hpr-knowledge-base/hpr_transcripts/hpr3440.txt

Episode: 3440
Title: HPR3440: Lighten Layer Modes
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3440/hpr3440.mp3
Transcribed: 2025-10-24 23:28:41

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This is Hacker Public Radio Episode 3444 Friday, the 8th of October 2021.
Tid's show is entitled, Light and Layer Modes, and is part of the series'
Gimp It Is Hosted by Aoka, and is about 19 minutes long, and carries a clean flag.
The summary is, we continue our look at the Layer Modes, and gimp with the Light and Modes.
This episode of HPR is brought to you by an honest host.com.
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Hello, this is Aoka, welcoming you to Hacker Public Radio and another exciting episode
in our ongoing Gimp series, and we are continuing our look at the different Layer Modes,
and today I want to look at the Layer Modes for Lightning.
Now, remember that the Layer Modes are ways of combining two layers so that they interact
with each other in some way.
So, the first Layer Modes I'm going to look at is something called Light and Only,
and Light and Only looks at each component of each pixel in the top and bottom layers,
and always selects the largest value for the combination.
That means it will always, at the very least, lighten since larger numbers are lighter than
smaller numbers. Now, in RGB space, pure black is 0000 in hexadecimal, and pure white is FFFFFFF.
Each component is a two-digit hexadecimal number, where the first two digits are red,
the next two are green, and the last two are blue.
So, as an example, if you had a pure red layer that would have the combination FFF0000,
if you had a pure blue layer that would have the formula 0000FFF.
Now, if you combine to them using Light and Only, the result would be an image where each pixel
had the number FFF00FF, and that, as it happens, is magenta, the combination of red and blue.
Now, you can do the experiment yourself in GAMP by simply creating some layers,
using the Light and Only mode to see what happens. I've done that, and you can see how that works
on my web page, which link is in the show notes for all of that, and that's just a good time to
mention that given that we're talking about a visual program in an audio medium, which is kind
of interesting. So, you know, a lot of the examples are the images, if you want to see the images
that I create in doing all of this, you want to go to the web page and take a look at that.
Now, again, if you, suppose you took a copy of the magenta image,
and then added a pure green layer 00FF00 using Light and Only mode, you would then get a pure
white image, because now every pixel would be FFFFFFFF. Now, the point of all of this is to focus
on the components. Now, I have seen people say, in shorthand, casually, well, Light and Only mode
selects the lightest pixel from each layer. Yeah, not entirely correct, you know. The fact is,
you cannot talk meaningfully about pixel lightness separate from the lightness of each component,
the red degree in the blue. Now, in a lot of cases, it probably doesn't make a difference,
but there are cases where it does. And that's why I always want to emphasize the correct way of
thinking about it is to think component by component. So, if you wanted to see what difference that
makes, you would have to do some analysis. And in comparing some of these modes, I sometimes do
that analysis. Now, the interesting thing is what happens when you're using images with some
content in them. Now, I picked kind of randomly two images that I've been using as examples here,
because they're very different. And I think that that sometimes is useful. And the very different
ones are an image of the face of a dog and an image of a toy with a wizard and a crystal ball.
So, with those two images, I did Light and Only. And what happens is, I got a bit of the dog
coming through because the side of the dog's head is in sunlight and is lighter than the shadow
side of the toy. And because the crystal ball is so dark, you see a bit of the dog come through
there as well, even though that part of the dog's head is in shadow in the image of the dog.
Now, it's worth noting that for Light and Only, the mode is commutative. And what that means for
those of you who haven't studied mathematics in a long time is that the order does not matter.
Now, for some layer modes, it is a very clear distinction between the top layer and the bottom
layer when you're doing this. But there are others where it doesn't. And those are called commutative
layer modes. And there's a number of them. Now, the next thing I want to talk about is Luma slash
luminance light and only. And this gets a little bit technical. I've got links in the show notes
again to a lot of this stuff. Luma and luminance are measures of brightness. The difference appearing
to be whether one measures it in black and white or in color. Now, Wikipedia is frequently my go-to
source for good basic explanations of these kinds of things. Which is good because in fact,
this is not documented at all in the GIMP documentation. Now, in video, in video, in video,
Luma represents the brightness in an image, the black and white or acromatic portion of the image.
Luma is typically paired with chrominance. Luma represents the acromatic image
while the chroma components represent the color information. Converting our prime G prime
B prime sources such as the output of a 3ccd camera into Luma and Chroma allows for Chroma
subsampling because human vision has finer spatial sensitivity to luminance, black and white
differences, and chromatic differences. Video systems can store and transmit chromatic
information at lower information, optimizing perceived detail at a particular bandwidth.
So, clearly what they're talking about is how can you compress a signal for broadcast?
And a lot of this was initially developed for that reason. Now, luminance is a weighted
average of the RGB values. So, again, going to Wikipedia, while Luma is more often encountered,
relative luminance is sometimes used in video engineering when referring to the brightness of a
monitor. The formula used to calculate relative luminance uses coefficients based on the CIE
color matching functions and the relevant standard chromaticities of red, green, and blue.
For example, the original NTSC primaries and for the REC 709 and SRGB primaries, the linear
combination based on pure colorometric considerations and the definition of relative luminance is
and these use different colors. Now, Y equals 0.2126, R is at 0.7152 and green is 0.0722.
So, there's a formula for all of this. The formula used to calculate Luma in the REC 709 spec
arbitrarily also uses the same coefficients, but with gamma compressed components.
And that's the R prime G prime B prime where the prime symbol denotes gamma compression.
Okay, now, what the heck is gamma compression? I found a wonderful explanation on a blog post.
Again, link on the show notes, what every coder should know about gamma that I thought made
it pretty clear. The idea is that brightness is not perceived in a linear fashion, but instead
follows a power relationship. And so, the blogger who wrote this says the 24-bit RGB color
model used on virtually all consumer level electronic devices uses 8-bit gamma-encoded values
per channel to represent light intensities. If you recall what we discussed earlier, that means
that pixels with RGB, such as 128, 128, 128, will not emit approximately 50% of the light
energy of pixels with an RGB value of 255, 255, 255, but only about 22%.
Now, that makes perfect sense. Because of the non-linear nature of human vision,
a light source needs to be attenuated to about 22% of its original light intensity to appear
half as bright to humans. RGB 128, 128, 128 appears to be half as bright as RGB 255, 255, 255 to us.
Now, if you find this confusing, reflect a bit on it because it's crucial to have a solid
understanding of what has been discussed so far. Trust me, it will only get more confusing.
Of course, gamma encoding is always done with the assumption that the images ultimately meant
to be viewed by humans on computer screens. In some way, you can think of it as a lossy MP3-like
compression, but for images. For other purposes, like scientific analysis or images meant for
further post-processing, using floats and sticking with linear scale is often a much better choice.
So, this is all from John Novak, Link in the Show Notes.
So, what this boils down to is that most computer monitors utilize gamma compressed,
or sometimes called gamma corrected values, for displaying RGB colors. And that's a good thing
for most purposes. Now, this makes for a small difference when using Luma Luminance Lighting Only mode.
Since this mode will work with the perceived value of the color values rather than the absolute
value of them, the main difference is that Luminance is taking into account the brightness of the color.
So, I did a combination again of the the dog and the toy, this time using Luma Luminance.
And if you're just looking at that compared to the light and only, it's very hard to see a
difference. There is a way to get an objective measure of what the difference is, and there's another
layer mode that does that, it's called the difference layer mode. I can take any two images,
put them on layers and use the difference layer mode to see how different they are.
And when I take the two images and use that difference layer mode, and I've put this on the
web page, you can see the result. It does pick up differences between them.
Now, assuming the black is identical in the two images, and both had white backgrounds,
it makes sense that we're seeing a lot of black in the difference image. But where we see color
other than black, there is a difference. So, anything black is identical in the two images,
anything that's not black represents a difference.
Now, screen mode. This actually has a documented formula. Yay! It's a little bit complicated.
Again, we're working with the RGB values, which range from 0 to 255. 0 is black, 255 is full color.
And again, you do this with the R value, the G value, and the B value separately.
That's happening in the background. You may not be aware of it, but you have to do it that way.
So, the formula says the resulting image, if you use screen mode, you start with a value of 255,
and then subtract from that a complicated expression where you first take 255 and subtract the top layer,
whatever that value is, and multiply that by 255 minus the bottom layer, whatever that is.
So, they're multiplied together and then divided by 255, and then that whole thing is subtracted from 255.
You can see the formula on my web page or in the game documentation.
So, what does that mean? Let's take a look at a calculation here. Let's say one image had a red
value of 104. It gets inverted as 255 minus 104, which is 151. Then do the same thing for the
red value in the other image. So, let's say the other image had a red value of 87.
Inverting that by subtracting from 255 gives us 168. Multiplying them together, we get 151 times 168,
and that is 25368. Then we divide by 255, giving us 99. Then we re-invert that because we subtract it
from 255 and get 146. So, we started with red values of 104 and 87 and wind up with 146.
So, the red level is distinctly higher now than either of the starting values.
So, in essence, this mode will make dark colors fade more than light colors and can be useful
for removing dark backgrounds. Again, this is another example of a commutative layer mode.
The order doesn't matter. Whether it's top or bottom, the calculation is the same no matter which
order you do them in. In general, this is going to give a more washed out appearance to the resulting
image. Again, I've got examples on my web page. Now, I tried using this mode using the image
from the Luma Luminance layer mode and the dark image I got from using the Difference mode.
Now, as you can see, the black background, if you take a look at the image on my website, is completely gone.
Another interesting use of this mode can be seen in a tutorial I ran across from a fellow named
Chris Parker, who posts a lot of tutorials on YouTube, and he uses it to create a soft glow effect
for photography, such as wedding photos. He duplicates the image, then adds Gaussian blur to the duplicate,
then he uses the screen mode to combine the two, and fine-tunes the result using opacity.
If you want to see his tutorial, I put a link in the show notes for that.
Now, the last of the lighten modes for this tutorial is addition, which is pretty straight forward.
You simply add the values from the two images, but the maximum is 255.
So, you can't go above that. So, if we had the values for red, as in the screen example above,
adding them would give us a value of 191. Since this is less than 255, we can keep this value.
But, suppose the values were 104 and 187 in the two input pictures.
Then the total would be 291, and we round it back down to 255.
Now, this is somewhat similar to the screen mode in effect, but there are differences if you look
closely at the two images, such as in the base of the globe, the crystal ball, the wizard's hat,
the wizard's beard. There are some small differences there.
Now, there is in fact one more lighten mode, but I'm going to treat that in a separate tutorial.
It's paired with a darken mode, and also has a couple of tools doing similar things.
So, our next gimp tutorial is going to be on Dodge and Burn, where Dodge is the lighten,
and Burn is the darken. Interesting stuff there, too, I think. So, this is Ahuka for Hacker Public
Radio, signing off, and is always encouraging you to support FreeSoftware. Bye-bye.
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