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gimp and astrophotography

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[Q] Primary colours Daniel Carrera 30 Mar 09:01
  [Q] Primary colours Bob Lockie 30 Mar 09:09
  gimp and astrophotography Gene Imes 30 Mar 16:21
[Q] Primary colours Fay 30 Mar 10:18
001101c2f68c$b8053ac0$fef83... 07 Oct 20:15
  [Q] Primary colours Daniel Carrera 30 Mar 10:03
   [Q] Primary colours Scott Lamb 30 Mar 10:30
    [Q] Primary colours Daniel Carrera 30 Mar 10:56
Daniel Carrera
2003-03-30 09:01:59 UTC (over 17 years ago)

[Q] Primary colours

Hello,
This is more of a curiosity question. When I was in elementary school I was taught that the primary colours were yellow, blue and red. I used those to create "secondary colours": green, orange, purple.

Today I know that the "primary colours" are red, green and blue since the human retina has 3 kinds of cones which detect precisely those colours. I also know that, when using paints, you use cyan, magneta and yellow because paint colours are subtractive and these are the complements of the primary colours:

cyan = white - red magneta = white - green
yellow = white - blue

I would like to reconcile this, with what I was taught in school. My grade-school teaching got yellow right, and I can see that "cyan" could be taught as "blue" to a kid. But magneta doesn't look like red.

Did my grade-school teacher lie to me?

Thanks,

Bob Lockie
2003-03-30 09:09:56 UTC (over 17 years ago)

[Q] Primary colours

On 03/30/03 02:01 Daniel Carrera spoke thusly

Hello,
This is more of a curiosity question. When I was in elementary school I was taught that the primary colours were yellow, blue and red. I used those to create "secondary colours": green, orange, purple.

Today I know that the "primary colours" are red, green and blue since the human retina has 3 kinds of cones which detect precisely those colours. I also know that, when using paints, you use cyan, magneta and yellow because paint colours are subtractive and these are the complements of the primary colours:

cyan = white - red magneta = white - green
yellow = white - blue

I would like to reconcile this, with what I was taught in school. My grade-school teaching got yellow right, and I can see that "cyan" could be taught as "blue" to a kid. But magneta doesn't look like red.

Did my grade-school teacher lie to me?

Magenta looks purple.
"red" is easier to pronounce. :-)

Thanks,

Daniel Carrera
2003-03-30 10:03:20 UTC (over 17 years ago)

[Q] Primary colours

On Sun, Mar 30, 2003 at 05:19:35PM +1000, Fay wrote:

Hi Daniel,
I don't think that either set of colours has to do with how our eyes work.

Actually, it does.
Recall:
- Colours are frequencies of light.
- White is the sum of all frequencies, and black is no frequency. - The human retina has cones for red green and blue. - The table:
cyan = white - red
magneta = white - green
yellow = white - blue

With this we can explain all of the 'weird' things you mentioned:

If you mix red and green light you get yellow.

If you mix red and green light your retina perceives all colours except blue, which is equal to yellow (see above).

If you mix red and green paint you get brown.

True green paint absorbs red and blue and reflects green. The "green" paint you are using probably reflects lots of red light and absorbs part of the green.

For instance, if you have paint that absorbs: - All of the blue light.
- 50% of the red light.
- 25% of the green light.
The paint will look perfectly green to you. Try it in GIMP, this colour corresponds to #80c000

Suppose that you mix this "green" with true red paint (absorbs blue and green and reflects red). The resulting paint will absorb: - All of the blue light.
- 25% of the red light.
- 70% of the green light.

Thus, the colour reflects: - 75% of the red light.
- 30% of the green light.

This colour corresponds to #c04c00. Try it in GIMP, it's brown.

If you mix all the colours of paint together you get something very dark.

True green paint absorbs blue and red. True red paint absorbs green and blue. True blue paoint absorbs green and red.

Mix these together and you get paint that absorbs 77% of the light from each of the three. In other words, it reflects 33% of the light from all three.

This colour is #545454. Try it in GIMP, it's very dark (gray). If you don't use pure red/green/blue paint you will not get exactly gray. You will get some very dark colour.

If you paint a spinning top with segments of all colours and spin it you see something approaching white.

If the spinning top has equal segments of red green and blue, it will receive equal quantities of all three primary colours. Thus it will look light (almost white). The more intense the colours the closer to white.

If the colours are not pure, or are not equally represented, the final colour will shift from pure white as well.

Hey. It's weird, isn't it?

I would call it "fascinating".

Note: I'm a scientist, I find this sort of thing fascinating.

Cheers,

Fay
2003-03-30 10:18:11 UTC (over 17 years ago)

[Q] Primary colours

Thank you for all of that. I'll experiment as you've outlined for me. Daniel wrote
m a little old lady with four years of high school and *I* find it fascinating too!

Scott Lamb
2003-03-30 10:30:36 UTC (over 17 years ago)

[Q] Primary colours

Daniel Carrera wrote:

On Sun, Mar 30, 2003 at 05:19:35PM +1000, Fay wrote:

Hi Daniel,
I don't think that either set of colours has to do with how our eyes work.

Actually, it does.
Recall:
- Colours are frequencies of light.
- White is the sum of all frequencies, and black is no frequency. - The human retina has cones for red green and blue.

[...]

If you mix red and green light you get yellow.

If you mix red and green light your retina perceives all colours except blue, which is equal to yellow (see above).

That's not quite the way I would have put it. Your eye has cones that have different frequency responses roughly forming normal curves centered on red, green, and blue. (See ) If
your eye really _just_ saw the red, green, and blue, you would be unable to see actual yellow light; I guess you'd sort of see on three axii rather than a spectrum of color, if that makes sense. But it doesn't work that way, so a certain wavelength of yellow can be faked with certain levels of the peak sensitivies. It's indistinguishable by eye. (Unless you're a tetrachromat.
)

That aside, I think you're correct...and your grade school teacher lied. Sorry. ;)

Scott Lamb

Daniel Carrera
2003-03-30 10:56:51 UTC (over 17 years ago)

[Q] Primary colours

On Sun, Mar 30, 2003 at 02:30:36AM -0600, Scott Lamb wrote:

If you mix red and green light your retina perceives all colours except blue, which is equal to yellow (see above).

That's not quite the way I would have put it. Your eye has cones that have different frequency responses roughly forming normal curves centered on red, green, and blue.

Yes, I know. I made a first-degree approximation to keep the explanation simpler. As it was, it took about a couple pages. I also ignored the complication that different frequencies are interpreted with different intensities.

The model of a vector space with 3 independent vectors (red, green and blue) is sufficient to describe the behaviour that we see.

The fact that the frequencies seen by the three kinds of cones form normal curves means that you can add and subtract colours the way I did. It is precisely this normality that makes the vector-space model of the spectrum work so well (though not perfectly).

I guess you'd sort of see on three axii rather than a spectrum of color, if that makes sense. But it doesn't work that way, so a certain wavelength of yellow can be faked with certain levels of the peak sensitivies. It's indistinguishable by eye.

I know. But this is an example of why the vector-space model works. Because of the normality of the curves, the yellow colour is picked up by both the 'red' and 'green' cones, but not with less intensity each, thus sort-of justifying the equation:

yellow = red + green

Besides, I'm a mathematecian. Vector spaces are my idea of "intuitive". :)

(Unless you're a tetrachromat.
)

That's really cool!
I can't believe that terachromats are all women! (now *that* is weird).

Cheers,

Gene Imes
2003-03-30 16:21:10 UTC (over 17 years ago)

gimp and astrophotography

Anyone know of any location with instruction on using gimp for processing astrophotography images?

Thanks,

Gene