hpr-knowledge-base/hpr_transcripts/hpr3540.txt

Episode: 3540
Title: HPR3540: HSV Components Layer Modes
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3540/hpr3540.mp3
Transcribed: 2025-10-25 01:09:42

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This is Hacker Public Radio Episode 3544 Friday, the 25th of February 2022.
Today's show is entitled, HSV components layer modes and is part of the series
Gimp It is hosted by Aoka and is about 20 minutes long and carries a clean flag.
The summary is more on layer modes in Gimp with the HSV components modes.
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Hello, this is Aoka, welcoming you to Hacker Public Radio and another exciting episode
in our ongoing Gimp series and what I want to do today is talk about HSV components layer modes.
We've got this and then one more section and then we complete layer modes and move on to
discussion of something else. So these two new modes have not yet been documented unfortunately
but I have been able to get a lot of information about them from a variety of sources.
One of them is the Linux Topia Gimp Guide link in the show notes and Mike Davies of course with his
review of the layer modes on his YouTube channel. Again a link in the show notes for all of this.
HSV, what does that mean? Well HSV stands for Hue, Saturation and Value and this opens up I think the
whole topic of color models which we've kind of talked about but let's be a little more systematic
about it. The one we've used mostly so far is what's called the RGB model and that's the model
that's used for web graphics so it's probably the one you're most familiar with encountering
and it is the default model in Gimp. It is also the model used in televisions.
Now you can see this with a good magnifying glass or a neat trick is you can use your cell phone
camera and just keep zooming in. You don't have to take a picture just keep zooming in and you
will eventually magnify it enough that you will see the individual red, green and blue
component dots in the picture. You have to zoom in a lot with modern TVs that tend to be high
deaf. Back in the old days when I had a cathode ray color TV it was a lot easier. You didn't even need
to zoom in that much. You just needed to stand close to it to see it. But the RGB model is the
default model for broadcast television. Now this is what we call an additive model because the three
primary colors are added together to produce all the other colors. So if you max out your three
primary colors, full red, full green, full blue you get pure white or if you set them all to zero
you get pure black. Now if you just max out two of the primary colors you'll get one of the
secondary colors. So red plus blue gives you magenta, red plus green gives you yellow and green
plus blue gives you cyan. And that brings us to the CMYK color model. Now this is what we call a
subtractive color model and is used in printing. For example, if you wanted to prepare a piece to
be printed at a commercial printer you would need to prepare it using the CMYK color model.
My wife spent many years working in the commercial printing industry so I've had some insight into
how a lot of that works. And of course anyone using an HUT printer knows those are the colors of
ink you need if you want to print. Now this is called subtractive because what you're doing
really is you're starting with a white piece of paper and the fact that it's white means it
reflects all colors of light equally. What the inks do is actually subtract certain colors from the
reflection by absorbing them. So the cyan ink absorbs red but it will reflect green and blue.
The magenta ink absorbs green light but reflects red and blue. And the yellow ink absorbs blue
but reflects red and green. Now theoretically combining all three inks should give you a very
nice black but as now there's two problems with this the the inks don't absorb perfectly so
they will still let some color come through although it may not be enough for you to notice but
for commercial printers that's an important thing. And for many purposes such as printing text
you just want to be black anyway why use three inks to get that'd be very wasteful. So having the
the black ink by itself is a useful part of the process. Okay so we've looked at the two models
that people may be familiar with to some degree already so what's this HSV color model.
Now this describes color in terms of three values. The first hue describes color using both RGB
and CMY colors and is often pictured using a cone or a cylinder. The colors fill a 360 degree
circle with red starting at zero degrees green starting at 120 degrees and blue starting at 240
degrees and of course midway between the primary colors you get the three secondary colors.
So hue is the color part of that. Saturation is the amount of gray in the color and the lower the
saturation the more gray. While higher saturation makes for more vibrant colors one way to turn a
color image to a black and white image for instance is to reduce the saturation to zero
though that may not be the best way to do it. Finally value is a measure of the brightness
or how much light is emitted from an area. The idea of the HSV model as the creators put it was to
more closely model how humans perceive colors. And it's one of the things that's kind of important
looking at this because things like RGB are you know very technical they're how things are
measured by instruments and things like that. How people perceive things when they're looking at them
can be different. So if you want to see some of this stuff if you go into GIMP and one of the things
you're doing all the time is you're picking your foreground and background colors and that's just
something you'll do probably 40 or 50 times in a session just to do the stuff you want to do when
you're working with images. Well when you click the foreground a little window pops up that says
change foreground color and you've probably done that a bunch of times and you could type in an
HTML notation or use selectors of various kinds but if you take a look in this window in the upper
right you'll see LCH and HSV which are these two alternate color models we're talking about HSV
this time we'll talk about LCH next time. So if you click the HSV you are going to see
those values so you know you do that you click the HSV first you'll see the red green and blue
which you're used to and you probably never paid attention to the others but right under it
you'll see HSV and you can manipulate those and you can do it in a variety of ways you have
you can just type directly in there to type in a numeric value between 0 and 100
you could use up and down arrow buttons to change the numbers or you can use a slider
so if you take a look at the left side of this window you see two things you see a rectangle a
large rectangle with what looks like cross hairs in it and then next to it you see a very thin
vertical rectangle with what looks like a color spectrum so you can set the hue by looking at
that color spectrum and you'll see there's a little horizontal bar on it that you can move up and
down and choose where you want to be in there. Similarly with saturation you can do that again by
typing in a number or hitting an up and down arrow button or use the take a look at the large
rectangle and use the vertical bar of the cross hairs and move that around to set saturation
and value you'd look at the horizontal bar of the cross hairs and move that up or down
so if you want to learn more about the general topic of color models which I've introduced here
there's I put a link in the show notes to a Wikipedia article that you might find interesting
now let's get to these layer modes HSV hue is the first one and this takes the hue value from
the top layer and combines it with the saturation and value from the bottom layer now to illustrate
how you might do this I created a layer and filled it with a brown color and the brown color I
picked had the HTML color value of 85 73 69 and I made that the top layer and in the bottom
I used my toy image now you you always set layer modes on the top layer so I make sure that the toy
image is just normal and then the color layer above that I set to HSV hue well it it had a brown
color so you'd expect it's going to make the image a little browner and it does that so what was
originally mostly a blue image is now mostly brown what was yellow in the original has migrated
to orange but in most other respects the image is pretty much the same it just the you know the color
tones have just shifted a little now there has to be a color difference to see any effect
so I tried with the dog image again using that brown layer and I couldn't see any difference at all
because the dog image was really all brown to begin with so I said well let's go change that
so the color layer I shifted it from the brown to a green and now I've got a very greenish looking
dog when I do that now the next one is HSV saturation so HSV saturation takes the saturation
of the top layer and combines that with the hue and value of the bottom layer so what it does
is change the intensity of the colors and the combined image now I went back to my green color
layer and pushed the saturation up to 100 then I combined it with the dog image using HSV saturation
mode it did a very interesting image some of the colors that were in the original image are kind
of there but they're more intense some colors that were not really noticeable in the original image
are now very noticeable and two things that stood out in the in the picture of the dog
under the eyes looked very black and the nose of the dog the muzzle looked very black
but black is rarely a pure color more often it's a very dark version of other colors
so what we see here by pushing up the saturation is that what was black in this particular case is
now looking very blue interesting effect so clearly there were shades of a very muted dark blue
in those blacks originally and now with the saturation maxed out they they really pop out
similarly some of the brown in the original image was kind of a reddish brown and it's a lot
more reddish now so it really brought that out now the color layer I used was green you don't really
see much of that here because there wasn't much green in the original image a little bit I can see
it's not from the dog itself but from the background of the picture I did the same thing with the
toy image again with the green layer using HSE saturation and the wizard appeared gray in the
original yeah I mean no the wizard was was blueish but it's now very dark blue the shadow was
gray in the original but it obviously must have had a bit more red in it because now it's very red
you know the gray pedestal holding the crystal ball is now a very green pedestal
and the background you can see colors little splotches of color in the background and then the
background was white but it was not a pure white it had some color in it obviously and the
maximized saturation brought that all out now next one is HSL color now this is a slightly
different color model called the HSL model which has the same hue and saturation but instead of
value uses lightness the lightness is an attempt to match the way paint smics in the real world
with more white giving lighter colors and more black giving darker colors you've ever watched a
painter and I don't mean a house painter I mean a fine art painter you know someone with a palette
living in an artist's garret kind of thing watch the way they mix paints when they're
working on a painting and they'll put in a little bit of a red for instance and then mix in
some white and the red will start going more and more towards pink as they mix the white in so
lightness is an attempt to model that sort of thing more white makes it lighter more black mix
it darker so in this mode the hue and saturation of the top layer mixes with the lightness of the
bottom layer now to colorize a black and white photo that's something you could do with this very
nicely because the black and white basically it's all lightness anyway it's just you know how intense
is the is the white and you know how much black is there so that's all your measuring in the bottom
layer anyway so let's try colorizing the photo so I wanted to get a nice black and white image
I went to a site called snappy goat link on this to show notes which is 13 million public domain
or CC zero images and I grabbed a black and white photo of a little girl
then I created a color layer with green again because I like green now I combine them and I get a
green image of a girl now if I was seriously trying to color this image what I might do is keep
the green for the vegetation in the background because that actually does not look all that unnatural
I might cut down the opacity a little bit some things like that and then I might want to use a
layer mask to get rid of the green on the girl herself you know separately color her hair which is
probably blondish and then her skin would be flesh tone etc but this isn't a tutorial on how to
colorize black and white photos now the next one I want to cover is called HSV value as you
probably guessed the HSV value mode keeps the value of the top layer and combines that with the
hue and saturation of the bottom layer recalled that value is a measure of the brightness as
expressed by the shades of gray so a good way to illustrate this is by using a grayscale gradient
as the top layer so I created a linear gradient using black as the foreground color and white as
the background color and combined it with the dog image and I got one that starts off almost black
on the edge lightens up as you move towards the right and the dog starts to come through but it's
you know through a through a glass darkly did the same thing with the toy image
and again you know it starts off virtually black on the left hand side and lightens up as you move
to the right but in both cases it's it's pretty muted now interesting would be to take this
particular layer mode the HSV value mode and do it with a pure white color layer and what that
does is gives a very interesting washed out effect to the combination so I did it with the the toy
image and it's it's like the color is is very muted some parts of the image almost seem to
disappear if they're so light now what would happen if you did it with a pure black layer
well if you think about it the result and you can try the experiment as you get a pure black image
no light can escape from a black hole so with that this is a hookah for Hacker Public Radio
signing off and as always encouraging you to support free software bye bye
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