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Mainly Tech projects on Python and Electronic Design Automation.

Wednesday, November 21, 2012

RGB spectrum. 3D to 2D with smooth transition?

I was messing about with HTML red, green, blue triplets of 0..255 values each that are used for specifying HTML colours (with a u as I am English). I decided that I wanted to be able to take a spread of rgb values and arrange them in a spectrum.

Easily said, but hard to do. I went down a blind alley of converting to Hue, Saturation, Luminance; but could do nothing with that. I decided to ignore our eyes different sensitivities and any difference in 'purity' of pure red/green/blue from monitors and decided to try and work out mathematically how to create such a spectra.

The values for the three colours are independently variable over their ranges and so I got to thinking of the available colours as a colour cube. Think of a a 3D graph with three perpendicular axes red, green and blue. You can do the physics trick of taking your hand; stick the thumb straight up, the finger directly below the thumb straight out in front and the second finger below the thumb at right angles to them both pointing towards your body. There you have it! three perpendicular axes for red, green and blue, with the origin being towards your palm.  

The available colours fill the cube of integers (0, 0, 0) up to (255, 255, 255). I want to create a linear 'spectrum'.

If you consider the three points R, G, B, = (255, 0, 0), (0, 255, 0), (0, 0, 255) respectively,  they are the points of maximum r,g,b intensity, unsullied by other colours. If you cut the cube to expose the plane linking those three points then I would ideally like a spectra that  traversed R->G->B (->R); but what about the off-plane colour triplets?

If given a list of colour triplets to sort into a spectrum I decided to 
  1. Find out which of the three cardinal points/primary colours R, G, or B each triplet was closest-to.
  2. Sort all the triplets on their closeness to a primary.
  3. Extract three lists of the triplets for each primary colour in such a way that they would be sorted with the primaries near the centre of their list; and those with more of the next primary to the 'right' and to the previous primary to the 'left'.
    For example the Red list would have reddy-greens to the right and reddy-blues to the left; the green list would have greeny-reds to the left and greeny-blues to the right; similarly the blue list would have bluey-greens to the left and bluey-reds to the right.   
  4. Join the three lists in the R, G,B order - assuming that B wraps-round to R to form the spectrum.

The Code


 1 
 2 '''
 3 Generate rgb colours in order
 4 '''
 5 
 6 from pprint import pprint as pp
 7 from collections import namedtuple
 8 from itertools import product, groupby
 9 
10 
11 Rgb = namedtuple('Rgb', 'r g b')
12 # red, green, blue, dark, light. 'cardinal' points
13 RGBDL = ( Rgb(255, 0, 0), Rgb(0, 255, 0), Rgb(0, 0, 255), Rgb(0, 0, 0), Rgb(255, 255, 255) )
14 RGB   = ( Rgb(255, 0, 0), Rgb(0, 255, 0), Rgb(0, 0, 255) )
15 
16 def dist(p, q):
17     'dist squared between two points'
18     return sum((pp - qq)**2 for pp,qq in zip(p,q))
19 
20 def cmp(x,y):
21     return 1 if x>y else (0 if x==y else -1)
22 
23 def rgbsort2(rgblist):
24     'reddest to greenest to bluest (... to redest, circularly)'
25     # , , 
26     # cindex = 0 for red; 1 for green; 2 for blue.
27     Closest = namedtuple('Closest', 'distance, cindex, col')
28     # find Closest info for all colours
29     colourdistances = []
30     for col in rgblist:
31         # distance to each cardinal colour in turn
32         rgbdistances = tuple(dist(col, cardinal) for cardinal in RGB)
33         distance = min(rgbdistances)        # closest
34         cindex = rgbdistances.index(distance)
35         colourdistances.append(Closest(distance, cindex, col))
36     # Sort closest to a cardinal point first 
37     colourdistances.sort()
38 
39     # clist accumulates redder, greener, bluer colours separately.
40     # Most intense colour towards the middle of each sublist.
41     clist = [list() for i in range(3)]
42 
43     # Accumulate the sub-spectra for each cardinal colour
44     for _, cindex, col in colourdistances:
45         # add right if closest to next colour cardinal point
46         addright = col[(cindex + 1) % 3] > col[(cindex + 2) % 3]
47         if addright:
48             clist[cindex].append(col)
49         else:
50             clist[cindex].insert(0, col)
51     # Final spectrum
52     return clist[0] + clist[1] + clist[2]
53 
54 def rgb2HTML(colours):
55     f, ht = 4, 50

...


67     return txt
68 
69 
70 if __name__ == '__main__':
71     if 1:
72         # See http://www.lynda.com/resources/webpalette.aspx# for info on
73         # HTML colours without dithering on Windows & Mac.
74         nondithers = (0xFF, 0xCC, 0x99, 0x66, 0x33, 0x00)
75     else:
76         nondithers = tuple(range(0,256,32))
77     rgblist = [Rgb(*rgb) for rgb in product(*(nondithers,)*3)]
78     spectra2 = rgbsort2(rgblist)
79     #assert spectra == spectra2
80     with open('rgbgen.htm', 'w') as f:
81         f.write(rgb2HTML(spectra2))
82 
83 










I have had to miss-out the guts of the rgb2HTML function as Blogger has problems with rendering the HTML tags 





The result


I get some sort of progression, but could do better:






#FFFFFF
(255,255,255)#FFCCFF
(255,204,255)#FF99FF
(255,153,255)#CCCCCC
(204,204,204)


#FFCCCC
(255,204,204)#FF66FF
(255,102,255)#FF33FF
(255,51,255)#CC99CC
(204,153,204)


#FF99CC
(255,153,204)#FF00FF
(255,0,255)#000000
(0,0,0)#999999
(153,153,153)


#CC66CC
(204,102,204)#FF66CC
(255,102,204)#CC9999
(204,153,153)#FF9999
(255,153,153)


#CC33CC
(204,51,204)#333333
(51,51,51)#FF33CC
(255,51,204)#CC00CC
(204,0,204)


#996699
(153,102,153)#666666
(102,102,102)#330033
(51,0,51)#FF00CC
(255,0,204)


#330000
(51,0,0)#CC6699
(204,102,153)#993399
(153,51,153)#663366
(102,51,102)


#FF6699
(255,102,153)#990099
(153,0,153)#660066
(102,0,102)#996666
(153,102,102)


#CC3399
(204,51,153)#663333
(102,51,51)#FF3399
(255,51,153)#CC0099
(204,0,153)


#660033
(102,0,51)#FF0099
(255,0,153)#CC6666
(204,102,102)#993366
(153,51,102)


#660000
(102,0,0)#FF6666
(255,102,102)#990066
(153,0,102)#CC3366
(204,51,102)


#993333
(153,51,51)#FF3366
(255,51,102)#CC0066
(204,0,102)#990033
(153,0,51)


#FF0066
(255,0,102)#990000
(153,0,0)#CC3333
(204,51,51)#FF3333
(255,51,51)


#CC0033
(204,0,51)#FF0033
(255,0,51)#CC0000
(204,0,0)#FF0000
(255,0,0)


#FF3300
(255,51,0)#CC3300
(204,51,0)#FF6600
(255,102,0)#993300
(153,51,0)


#CC6600
(204,102,0)#FF6633
(255,102,51)#CC6633
(204,102,51)#996600
(153,102,0)


#996633
(153,102,51)#FF9900
(255,153,0)#663300
(102,51,0)#CC9900
(204,153,0)


#FF9933
(255,153,51)#CC9933
(204,153,51)#666600
(102,102,0)#999900
(153,153,0)


#FF9966
(255,153,102)#666633
(102,102,51)#999933
(153,153,51)#CC9966
(204,153,102)


#FFCC00
(255,204,0)#333300
(51,51,0)#999966
(153,153,102)#CCCC00
(204,204,0)


#FFCC33
(255,204,51)#CCCC33
(204,204,51)#FFCC66
(255,204,102)#CCCC66
(204,204,102)


#FFCC99
(255,204,153)#FFFF00
(255,255,0)#CCCC99
(204,204,153)#FFFF33
(255,255,51)


#FFFF66
(255,255,102)#FFFF99
(255,255,153)#FFFFCC
(255,255,204)#CCFFCC
(204,255,204)


#CCFF99
(204,255,153)#CCFF66
(204,255,102)#99CC99
(153,204,153)#99FF99
(153,255,153)


#CCFF33
(204,255,51)#CCFF00
(204,255,0)#003300
(0,51,0)#99CC66
(153,204,102)


#99FF66
(153,255,102)#669966
(102,153,102)#99CC33
(153,204,51)#336633
(51,102,51)


#99FF33
(153,255,51)#99CC00
(153,204,0)#336600
(51,102,0)#99FF00
(153,255,0)


#66CC66
(102,204,102)#669933
(102,153,51)#006600
(0,102,0)#66FF66
(102,255,102)


#669900
(102,153,0)#66CC33
(102,204,51)#339933
(51,153,51)#66FF33
(102,255,51)


#66CC00
(102,204,0)#339900
(51,153,0)#66FF00
(102,255,0)#009900
(0,153,0)


#33CC33
(51,204,51)#33FF33
(51,255,51)#33CC00
(51,204,0)#33FF00
(51,255,0)


#00CC00
(0,204,0)#00FF00
(0,255,0)#00FF33
(0,255,51)#00CC33
(0,204,51)


#00FF66
(0,255,102)#009933
(0,153,51)#00CC66
(0,204,102)#33FF66
(51,255,102)


#33CC66
(51,204,102)#009966
(0,153,102)#00FF99
(0,255,153)#339966
(51,153,102)


#006633
(0,102,51)#00CC99
(0,204,153)#33FF99
(51,255,153)#33CC99
(51,204,153)


#006666
(0,102,102)#009999
(0,153,153)#66FF99
(102,255,153)#336666
(51,102,102)


#339999
(51,153,153)#66CC99
(102,204,153)#00FFCC
(0,255,204)#003333
(0,51,51)


#00CCCC
(0,204,204)#33FFCC
(51,255,204)#669999
(102,153,153)#33CCCC
(51,204,204)


#66FFCC
(102,255,204)#66CCCC
(102,204,204)#00FFFF
(0,255,255)#99FFCC
(153,255,204)


#33FFFF
(51,255,255)#99CCCC
(153,204,204)#66FFFF
(102,255,255)#99FFFF
(153,255,255)


#CCFFFF
(204,255,255)#CCCCFF
(204,204,255)#99CCFF
(153,204,255)#66CCFF
(102,204,255)


#9999CC
(153,153,204)#9999FF
(153,153,255)#33CCFF
(51,204,255)#00CCFF
(0,204,255)


#000033
(0,0,51)#6699CC
(102,153,204)#6699FF
(102,153,255)#666699
(102,102,153)


#3399CC
(51,153,204)#333366
(51,51,102)#3399FF
(51,153,255)#0099CC
(0,153,204)


#003366
(0,51,102)#6666CC
(102,102,204)#336699
(51,102,153)#0099FF
(0,153,255)


#000066
(0,0,102)#6666FF
(102,102,255)#006699
(0,102,153)#3366CC
(51,102,204)


#333399
(51,51,153)#3366FF
(51,102,255)#0066CC
(0,102,204)#003399
(0,51,153)


#0066FF
(0,102,255)#000099
(0,0,153)#3333CC
(51,51,204)#3333FF
(51,51,255)


#0033CC
(0,51,204)#0033FF
(0,51,255)#0000CC
(0,0,204)#0000FF
(0,0,255)


#3300FF
(51,0,255)#3300CC
(51,0,204)#6600FF
(102,0,255)#330099
(51,0,153)


#6600CC
(102,0,204)#6633FF
(102,51,255)#6633CC
(102,51,204)#660099
(102,0,153)


#663399
(102,51,153)#9900FF
(153,0,255)#330066
(51,0,102)#9900CC
(153,0,204)


#9933FF
(153,51,255)#9933CC
(153,51,204)#9966FF
(153,102,255)#9966CC
(153,102,204)


#CC00FF
(204,0,255)#CC33FF
(204,51,255)#CC66FF
(204,102,255)#CC99FF
(204,153,255)













Posted by
Paddy3118


at
12:57 pm


































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