levels, curves, posterize, and desaturate.
Now, the fact that the nine tools not in the toolbox
also do not have any keyboard shortcuts,
should tell you that the Gimp project designers do not think you will use these tools all that often,
and they may be right about that.
The number one use case for Gimp and for Photoshop come to that is to work on photos.
That's why it got called Photoshop in the first place.
And these tools may not be used much for photo work,
but let's review them anyway so that we have some sense of what they can do.
There will be times you will want to use them.
Now, just a side note before we get into the details about Raster Graphics,
and we've raised this point before, we'll raise it again.
I mean, you really want to understand that different graphics programs work in different ways.
Raster Graphics, which is what Gimp is designed for,
is based on individual pixels.
And so, everything you're doing is operating at a pixel level.
Pixel stands for Picture Element. It's a dot.
Now, vector graphics, by contrast, are described by equations.
And so, vector graphics can be scaled up or down without any change in any loss of information or what have you.
The look equally good at any scale.
In fact, we refer to them frequently as scalable vector graphics,
which is an SVG file extension.
Now, pixels are perfectly described as well by a series of numbers.
And those numbers are going to describe things like the location of the pixel,
the different color values, and so on.
And each pixel in an image is perfectly described by these numbers.
A program like Gimp uses clever algorithms to manipulate those numbers.
And if you think about it, that's about all a computer program can do when you come right down to it.
So, color tools, let's start with Color Picker.
And that has the keyboard shortcut of O.
This tool that you click on a pixel anywhere in an image and copy the color numbers.
Gimp operates in RGB space right now.
CMYK, which is the alternative, will arrive someday.
There is a plugin that partially fixes this, called Separate Plus.
But if you want to do top quality color print production,
honestly, Gimp is not the best tool right now.
It is really designed for graphics that will show up on websites, computer screens generally.
Now, Gimp is RGB, that means red, green, blue.
CMYK, cyan, magenta, yellow, and black.
And if you've ever used like an inkjet printer, a color inkjet printer,
you know those are the four colored inks that you have to have to create images.
So, the color picker picks up the RGB values on a pixel when you click on it.
And if you select the Use Info window at the bottom,
remember for every tool, there is a tool option at the bottom
for all the ones that are in the toolbox at any rate.
And so, take a look at the options for the tool.
And on the color picker, there's one that says Use Info window,
and it's going to give you the numbers.
So, you can see exactly what they are.
And give it to you in a variety of ways.
It will give you the red, green, and blue as raw numbers.
And then how that translates into percentages.
So, how do we interpret that?
Well, remember, every color is described as an 8-bit number.
And that means there are values from 0 to 255.
So, let's say red, just hypothetically, red had a value of 81.
Well, 81, where does 81 fall between 0 and 255?
It falls 32% of the way along.
So, you can take a look at this window.
You would see, in this particular case, you would see red 81,
and then next to it, red 32%.
So, two ways of looking at the same thing.
That also give you the hex code, which is an alternative way of describing colors.
And the hex code is going to be basically a combination of six numbers and or letters.
The hexadecimal runs from 0 to F.
So, you're counting 16, you're trying to a base 16.
And since our numbers are base 10, they only go from 0 to 9.
They add A, B, C, D, E, F that gets you from 0 to 15.
So, anyway, that's enough about that.
Now, with the color picker, the information window will tell you what the color is.
But the color picker will then let you add it to the foreground color or the background color.
And we've talked about those concepts before.
You can add it to the palette.
And the palette is when you're working with colors are the kind of the presets that you can just click on and use that color without having to dial up all of the values.
So, for sampling, you can use more than one pixel by selecting average or emerged.
Now, I think of these as horizontal versus vertical.
If you select average, you can select all of the pixels around the one you click on and then the radius setting determines how many it picks.
So, you could say select all the pixels up to two pixels away from the one that I clicked on and average that out.
So, that's the horizontal approach.
Merged selects the same pixel in all layers of a multi-layer image.
Now, I always think of layers as a vertical stack.
So, to me, this option drills vertically through the stack and picks it that way.
Now, the color picker is a tool you use frequently.
Hence, it is in the toolbox and has a keyboard shortcut.
The next tool, color balance, lets you adjust the relative values of the RGB numbers to correct an imbalance.
For example, I scanned in some old slides.
These slides are about 50 years old and haven't not necessarily been treated perfectly well that whole time.
And one of the things I found when I got a slide scanner recently, and my slide scanner is a Xerox Scanza, works pretty well, but the slides all came out very green.
Now, with this color balance, I might be able to moderate that.
Now, when you're working with many of these tools, color balance among them, make sure that you have the preview box checked.
And that way, you can see your photo changing as you move the sliders around, and that's going to make it a lot easier to work with.
Now, the next tool is Hue Saturation.
Now, saturation is essentially the intensity of color in general.
If you reduce the saturation to 0 in Gimp, you'll end up with a grayscale image.
So there's no intensity of color.
Increasing the saturation can improve some pictures, but if you go too far, it will end up being quite garish.
Now, Hue refers to the dominant color.
And again, this could be helpful with my overly green slides.
But use this with layers and a selected object, and you can change the color of that object.
Now, think of this like those online shopping tools, where you can see the same shirt in eight different colors.
So basically, someone could do a photograph of someone wearing a shirt, or just a photograph of the shirt itself, and then do a selection with any of the selection tools, and then use with that selection, change the hue to change the color.
Now, if you want to see more on these concepts, I've put a link in the show notes to a page at Lee Contnor.
And I think that goes into a little more detail.
Next tool is Colorize.
This basically just lets you see the images if there was a colored filter in front of it.
So this could be useful.
As an example, if you wanted to take a photo and give it a little bit of that old-time seepia tone look, by putting it kind of a brownish color in front of the image.
Next tool, Brightness Contrast.
Now, I remember back in the days of Black and White Television, and you kids out there ask your grandparents about it.
And in those days we had a brightness knob, and I heard a comedian tell a joke about this saying how dumb most TV is.
He said, I tried increasing the brightness, but it didn't work.
So, what these settings do is increase the overall amount of light, which is brightness, and how much contrast there is between light and dark areas.
Now, since this is working on the entire image, it is a bit crude.
But can be helpful in seeing what you have to work with if the image is over or under exposed.
To do a better job, you would want to work with the Levels tool or the Curves tool.
Threshold.
Now, this turns the image into a purely black and white image, and I do mean pure black or pure white. There are no shades of gray.
Every pixel is either pure black or pure white.
Now, what the threshold setting does is it takes a look at the image and decides, okay, where do I flip from white to black?
What's the threshold that determines that? That's why it's called threshold.
So, as you move the slider towards the left, towards the setting of zero, more and more pixels turn pure white.
And as you move it to the right, towards 255, more and more pixels turn pure black.
An interesting effect, but by itself, probably of limited usefulness.
As with all tools, their full usefulness starts to be revealed when we get to layers.
And we are going to get there. I'm just trying to build things slowly here.
Next tool, Levels.
Now, this tool lets you adjust hue and contrast by channels.
And when we say channels, we mean red, green, and blue. Those are the three channels.
There's actually a fourth channel that pops up in some places, and that's the alpha channel, and that's for transparency.
Which, again, for computer screens and web graphics and things like that, it's very useful to have a transparency channel.
But anyway, this is going to operate on red, green, blue.
And you can select which channel you're operating on. Now, if you just leave it at value, it's affecting all colors equally.
It's like selecting all of them.
But you can pick a specific color and increase or decrease the color without affecting the other channels.
So this is a bit easier to work with than the hue saturation tool in that respect.
Now, note that if you increase one color, you decrease the complementary color and vice versa.
For example, adding more red would also mean reducing the amount of its complementary color cyan.
Now, complementary colors and all of that is a whole topic in itself.
There's a link in the show notes if you want to investigate that more.
But, you know, a lot of this has to do with additive versus subtractive color and things like this.
And, you know, we could do a whole series on color theory, but I'm not the one to do it.
Curves tool. This is the most useful tool, or one of the most, for adjusting the color and brightness of an image,
because you can get smooth transitions in the changes you make.
Now, when you select this tool, a window pops up with a 45-degree line by selecting a graph.
At the 45-degree line, that means everything is linear. It's a perfect straight line transition all the way along.
Now, you make changes by clicking and dragging the line to create a smooth curve.
Now, what you're doing is you're adjusting the color and brightness.
When you make it a curve, instead of a linear change in color and brightness, you start following the curve.
And, this makes your changes less abrupt, since nearby values are dragged along in the curve.
As before, you have various channels, value will affect the entire image, red, green, and blue affect those colors, and alpha affects the transparency.
Now, transparency, particularly, is going to be helpful when we get to layers.
Posterize. This tool reduces the number of colors in your image by making closely similar pixels have the identical color.
The fewer the colors, the more blotchy the image. Whereas, at the highest setting, it's pretty close to the original image.
The posterize level is basically the exponent in determining the number of colors.
Since there are three channels, a setting of three will give you eight colors. Two to the third power is eight.
The maximum is 256, which is giving each color the full range of values.
Remember, the values run from zero to 255. 256 is two to the eighth power, so it's an eight-bit number.
Disaturate. This turns the image into a grayscale or black and white image.
However, the original RGB values are all preserved, which can be handy for later editing.
So, this would be preferable in your work process to using the hue tool to turn everything down.
So, this completes our look at the various color tools that GIMP offers, and there are quite a few of them.
So, as with all of the other tools, a little practice will help a lot in getting comfortable with these color tools.
I recommend opening a photograph in GIMP and giving each of them a try. That will make things a lot clearer.
We still have a few more topics to address in getting the basics going.
Next up will be miscellaneous tools, then brushes, and then the really big one layers.
So, there are still lots to go before we get into actual photo work.
So, please come along on the journey with me.
So, this is Ahuka for Hacker Public Radio.
Signing off and reminding you is always to support FreeSoftware. Bye-bye!
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