hpr-knowledge-base/hpr_transcripts/hpr3560.txt

Episode: 3560
Title: HPR3560: LCh Components Layer Modes
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3560/hpr3560.mp3
Transcribed: 2025-10-25 01:24:08

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This is Hacker Public Radio Episode 3,564 Friday, 25 March 2022.
Tid's show is entitled, LCH Components Layer Modes and is part of the series'
Gimp It is the 240th Show of Auka and is about 13 minutes long and carries a clean flag.
The summary is, more on Layer Modes in Gimp with the LCH Components modes.
This episode of HPR is brought to you by An Honesthost.com.
Get 15% discount on all shared hosting with the offer code HPR15.
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Hello, this is Auka welcoming you to Hacker Public Radio and an
other exciting episode in our ongoing series on Gimp and this is going to be on LCH Components Layer Modes.
Now, LCH is yet another color model. It's the last one we're going to look at,
but in this case it breaks down the components into lightness,
Chroma and Hue. Now, it is based on how people perceive differences in color.
Technically, it is a polar transform of something called the LAB color space,
where L stands for lightness and A and B are opposing color pairs.
A is the pair of red green and B is the pair of blue yellow.
Now, this model is a perceptual model based on how the human eye works to perceive colors.
The idea is that the human brain processes color information from the cones and rods of the eye
in an antagonistic manner. So, the A can be thought of as a spectrum running from red to green
and B similarly running from blue to yellow. While the axes for these are unbounded in theory,
as a practical matter is normal to combine the values to a range of minus 128 to plus 127.
Now, LCH takes this model and turns it into polar coordinates to map onto a cylinder.
This is the last of the color models we will look at in this series, but if you want to investigate
this further, I've put a link in the show notes to a Wikipedia article that discusses color models
that you might find interesting. But the basic idea is that color models can focus on
fairly practical for reproducing color, like RGB and CMYK,
or they can focus more on how humans perceive color, which is HSV and LCH.
Now, what we want to do in this tutorial is to look at how LCH can be used to blend layers
in GIMP 2.10. Specifically, I'm using, and I've mentioned this before, I'm using GIMP 2.10.24
on Kabuntu Linux 20.04 LTS. Now, LCH hue. This layer mode keeps the hue of the top layer while
taking the lightness and chroma of the bottom layer. This makes it very similar to HSV hue,
therefore, but there is a subtle difference because of how the colors are modeled.
So, I made a color layer as I did with the HSV hue, filled it with the same color, 85, 73, 69,
and I made the color layer the top layer, the toy the bottom layer, and did an image using LCH hue.
Well, I compare that with what I get with HSV hue and I can see differences.
They're very similar, but there are certain things where yellow and orange appear, for instance,
you can see differences there. You know, the hands of the wizard, remember, in the original photo,
they were yellow. With LCH hue, they're kind of a light orange, with HSV hue, they're kind of a dark orange.
In fact, everything just looks slightly darker in the HSV hue image. So, you know, they're similar,
but not identical in any sense. Now, the next one is the LCH chroma. Now, this again is similar to
its HSV counterpart, which in this case would be HSV saturation. The LCH chroma takes the chroma
information from the top layer and combines it with the lightness and hue from the bottom layer.
So, again, I repeated what I did in HSV saturation images. I did a dog image combined with a pure green
layer, 0, 0, F, 0, 0. And again, primary colors that were not noticeable in the original,
such as the blue in the dog's nose and under the dog's eyes, have been brought out here,
but you can see a difference. It's a distinctly redder image. The HSV saturation image is a lot darker
than the LCH chroma image. The chroma one, you've got bright reds, you've got more yellow coming
through. So, it's a distinctly different kind of image. Then I did this similar thing with the toy
image and compared that with what I did with HSV. So, I did my LCH chroma and I got a very bright image.
The wizard is a much brighter blue. The hands are back to looking yellow again,
but they're a bright yellow. Everything is just brighter here. And in comparison with the HSV
saturation image, you can see a big difference and particularly more yellow. And you see that in
the background, more than anything, because there were specks of yellow in the background when I
did HSV saturation. And remember, what that comes from is very rarely does an image have pure white
or pure black. And so, what you may perceive that way turns out to be just slightly different.
And so, you're picking up colors that are coming through. Well, with the toy image,
a lot of yellow came through. Okay, LCH color. Now, this is kind of a counterpart to the HSL color
that we did in the HSV section. So, again, that means that you want to take a look at something
like colorizing a black and white image. So, I went back to the photo of the little girl and repeated
the process. Well, it looks very similar to the HSL color vision, but I can easily see that there
is a difference. And the difference is the LCH color is just a little darker than the HSL color.
So, the point is not that LCH is always lighter than HSB, but it's generally going to be different.
Now, LCH lightness. This mode takes the lightness of the top layer and combines it with the
chroma and hue of the bottom layer and is analogous to the HSV value mode. So, I repeated my steps
by creating a linear grayscale gradient with black as my foreground color and white as my background
color and use that as the top layer to get an image of the dog. Well, again, you see it starts off
virtually black on the left side and lightens as it moves to the right, but the detail has almost
entirely disappeared. You're just getting splotches of color with this. Doing it with the toy image,
again, a very similar kind of effect. So, just a lot less detail using the LCH lightness.
And I did it with a pure white layer. I remember that got an interesting kind of washed out effect.
In the HSV value. Here, the image almost disappears. It's so light, you can see
bits of it. So, luminance. Now, luminance is similar to lightness, but the difference is
how the lightness is perceived. I got a link in the show notes to Wikipedia article. I'm going to
quote just a little bit from it. Luminance is often used to characterize emission or reflection
from flat, diffuse surfaces. Luminance levels indicate how much luminous power could be
detected by the human eye, looking at a particular surface from a particular angle of view.
Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle
of interest is the solid angle subtended by the eye's pupil. Now, when you use it as a layer mode,
the effect is the opposite of the LCH color mode. And only slightly different from the LCH
lightness mode. The difference being that the luminance is how the eye perceives it,
whereas lightness is how it's measured by instruments.
Now, if you recall from our previous discussion of the HSL color mode, that takes the
hue and saturation from the top layer and mixes it with the lightness of the bottom layer.
And in LCH color, it takes the chroma and hue of the top layer and mixes it with the lightness
of the bottom layer. And then we use some in colorizing black and white photos because the only
information in the black and white photo is how light or dark each pixel is.
So, these modes take the information of the black and white photo and combine it with color
information we put in from the top layer. In the luminance mode, as in the lightness mode,
this is reversed. So, if we put the black and white photo on top and use the lightness mode,
would we get the same result? Remember, the LCH color used lightness, not luminance. So,
I'm going to do it first with lightness just to illustrate the point. And the point is,
they are exactly the same. And I proved they're exactly the same,
because I took both of these images and combined them using difference mode and got a pure black
image. What that indicates is that they're completely identical. Now, when I do it with luminance,
it's slightly different. And I could sort of see the difference when I just switch back and forth
in GIMP and saw that the luminance image was just slightly darker. But again, proof is a good
thing. So, I used the difference mode between the two images. And what I got was a very interesting
image. Both of the starting images were green, but the difference between them, it's starting to
look like a very dark purple. So, it's extremely dark image with bits of somewhat lighter stuff
coming through. You can kind of make out the original image very fuzzly.
So, this concludes our look at all of the layer modes in GIMP. We've looked at a lot of information
regarding layers, because they're central to how GIMP operates. We could perhaps spend more time
digging into all of the other tools, but I don't want to do that right now. My primary objective when
I started was to work on photos. So, I think I will move in that direction and explore other tools
when they come up. So, this is a hookah for hacker public radio thanking you, and as always
encouraging you to support free software. Bye-bye.
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