RGB HSL Conversion formulas

comes from : http://www.paris-pc-gis.com/MI_Enviro/Colors/color_models.htm#6-4%20R,%20G,%20B%20%3C%3E%20H,%20S,%20L

or : http://130.113.54.154/~monger/hsl-rgb.html

RGB > HSL

1 - Convert the RBG values to the range 0-1

Example: from the video colors page, colorbar red has R=83%, B=7%, G=7%, or in this scale, R=.83, B=.07, G=.07

2 - Find min and max values of R, B, G

In the example, maxcolor = .83, mincolor=.07

3 - L = (maxcolor + mincolor)/2

For the example, L = (.83+.07)/2 = .45

4 - If the max and min colors are the same (i.e. the color is some kind of grey), S is defined to be 0, and H is undefined but in programs usually written as 0. (all is done)

5 - Otherwise, test L.

If L < 0.5, S=(maxcolor-mincolor)/(maxcolor+mincolor)

If L >=0.5, S=(maxcolor-mincolor)/(2.0-maxcolor-mincolor)

For the example, L=0.45 so S=(.83-.07)/(.83+.07) = .84

6 - Compute

If R=maxcolor, H = (G-B)/(maxcolor-mincolor)

If G=maxcolor, H = 2.0 + (B-R)/(maxcolor-mincolor)

If B=maxcolor, H = 4.0 + (R-G)/(maxcolor-mincolor)

For the example, R=maxcolor so H = (.07-.07)/(.83-.07) = 0

7 - To use the scaling shown in the video color page, convert L and S back to percentages, and H into an angle in degrees (ie scale it from 0-360).

8 - From the computation in step 6, H will range from 0-6. RGB space is a cube, and HSL space is a double hexacone, where L is the principal diagonal of the RGB cube. Thus corners of the RGB cube; red, yellow, green, cyan, blue, and magenta, become the vertices of the HSL hexagon. Then the value 0-6 for H tells you which section of the hexagon you are in. H is most commonly given as in degrees, so to convert

H = H*60.0

9 - If H is negative, add 360 to complete the conversion.

HSL < RGB

1 - If S=0, define R, G, and B all to L (all is done)

 

2 - Otherwise, test L.

If L < 0.5, temp2=L*(1.0+S)

If L >= 0.5, temp2=L+S - L*S

 

In the colorbar example for colorbar green, H=120, L=52, S=79, so converting to the range 0-1, L=.52, S=.79, so temp2=(.52+.79) - (.52*.79) = .899

3 - Compute

temp1 = 2.0*L - temp2

In the example, temp1 = 2.0*.52 - .899 = .141

4 - Convert H to the range 0-1

            In the example, H=120/360 = .33

 

5 - For each of R, G, B, compute another temporary value, temp3, as follows:

            for R, temp3=H+1.0/3.0

            for G, temp3=H

            for B, temp3=H-1.0/3.0

            if temp3 < 0, temp3 = temp3 + 1.0

if temp3 > 1, temp3 = temp3 - 1.0

 

            In the example, Rtemp3=.33+.33 = .66, Gtemp3=.33, Btemp3=.33-.33=0

 

7 - For each of R, G, B, do the following test:

            If 6.0*temp3 < 1, color=temp1+(temp2-temp1)*6.0*temp3

            Else if 2.0*temp3 < 1, color=temp2

            Else if 3.0*temp3 < 2, color=temp1+(temp2-temp1)*((2.0/3.0)-temp3)*6.0

            Else color=temp1

 

In the example,  

3.0*Rtemp3 < 2 so R=.141+(.899-.141)*((2.0/3.0-.66)*6.0=.141

2.0*Gtemp3 < 1 so G=.899

6.0*Btemp3 < 1 so B=.141+(.899-.141)*6.0*0=.141

8 - Scale back to the range 0-100 to use the scaling shown in the video color page

            For the example, R=14, G=90, B=14  Color in video