Every now and then I need to create a series of contrasting colors. For instance, in my mapping app PyTopo, when displaying several track logs at once, I want them to be different colors so it's easy to tell which track is which.
Of course, I could make a list of five or ten different colors and cycle through the list. But I hate doing work that a computer could do for me.
Choosing random RGB (red, green and blue) values for the colors, though, doesn't work so well. Sometimes you end up getting two similar colors together. Other times, you get colors that just don't work well, because they're so light they look white, or so dark they look black, or so unsaturated they look like shades of grey.
What does work well is converting to the HSV color space:
hue, saturation and value.
Hue is a measure of the color -- that it's red, or blue, or
yellow green, or orangeish, or a reddish purple.
Saturation measures how intense the color is: is it a bright, vivid
red or a washed-out red? Value tells you how light or dark it is: is
it so pale it's almost white, so dark it's almost black, or somewhere
in between? (A similar model, called HSL, substitutes Lightness for Value,
but is similar enough in concept.)
If you're not familiar with HSV, you can get a good feel for it by playing with GIMP's color chooser (which pops up when you click the black Foreground or white Background color swatch in GIMP's toolbox). The vertical rainbow bar selects Hue. Once you have a hue, dragging up or down in the square changes Saturation; dragging right or left changes Value. You can also change one at a time by dragging the H, S or V sliders at the upper right of the dialog.
Why does this matter? Because once you've chosen a saturation and value, or at least ensured that saturation is fairly high and value is somewhere in the middle of its range, you can cycle through hues and be assured that you'll get colors that are fairly different each time. If you had a red last time, this time it'll be a green, or yellow, or blue, depending on how much you change the hue.
How does this work programmatically?
PyTopo uses Python-GTK, so I need a function that takes a gtk.gdk.Color
and chooses a new, contrasting Color. Fortunately, gtk.gdk.Color already
hue, saturation and
value built in.
Color.hue is a floating-point number between 0 and 1,
so I just have to choose how much to jump. Like this:
def contrasting_color(color): '''Returns a gtk.gdk.Color of similar saturation and value to the color passed in, but a contrasting hue. gtk.gdk.Color objects have a hue between 0 and 1. ''' if not color: return self.first_track_color; # How much to jump in hue: jump = .37 return gtk.gdk.color_from_hsv(color.hue + jump, color.saturation, color.value)
What if you're not using Python-GTK?
[ 13:27 Sep 27, 2015 More programming | permalink to this entry | ]