How to get the wavelength of a pixel using RGB? - rgb

Question

I have a project that would classify the color of a pixel. Whether it is red,violet, orange or simply any color in the color wheel. I know that there are over 16 million color combination for pixels. But I was able to read a web page that says its possible for me to do my project using the wavelengths of color. Please give me the formula to compute for the wavelength using RGB values. Thanks!

Answer 1

A pure color has a wavelength (any single color LED will have a specific wavelength).
Red, green and blue each have a range of wavelength. However, when you make an RGB color, you add these wavelengths together, which will NOT give you a new wavelength. The eye can't distinguish a yellow composed of one wavelength from that of adding red and green (just how the eye works).
I'd recommend reading up on color theory
http://en.wikipedia.org/wiki/RGB_color_model

Answer 2

Well RGB for a monitor maps to 3 independant levels of Red Green and Blue light, so there are (mostly) 3 distinct wavelengths present of any one percieved colour.
BUT If you can convert your RGB colour value to its equivilent HSL, the H part (Hue) is the dominant colour in so far as wavelength goes if you are prepared to ignore the saturation (think of it as whiteness).
Based on that you could approximate the dominante wavelength of a colour based on its H value.
Red light is roughly 630–740nm wavelength, Violet is roughly 380–450nm.

Answer 3

Working out wavelength is a bit tricky, and as Goblin mentioned, not always possible (another example is the colour obtained by mixing equal amounts of red and blue light. That purple has no single wavelength).
But if all you want to do is identify the colour by name, then the HSV model would be a good one to use. HSV is Hue (where the colour is around the colour wheel), Saturation (how much colour there is as opposed to being a shade of black/grey/white) and Value (how bright or dark the pixel is). In this case Hue is probably exactly what you want.
If you are using a .NET language, then you're in luck. See the Color.GetHue Method which does all the work for you.
Otherwise, see HSV at Wikipedia for more details.
In essence, if you have R, G and B as floats ranging from 0.0 to 1.0 (instead of ints from 0 to 255 for example), then:
M = max(R, G, B)
m = min(R, G, B)
C = M-m
if M = m then H' is undefined (The pixel is some shade of grey)
if M = R then H' = (G-B)/C mod 6
if M = G then H' = (B-R)/C + 2
if M = B then H' = (R-G)/C + 4
When converting RGB to HSV you then multiply H' by 60 degrees, but for your purposes H' is probably fine. It will be a float ranging from 0 to 6 (almost). 0 is Red (as is 6). 1 is Yellow, with values between 0 and 1 being shaded between Red and Yellow. So 0.5 would be Orange. The important landmarks are:
0 - Red
1 - Yellow
2 - Green
3 - Cyan
4 - Blue
5 - Purple
6 - Red (again)
Hope that helps.

Answer 4

http://en.wikipedia.org/wiki/Visible_spectrum
It is possible. See above. Gray background apparently makes it easier. You might get something like that on your own, and even improve on it. But to do it accurately will cost major dollars. U will need a colorimetry expert, a calibrated monitor and viewing environment (since what the dominant wavelength of your pixel is just means what monochromatic wavelength it approximates on your calibrated monitor in your calibrated viewing environment). All this will be a few thousand dollars. The work done at the above link, shown on wikipedia, does not seem that accurate but it is probably what you want.

Answer 5

Just convert the RGB to HSV then get the HSV value to degrees and this is the answer:
650 - 250 / 270 * D
where D is the degrees.
Considering...
Violet has a 380–450 nm wavelength, &
Blue has a 450–495 nm wavelength, &
Green has a 495–570 nm wavelength, &
Yellow has a 570–590 nm wavelength, &
Orange has a 590–620 nm wavelength, &
Red has a 620–750 nm wavelength,
then you just need to check if it is in those ranges, then you can classify it.
Hope this helps!