RGB value to HSL converter

Question

Google maps api v3 allows "styles" to be applied to the map, including setting the color of various features. However, the color format it uses is HSL (or what seems like it):

• hue (an RGB hex string)
• lightness (a floating point value between -100 and 100)
• saturation (a floating point value between -100 and 100)

(from the docs)

I managed to find RGB to HSL converters online, but I am unsure how to specify the converted values in a way that google maps will accept. For instance, a typical HSL value given by a converter would be: 209° 72% 49%

How does that HSL value map to the parameters I specified from the google maps api? i.e. how does a hue degree value map to an RGB hex string and how does a percentage map to a floating point value between -100 and 100?

I am still uncertain how to do the conversion. I need to, given an RGB value, quickly convert it to what google maps expects so that the color will be identical...

Answer 1

From the linked page:

Note: while hue takes an HTML hex color value, it only uses this value to determine the basic color (its orientation around the color wheel), not its saturation or lightness, which are indicated separately as percentage changes. For example, the hue for pure green may be defined as "#00ff00" or "#000100" within the hue property and both hues will be identical. (Both values point to pure green in the HSL color model.) RGB hue values which consist of equal parts Red, Green and Blue — such as "#000000" (black) and "#FFFFFF" (white) and all the pure shades of grey — do not indicate a hue whatsoever, as none of those values indicate an orientation in the HSL coordinate space. To indicate black, white or grey, you must remove all saturation (set the value to -100) and adjust lightness instead.

At least as I read it, that means you need to convert your angle based on a color wheel. For example, let's assume 0 degrees is pure red, 120 degrees is pure blue and 240 degrees is pure green. You'd then take your angle, figure out which two primaries it falls between, and interpolate to determine how much of each primary to use. In theory you should probably use a quadratic interpolation -- but chances are that you can get by reasonably well with linear.

Using that, 90 degrees (for example) is 90/120 = 3/4^ths of the way from red to blue, so your hex number for the hue would be 0x00010003 -- or any other number that had green set to 0, and a 1:3 ratio between red and blue.

Answer 2

Since the hue argument expects RGB, you can use the original color as the hue.

rgb2hsl.py:

#!/usr/bin/env python

def rgb2hsl(r, g, b):
    #Hue: the RGB string
    H = (r<<16) + (g<<8) + b
    H = "0x%06X" % H

    #convert to [0 - 1] range
    r = float(r) / 0xFF
    g = float(g) / 0xFF
    b = float(b) / 0xFF

    #http://en.wikipedia.org/wiki/HSL_and_HSV#Lightness
    M = max(r,g,b)
    m = min(r,g,b)
    C = M - m

    #Lightness
    L = (M + m) / 2

    #Saturation (HSL)
    if L == 0:
        S = 0
    elif L <= .5:
        S = C/(2*L)
    else:
        S = C/(2 - 2*L)

    #gmaps wants values from -100 to 100
    S = int(round(S * 200 - 100))
    L = int(round(L * 200 - 100))

    return (H, S, L)


def main(r, g, b):
    r = int(r, base=16)
    g = int(g, base=16)
    b = int(b, base=16)
    print rgb2hsl(r,g,b)

if __name__ == '__main__':
    from sys import argv
    main(*argv[1:])

Example:

$ ./rgb2hsl.py F0 FF FF
('0xF0FFFF', 100, 94)

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Result:

Below is a screenshot showing the body set to a rgb background color (#2800E2 in this case), and a google map with styled road-geometry, using the values calculated as above ('0x2800E2', 100, -11).

It's pretty clear that google uses your styling to create around six different colors centered on the given color, with the outlines being closest to the input. I believe this is as close as it gets.

---

From experimentation with: http://gmaps-samples-v3.googlecode.com/svn/trunk/styledmaps/wizard/index.html

For water, gmaps subtracts a gamma of .5. To get the exact color you want, use the calculations above, and add that .5 gamma back.

like:

{
  featureType: "water",
  elementType: "geometry",
  stylers: [
    { hue: "#2800e2" },
    { saturation: 100 },
    { lightness: -11 },
    { gamma: 0.5 },
  ]
}