I wrote the two methods below to automatically select N distinct colors. It works by defining a piecewise linear function on the RGB cube. The benefit of this is you can also get a progressive scale if that's what you want, but when N gets large the colors can start to look similar. I can also imagine evenly subdividing the RGB cube into a lattice and then drawing points. Does anyone know any other methods? I'm ruling out defining a list and then just cycling through it. I should also say I don't generally care if they clash or don't look nice, they just have to be visually distinct.
public static List<Color> pick(int num) {
List<Color> colors = new ArrayList<Color>();
if (num < 2)
return colors;
float dx = 1.0f / (float) (num - 1);
for (int i = 0; i < num; i++) {
colors.add(get(i * dx));
}
return colors;
}
public static Color get(float x) {
float r = 0.0f;
float g = 0.0f;
float b = 1.0f;
if (x >= 0.0f && x < 0.2f) {
x = x / 0.2f;
r = 0.0f;
g = x;
b = 1.0f;
} else if (x >= 0.2f && x < 0.4f) {
x = (x - 0.2f) / 0.2f;
r = 0.0f;
g = 1.0f;
b = 1.0f - x;
} else if (x >= 0.4f && x < 0.6f) {
x = (x - 0.4f) / 0.2f;
r = x;
g = 1.0f;
b = 0.0f;
} else if (x >= 0.6f && x < 0.8f) {
x = (x - 0.6f) / 0.2f;
r = 1.0f;
g = 1.0f - x;
b = 0.0f;
} else if (x >= 0.8f && x <= 1.0f) {
x = (x - 0.8f) / 0.2f;
r = 1.0f;
g = 0.0f;
b = x;
}
return new Color(r, g, b);
}
This question is related to
java
colors
color-scheme
color-picker
The answer is
I think this simple recursive algorithm complementes the accepted answer, in order to generate distinct hue values. I made it for hsv, but can be used for other color spaces too.
It generates hues in cycles, as separate as possible to each other in each cycle.
/**
* 1st cycle: 0, 120, 240
* 2nd cycle (+60): 60, 180, 300
* 3th cycle (+30): 30, 150, 270, 90, 210, 330
* 4th cycle (+15): 15, 135, 255, 75, 195, 315, 45, 165, 285, 105, 225, 345
*/
public static float recursiveHue(int n) {
// if 3: alternates red, green, blue variations
float firstCycle = 3;
// First cycle
if (n < firstCycle) {
return n * 360f / firstCycle;
}
// Each cycle has as much values as all previous cycles summed (powers of 2)
else {
// floor of log base 2
int numCycles = (int)Math.floor(Math.log(n / firstCycle) / Math.log(2));
// divDown stores the larger power of 2 that is still lower than n
int divDown = (int)(firstCycle * Math.pow(2, numCycles));
// same hues than previous cycle, but summing an offset (half than previous cycle)
return recursiveHue(n % divDown) + 180f / divDown;
}
}
I was unable to find this kind of algorithm here. I hope it helps, it's my first post here.
I would try to fix saturation and lumination to maximum and focus on hue only. As I see it, H can go from 0 to 255 and then wraps around. Now if you wanted two contrasting colours you would take the opposite sides of this ring, i.e. 0 and 128. If you wanted 4 colours, you would take some separated by 1/4 of the 256 length of the circle, i.e. 0, 64,128,192. And of course, as others suggested when you need N colours, you could just separate them by 256/N.
What I would add to this idea is to use a reversed representation of a binary number to form this sequence. Look at this:
0 = 00000000 after reversal is 00000000 = 0
1 = 00000001 after reversal is 10000000 = 128
2 = 00000010 after reversal is 01000000 = 64
3 = 00000011 after reversal is 11000000 = 192
... this way if you need N different colours you could just take first N numbers, reverse them, and you get as much distant points as possible (for N being power of two) while at the same time preserving that each prefix of the sequence differs a lot.
This was an important goal in my use case, as I had a chart where colors were sorted by area covered by this colour. I wanted the largest areas of the chart to have large contrast, and I was ok with some small areas to have colours similar to those from top 10, as it was obvious for the reader which one is which one by just observing the area.
I have written a package for R called qualpalr that is designed specifically for this purpose. I recommend you look at the vignette to find out how it works, but I will try to summarize the main points.
qualpalr takes a specification of colors in the HSL color space (which was described previously in this thread), projects it to the DIN99d color space (which is perceptually uniform) and find the n that maximize the minimum distance between any oif them.
# Create a palette of 4 colors of hues from 0 to 360, saturations between
# 0.1 and 0.5, and lightness from 0.6 to 0.85
pal <- qualpal(n = 4, list(h = c(0, 360), s = c(0.1, 0.5), l = c(0.6, 0.85)))
# Look at the colors in hex format
pal$hex
#> [1] "#6F75CE" "#CC6B76" "#CAC16A" "#76D0D0"
# Create a palette using one of the predefined color subspaces
pal2 <- qualpal(n = 4, colorspace = "pretty")
# Distance matrix of the DIN99d color differences
pal2$de_DIN99d
#> #69A3CC #6ECC6E #CA6BC4
#> 6ECC6E 22
#> CA6BC4 21 30
#> CD976B 24 21 21
plot(pal2)
This questions appears in quite a few SO discussions:
- Algorithm For Generating Unique Colors
- Generate unique colours
- Generate distinctly different RGB colors in graphs
- How to generate n different colors for any natural number n?
Different solutions are proposed, but none are optimal. Luckily, science comes to the rescue
Arbitrary N
- Colour displays for categorical images (free download)
- A WEB SERVICE TO PERSONALISE MAP COLOURING (free download, a webservice solution should be available by next month)
- An Algorithm for the Selection of High-Contrast Color Sets (the authors offer a free C++ implementation)
- High-contrast sets of colors (The first algorithm for the problem)
The last 2 will be free via most university libraries / proxies.
N is finite and relatively small
In this case, one could go for a list solution. A very interesting article in the subject is freely available:
There are several color lists to consider:
- Boynton's list of 11 colors that are almost never confused (available in the first paper of the previous section)
- Kelly's 22 colors of maximum contrast (available in the paper above)
I also ran into this Palette by an MIT student. Lastly, The following links may be useful in converting between different color systems / coordinates (some colors in the articles are not specified in RGB, for instance):
- http://chem8.org/uch/space-55036-do-blog-id-5333.html
- https://metacpan.org/pod/Color::Library::Dictionary::NBS_ISCC
- Color Theory: How to convert Munsell HVC to RGB/HSB/HSL
For Kelly's and Boynton's list, I've already made the conversion to RGB (with the exception of white and black, which should be obvious). Some C# code:
public static ReadOnlyCollection<Color> KellysMaxContrastSet
{
get { return _kellysMaxContrastSet.AsReadOnly(); }
}
private static readonly List<Color> _kellysMaxContrastSet = new List<Color>
{
UIntToColor(0xFFFFB300), //Vivid Yellow
UIntToColor(0xFF803E75), //Strong Purple
UIntToColor(0xFFFF6800), //Vivid Orange
UIntToColor(0xFFA6BDD7), //Very Light Blue
UIntToColor(0xFFC10020), //Vivid Red
UIntToColor(0xFFCEA262), //Grayish Yellow
UIntToColor(0xFF817066), //Medium Gray
//The following will not be good for people with defective color vision
UIntToColor(0xFF007D34), //Vivid Green
UIntToColor(0xFFF6768E), //Strong Purplish Pink
UIntToColor(0xFF00538A), //Strong Blue
UIntToColor(0xFFFF7A5C), //Strong Yellowish Pink
UIntToColor(0xFF53377A), //Strong Violet
UIntToColor(0xFFFF8E00), //Vivid Orange Yellow
UIntToColor(0xFFB32851), //Strong Purplish Red
UIntToColor(0xFFF4C800), //Vivid Greenish Yellow
UIntToColor(0xFF7F180D), //Strong Reddish Brown
UIntToColor(0xFF93AA00), //Vivid Yellowish Green
UIntToColor(0xFF593315), //Deep Yellowish Brown
UIntToColor(0xFFF13A13), //Vivid Reddish Orange
UIntToColor(0xFF232C16), //Dark Olive Green
};
public static ReadOnlyCollection<Color> BoyntonOptimized
{
get { return _boyntonOptimized.AsReadOnly(); }
}
private static readonly List<Color> _boyntonOptimized = new List<Color>
{
Color.FromArgb(0, 0, 255), //Blue
Color.FromArgb(255, 0, 0), //Red
Color.FromArgb(0, 255, 0), //Green
Color.FromArgb(255, 255, 0), //Yellow
Color.FromArgb(255, 0, 255), //Magenta
Color.FromArgb(255, 128, 128), //Pink
Color.FromArgb(128, 128, 128), //Gray
Color.FromArgb(128, 0, 0), //Brown
Color.FromArgb(255, 128, 0), //Orange
};
static public Color UIntToColor(uint color)
{
var a = (byte)(color >> 24);
var r = (byte)(color >> 16);
var g = (byte)(color >> 8);
var b = (byte)(color >> 0);
return Color.FromArgb(a, r, g, b);
}
And here are the RGB values in hex and 8-bit-per-channel representations:
kelly_colors_hex = [
0xFFB300, # Vivid Yellow
0x803E75, # Strong Purple
0xFF6800, # Vivid Orange
0xA6BDD7, # Very Light Blue
0xC10020, # Vivid Red
0xCEA262, # Grayish Yellow
0x817066, # Medium Gray
# The following don't work well for people with defective color vision
0x007D34, # Vivid Green
0xF6768E, # Strong Purplish Pink
0x00538A, # Strong Blue
0xFF7A5C, # Strong Yellowish Pink
0x53377A, # Strong Violet
0xFF8E00, # Vivid Orange Yellow
0xB32851, # Strong Purplish Red
0xF4C800, # Vivid Greenish Yellow
0x7F180D, # Strong Reddish Brown
0x93AA00, # Vivid Yellowish Green
0x593315, # Deep Yellowish Brown
0xF13A13, # Vivid Reddish Orange
0x232C16, # Dark Olive Green
]
kelly_colors = dict(vivid_yellow=(255, 179, 0),
strong_purple=(128, 62, 117),
vivid_orange=(255, 104, 0),
very_light_blue=(166, 189, 215),
vivid_red=(193, 0, 32),
grayish_yellow=(206, 162, 98),
medium_gray=(129, 112, 102),
# these aren't good for people with defective color vision:
vivid_green=(0, 125, 52),
strong_purplish_pink=(246, 118, 142),
strong_blue=(0, 83, 138),
strong_yellowish_pink=(255, 122, 92),
strong_violet=(83, 55, 122),
vivid_orange_yellow=(255, 142, 0),
strong_purplish_red=(179, 40, 81),
vivid_greenish_yellow=(244, 200, 0),
strong_reddish_brown=(127, 24, 13),
vivid_yellowish_green=(147, 170, 0),
deep_yellowish_brown=(89, 51, 21),
vivid_reddish_orange=(241, 58, 19),
dark_olive_green=(35, 44, 22))
For all you Java developers, here are the JavaFX colors:
// Don't forget to import javafx.scene.paint.Color;
private static final Color[] KELLY_COLORS = {
Color.web("0xFFB300"), // Vivid Yellow
Color.web("0x803E75"), // Strong Purple
Color.web("0xFF6800"), // Vivid Orange
Color.web("0xA6BDD7"), // Very Light Blue
Color.web("0xC10020"), // Vivid Red
Color.web("0xCEA262"), // Grayish Yellow
Color.web("0x817066"), // Medium Gray
Color.web("0x007D34"), // Vivid Green
Color.web("0xF6768E"), // Strong Purplish Pink
Color.web("0x00538A"), // Strong Blue
Color.web("0xFF7A5C"), // Strong Yellowish Pink
Color.web("0x53377A"), // Strong Violet
Color.web("0xFF8E00"), // Vivid Orange Yellow
Color.web("0xB32851"), // Strong Purplish Red
Color.web("0xF4C800"), // Vivid Greenish Yellow
Color.web("0x7F180D"), // Strong Reddish Brown
Color.web("0x93AA00"), // Vivid Yellowish Green
Color.web("0x593315"), // Deep Yellowish Brown
Color.web("0xF13A13"), // Vivid Reddish Orange
Color.web("0x232C16"), // Dark Olive Green
};
the following is the unsorted kelly colors according to the order above.
the following is the sorted kelly colors according to hues (note that some yellows are not very contrasting)
I think this simple recursive algorithm complementes the accepted answer, in order to generate distinct hue values. I made it for hsv, but can be used for other color spaces too.
It generates hues in cycles, as separate as possible to each other in each cycle.
/**
* 1st cycle: 0, 120, 240
* 2nd cycle (+60): 60, 180, 300
* 3th cycle (+30): 30, 150, 270, 90, 210, 330
* 4th cycle (+15): 15, 135, 255, 75, 195, 315, 45, 165, 285, 105, 225, 345
*/
public static float recursiveHue(int n) {
// if 3: alternates red, green, blue variations
float firstCycle = 3;
// First cycle
if (n < firstCycle) {
return n * 360f / firstCycle;
}
// Each cycle has as much values as all previous cycles summed (powers of 2)
else {
// floor of log base 2
int numCycles = (int)Math.floor(Math.log(n / firstCycle) / Math.log(2));
// divDown stores the larger power of 2 that is still lower than n
int divDown = (int)(firstCycle * Math.pow(2, numCycles));
// same hues than previous cycle, but summing an offset (half than previous cycle)
return recursiveHue(n % divDown) + 180f / divDown;
}
}
I was unable to find this kind of algorithm here. I hope it helps, it's my first post here.
If N is big enough, you're going to get some similar-looking colors. There's only so many of them in the world.
Why not just evenly distribute them through the spectrum, like so:
IEnumerable<Color> CreateUniqueColors(int nColors)
{
int subdivision = (int)Math.Floor(Math.Pow(nColors, 1/3d));
for(int r = 0; r < 255; r += subdivision)
for(int g = 0; g < 255; g += subdivision)
for(int b = 0; b < 255; b += subdivision)
yield return Color.FromArgb(r, g, b);
}
If you want to mix up the sequence so that similar colors aren't next to each other, you could maybe shuffle the resulting list.
Am I underthinking this?
This is trivial in MATLAB (there is an hsv command):
cmap = hsv(number_of_colors)
For the sake of generations to come I add here the accepted answer in Python.
import numpy as np
import colorsys
def _get_colors(num_colors):
colors=[]
for i in np.arange(0., 360., 360. / num_colors):
hue = i/360.
lightness = (50 + np.random.rand() * 10)/100.
saturation = (90 + np.random.rand() * 10)/100.
colors.append(colorsys.hls_to_rgb(hue, lightness, saturation))
return colors
Here's a solution to managed your "distinct" issue, which is entirely overblown:
Create a unit sphere and drop points on it with repelling charges. Run a particle system until they no longer move (or the delta is "small enough"). At this point, each of the points are as far away from each other as possible. Convert (x, y, z) to rgb.
I mention it because for certain classes of problems, this type of solution can work better than brute force.
I originally saw this approach here for tesselating a sphere.
Again, the most obvious solutions of traversing HSL space or RGB space will probably work just fine.
If N is big enough, you're going to get some similar-looking colors. There's only so many of them in the world.
Why not just evenly distribute them through the spectrum, like so:
IEnumerable<Color> CreateUniqueColors(int nColors)
{
int subdivision = (int)Math.Floor(Math.Pow(nColors, 1/3d));
for(int r = 0; r < 255; r += subdivision)
for(int g = 0; g < 255; g += subdivision)
for(int b = 0; b < 255; b += subdivision)
yield return Color.FromArgb(r, g, b);
}
If you want to mix up the sequence so that similar colors aren't next to each other, you could maybe shuffle the resulting list.
Am I underthinking this?
This OpenCV function uses the HSV color model to generate n evenly distributed colors around the 0<=H<=360º with maximum S=1.0 and V=1.0. The function outputs the BGR colors in bgr_mat:
void distributed_colors (int n, cv::Mat_<cv::Vec3f> & bgr_mat) {
cv::Mat_<cv::Vec3f> hsv_mat(n,CV_32F,cv::Vec3f(0.0,1.0,1.0));
double step = 360.0/n;
double h= 0.0;
cv::Vec3f value;
for (int i=0;i<n;i++,h+=step) {
value = hsv_mat.at<cv::Vec3f>(i);
hsv_mat.at<cv::Vec3f>(i)[0] = h;
}
cv::cvtColor(hsv_mat, bgr_mat, CV_HSV2BGR);
bgr_mat *= 255;
}
This is trivial in MATLAB (there is an hsv command):
cmap = hsv(number_of_colors)
Everyone seems to have missed the existence of the very useful YUV color space which was designed to represent perceived color differences in the human visual system. Distances in YUV represent differences in human perception. I needed this functionality for MagicCube4D which implements 4-dimensional Rubik's cubes and an unlimited numbers of other 4D twisty puzzles having arbitrary numbers of faces.
My solution starts by selecting random points in YUV and then iteratively breaking up the closest two points, and only converting to RGB when returning the result. The method is O(n^3) but that doesn't matter for small numbers or ones that can be cached. It can certainly be made more efficient but the results appear to be excellent.
The function allows for optional specification of brightness thresholds so as not to produce colors in which no component is brighter or darker than given amounts. IE you may not want values close to black or white. This is useful when the resulting colors will be used as base colors that are later shaded via lighting, layering, transparency, etc. and must still appear different from their base colors.
import java.awt.Color;
import java.util.Random;
/**
* Contains a method to generate N visually distinct colors and helper methods.
*
* @author Melinda Green
*/
public class ColorUtils {
private ColorUtils() {} // To disallow instantiation.
private final static float
U_OFF = .436f,
V_OFF = .615f;
private static final long RAND_SEED = 0;
private static Random rand = new Random(RAND_SEED);
/*
* Returns an array of ncolors RGB triplets such that each is as unique from the rest as possible
* and each color has at least one component greater than minComponent and one less than maxComponent.
* Use min == 1 and max == 0 to include the full RGB color range.
*
* Warning: O N^2 algorithm blows up fast for more than 100 colors.
*/
public static Color[] generateVisuallyDistinctColors(int ncolors, float minComponent, float maxComponent) {
rand.setSeed(RAND_SEED); // So that we get consistent results for each combination of inputs
float[][] yuv = new float[ncolors][3];
// initialize array with random colors
for(int got = 0; got < ncolors;) {
System.arraycopy(randYUVinRGBRange(minComponent, maxComponent), 0, yuv[got++], 0, 3);
}
// continually break up the worst-fit color pair until we get tired of searching
for(int c = 0; c < ncolors * 1000; c++) {
float worst = 8888;
int worstID = 0;
for(int i = 1; i < yuv.length; i++) {
for(int j = 0; j < i; j++) {
float dist = sqrdist(yuv[i], yuv[j]);
if(dist < worst) {
worst = dist;
worstID = i;
}
}
}
float[] best = randYUVBetterThan(worst, minComponent, maxComponent, yuv);
if(best == null)
break;
else
yuv[worstID] = best;
}
Color[] rgbs = new Color[yuv.length];
for(int i = 0; i < yuv.length; i++) {
float[] rgb = new float[3];
yuv2rgb(yuv[i][0], yuv[i][1], yuv[i][2], rgb);
rgbs[i] = new Color(rgb[0], rgb[1], rgb[2]);
//System.out.println(rgb[i][0] + "\t" + rgb[i][1] + "\t" + rgb[i][2]);
}
return rgbs;
}
public static void hsv2rgb(float h, float s, float v, float[] rgb) {
// H is given on [0->6] or -1. S and V are given on [0->1].
// RGB are each returned on [0->1].
float m, n, f;
int i;
float[] hsv = new float[3];
hsv[0] = h;
hsv[1] = s;
hsv[2] = v;
System.out.println("H: " + h + " S: " + s + " V:" + v);
if(hsv[0] == -1) {
rgb[0] = rgb[1] = rgb[2] = hsv[2];
return;
}
i = (int) (Math.floor(hsv[0]));
f = hsv[0] - i;
if(i % 2 == 0)
f = 1 - f; // if i is even
m = hsv[2] * (1 - hsv[1]);
n = hsv[2] * (1 - hsv[1] * f);
switch(i) {
case 6:
case 0:
rgb[0] = hsv[2];
rgb[1] = n;
rgb[2] = m;
break;
case 1:
rgb[0] = n;
rgb[1] = hsv[2];
rgb[2] = m;
break;
case 2:
rgb[0] = m;
rgb[1] = hsv[2];
rgb[2] = n;
break;
case 3:
rgb[0] = m;
rgb[1] = n;
rgb[2] = hsv[2];
break;
case 4:
rgb[0] = n;
rgb[1] = m;
rgb[2] = hsv[2];
break;
case 5:
rgb[0] = hsv[2];
rgb[1] = m;
rgb[2] = n;
break;
}
}
// From http://en.wikipedia.org/wiki/YUV#Mathematical_derivations_and_formulas
public static void yuv2rgb(float y, float u, float v, float[] rgb) {
rgb[0] = 1 * y + 0 * u + 1.13983f * v;
rgb[1] = 1 * y + -.39465f * u + -.58060f * v;
rgb[2] = 1 * y + 2.03211f * u + 0 * v;
}
public static void rgb2yuv(float r, float g, float b, float[] yuv) {
yuv[0] = .299f * r + .587f * g + .114f * b;
yuv[1] = -.14713f * r + -.28886f * g + .436f * b;
yuv[2] = .615f * r + -.51499f * g + -.10001f * b;
}
private static float[] randYUVinRGBRange(float minComponent, float maxComponent) {
while(true) {
float y = rand.nextFloat(); // * YFRAC + 1-YFRAC);
float u = rand.nextFloat() * 2 * U_OFF - U_OFF;
float v = rand.nextFloat() * 2 * V_OFF - V_OFF;
float[] rgb = new float[3];
yuv2rgb(y, u, v, rgb);
float r = rgb[0], g = rgb[1], b = rgb[2];
if(0 <= r && r <= 1 &&
0 <= g && g <= 1 &&
0 <= b && b <= 1 &&
(r > minComponent || g > minComponent || b > minComponent) && // don't want all dark components
(r < maxComponent || g < maxComponent || b < maxComponent)) // don't want all light components
return new float[]{y, u, v};
}
}
private static float sqrdist(float[] a, float[] b) {
float sum = 0;
for(int i = 0; i < a.length; i++) {
float diff = a[i] - b[i];
sum += diff * diff;
}
return sum;
}
private static double worstFit(Color[] colors) {
float worst = 8888;
float[] a = new float[3], b = new float[3];
for(int i = 1; i < colors.length; i++) {
colors[i].getColorComponents(a);
for(int j = 0; j < i; j++) {
colors[j].getColorComponents(b);
float dist = sqrdist(a, b);
if(dist < worst) {
worst = dist;
}
}
}
return Math.sqrt(worst);
}
private static float[] randYUVBetterThan(float bestDistSqrd, float minComponent, float maxComponent, float[][] in) {
for(int attempt = 1; attempt < 100 * in.length; attempt++) {
float[] candidate = randYUVinRGBRange(minComponent, maxComponent);
boolean good = true;
for(int i = 0; i < in.length; i++)
if(sqrdist(candidate, in[i]) < bestDistSqrd)
good = false;
if(good)
return candidate;
}
return null; // after a bunch of passes, couldn't find a candidate that beat the best.
}
/**
* Simple example program.
*/
public static void main(String[] args) {
final int ncolors = 10;
Color[] colors = generateVisuallyDistinctColors(ncolors, .8f, .3f);
for(int i = 0; i < colors.length; i++) {
System.out.println(colors[i].toString());
}
System.out.println("Worst fit color = " + worstFit(colors));
}
}
For the sake of generations to come I add here the accepted answer in Python.
import numpy as np
import colorsys
def _get_colors(num_colors):
colors=[]
for i in np.arange(0., 360., 360. / num_colors):
hue = i/360.
lightness = (50 + np.random.rand() * 10)/100.
saturation = (90 + np.random.rand() * 10)/100.
colors.append(colorsys.hls_to_rgb(hue, lightness, saturation))
return colors
This OpenCV function uses the HSV color model to generate n evenly distributed colors around the 0<=H<=360º with maximum S=1.0 and V=1.0. The function outputs the BGR colors in bgr_mat:
void distributed_colors (int n, cv::Mat_<cv::Vec3f> & bgr_mat) {
cv::Mat_<cv::Vec3f> hsv_mat(n,CV_32F,cv::Vec3f(0.0,1.0,1.0));
double step = 360.0/n;
double h= 0.0;
cv::Vec3f value;
for (int i=0;i<n;i++,h+=step) {
value = hsv_mat.at<cv::Vec3f>(i);
hsv_mat.at<cv::Vec3f>(i)[0] = h;
}
cv::cvtColor(hsv_mat, bgr_mat, CV_HSV2BGR);
bgr_mat *= 255;
}
Pretty neat with seaborn for Python users:
>>> import seaborn as sns
>>> sns.color_palette(n_colors=4)
it returns list of RGB tuples:
[(0.12156862745098039, 0.4666666666666667, 0.7058823529411765),
(1.0, 0.4980392156862745, 0.054901960784313725),
(0.17254901960784313, 0.6274509803921569, 0.17254901960784313),
(0.8392156862745098, 0.15294117647058825, 0.1568627450980392)]
If N is big enough, you're going to get some similar-looking colors. There's only so many of them in the world.
Why not just evenly distribute them through the spectrum, like so:
IEnumerable<Color> CreateUniqueColors(int nColors)
{
int subdivision = (int)Math.Floor(Math.Pow(nColors, 1/3d));
for(int r = 0; r < 255; r += subdivision)
for(int g = 0; g < 255; g += subdivision)
for(int b = 0; b < 255; b += subdivision)
yield return Color.FromArgb(r, g, b);
}
If you want to mix up the sequence so that similar colors aren't next to each other, you could maybe shuffle the resulting list.
Am I underthinking this?
HSL color model may be well suited for "sorting" colors, but if you are looking for visually distinct colors you definitively need Lab color model instead.
CIELAB was designed to be perceptually uniform with respect to human color vision, meaning that the same amount of numerical change in these values corresponds to about the same amount of visually perceived change.
Once you know that, finding the optimal subset of N colors from a wide range of colors is still a (NP) hard problem, kind of similar to the Travelling salesman problem and all the solutions using k-mean algorithms or something won't really help.
That said, if N is not too big and if you start with a limited set of colors, you will easily find a very good subset of distincts colors according to a Lab distance with a simple random function.
I've coded such a tool for my own usage (you can find it here: https://mokole.com/palette.html), here is what I got for N=7:
It's all javascript so feel free to take a look on the source of the page and adapt it for your own needs.
Here's an idea. Imagine an HSV cylinder
Define the upper and lower limits you want for the Brightness and Saturation. This defines a square cross section ring within the space.
Now, scatter N points randomly within this space.
Then apply an iterative repulsion algorithm on them, either for a fixed number of iterations, or until the points stabilise.
Now you should have N points representing N colours that are about as different as possible within the colour space you're interested in.
Hugo
Pretty neat with seaborn for Python users:
>>> import seaborn as sns
>>> sns.color_palette(n_colors=4)
it returns list of RGB tuples:
[(0.12156862745098039, 0.4666666666666667, 0.7058823529411765),
(1.0, 0.4980392156862745, 0.054901960784313725),
(0.17254901960784313, 0.6274509803921569, 0.17254901960784313),
(0.8392156862745098, 0.15294117647058825, 0.1568627450980392)]
I would try to fix saturation and lumination to maximum and focus on hue only. As I see it, H can go from 0 to 255 and then wraps around. Now if you wanted two contrasting colours you would take the opposite sides of this ring, i.e. 0 and 128. If you wanted 4 colours, you would take some separated by 1/4 of the 256 length of the circle, i.e. 0, 64,128,192. And of course, as others suggested when you need N colours, you could just separate them by 256/N.
What I would add to this idea is to use a reversed representation of a binary number to form this sequence. Look at this:
0 = 00000000 after reversal is 00000000 = 0
1 = 00000001 after reversal is 10000000 = 128
2 = 00000010 after reversal is 01000000 = 64
3 = 00000011 after reversal is 11000000 = 192
... this way if you need N different colours you could just take first N numbers, reverse them, and you get as much distant points as possible (for N being power of two) while at the same time preserving that each prefix of the sequence differs a lot.
This was an important goal in my use case, as I had a chart where colors were sorted by area covered by this colour. I wanted the largest areas of the chart to have large contrast, and I was ok with some small areas to have colours similar to those from top 10, as it was obvious for the reader which one is which one by just observing the area.
Here's a solution to managed your "distinct" issue, which is entirely overblown:
Create a unit sphere and drop points on it with repelling charges. Run a particle system until they no longer move (or the delta is "small enough"). At this point, each of the points are as far away from each other as possible. Convert (x, y, z) to rgb.
I mention it because for certain classes of problems, this type of solution can work better than brute force.
I originally saw this approach here for tesselating a sphere.
Again, the most obvious solutions of traversing HSL space or RGB space will probably work just fine.
Here's an idea. Imagine an HSV cylinder
Define the upper and lower limits you want for the Brightness and Saturation. This defines a square cross section ring within the space.
Now, scatter N points randomly within this space.
Then apply an iterative repulsion algorithm on them, either for a fixed number of iterations, or until the points stabilise.
Now you should have N points representing N colours that are about as different as possible within the colour space you're interested in.
Hugo
Everyone seems to have missed the existence of the very useful YUV color space which was designed to represent perceived color differences in the human visual system. Distances in YUV represent differences in human perception. I needed this functionality for MagicCube4D which implements 4-dimensional Rubik's cubes and an unlimited numbers of other 4D twisty puzzles having arbitrary numbers of faces.
My solution starts by selecting random points in YUV and then iteratively breaking up the closest two points, and only converting to RGB when returning the result. The method is O(n^3) but that doesn't matter for small numbers or ones that can be cached. It can certainly be made more efficient but the results appear to be excellent.
The function allows for optional specification of brightness thresholds so as not to produce colors in which no component is brighter or darker than given amounts. IE you may not want values close to black or white. This is useful when the resulting colors will be used as base colors that are later shaded via lighting, layering, transparency, etc. and must still appear different from their base colors.
import java.awt.Color;
import java.util.Random;
/**
* Contains a method to generate N visually distinct colors and helper methods.
*
* @author Melinda Green
*/
public class ColorUtils {
private ColorUtils() {} // To disallow instantiation.
private final static float
U_OFF = .436f,
V_OFF = .615f;
private static final long RAND_SEED = 0;
private static Random rand = new Random(RAND_SEED);
/*
* Returns an array of ncolors RGB triplets such that each is as unique from the rest as possible
* and each color has at least one component greater than minComponent and one less than maxComponent.
* Use min == 1 and max == 0 to include the full RGB color range.
*
* Warning: O N^2 algorithm blows up fast for more than 100 colors.
*/
public static Color[] generateVisuallyDistinctColors(int ncolors, float minComponent, float maxComponent) {
rand.setSeed(RAND_SEED); // So that we get consistent results for each combination of inputs
float[][] yuv = new float[ncolors][3];
// initialize array with random colors
for(int got = 0; got < ncolors;) {
System.arraycopy(randYUVinRGBRange(minComponent, maxComponent), 0, yuv[got++], 0, 3);
}
// continually break up the worst-fit color pair until we get tired of searching
for(int c = 0; c < ncolors * 1000; c++) {
float worst = 8888;
int worstID = 0;
for(int i = 1; i < yuv.length; i++) {
for(int j = 0; j < i; j++) {
float dist = sqrdist(yuv[i], yuv[j]);
if(dist < worst) {
worst = dist;
worstID = i;
}
}
}
float[] best = randYUVBetterThan(worst, minComponent, maxComponent, yuv);
if(best == null)
break;
else
yuv[worstID] = best;
}
Color[] rgbs = new Color[yuv.length];
for(int i = 0; i < yuv.length; i++) {
float[] rgb = new float[3];
yuv2rgb(yuv[i][0], yuv[i][1], yuv[i][2], rgb);
rgbs[i] = new Color(rgb[0], rgb[1], rgb[2]);
//System.out.println(rgb[i][0] + "\t" + rgb[i][1] + "\t" + rgb[i][2]);
}
return rgbs;
}
public static void hsv2rgb(float h, float s, float v, float[] rgb) {
// H is given on [0->6] or -1. S and V are given on [0->1].
// RGB are each returned on [0->1].
float m, n, f;
int i;
float[] hsv = new float[3];
hsv[0] = h;
hsv[1] = s;
hsv[2] = v;
System.out.println("H: " + h + " S: " + s + " V:" + v);
if(hsv[0] == -1) {
rgb[0] = rgb[1] = rgb[2] = hsv[2];
return;
}
i = (int) (Math.floor(hsv[0]));
f = hsv[0] - i;
if(i % 2 == 0)
f = 1 - f; // if i is even
m = hsv[2] * (1 - hsv[1]);
n = hsv[2] * (1 - hsv[1] * f);
switch(i) {
case 6:
case 0:
rgb[0] = hsv[2];
rgb[1] = n;
rgb[2] = m;
break;
case 1:
rgb[0] = n;
rgb[1] = hsv[2];
rgb[2] = m;
break;
case 2:
rgb[0] = m;
rgb[1] = hsv[2];
rgb[2] = n;
break;
case 3:
rgb[0] = m;
rgb[1] = n;
rgb[2] = hsv[2];
break;
case 4:
rgb[0] = n;
rgb[1] = m;
rgb[2] = hsv[2];
break;
case 5:
rgb[0] = hsv[2];
rgb[1] = m;
rgb[2] = n;
break;
}
}
// From http://en.wikipedia.org/wiki/YUV#Mathematical_derivations_and_formulas
public static void yuv2rgb(float y, float u, float v, float[] rgb) {
rgb[0] = 1 * y + 0 * u + 1.13983f * v;
rgb[1] = 1 * y + -.39465f * u + -.58060f * v;
rgb[2] = 1 * y + 2.03211f * u + 0 * v;
}
public static void rgb2yuv(float r, float g, float b, float[] yuv) {
yuv[0] = .299f * r + .587f * g + .114f * b;
yuv[1] = -.14713f * r + -.28886f * g + .436f * b;
yuv[2] = .615f * r + -.51499f * g + -.10001f * b;
}
private static float[] randYUVinRGBRange(float minComponent, float maxComponent) {
while(true) {
float y = rand.nextFloat(); // * YFRAC + 1-YFRAC);
float u = rand.nextFloat() * 2 * U_OFF - U_OFF;
float v = rand.nextFloat() * 2 * V_OFF - V_OFF;
float[] rgb = new float[3];
yuv2rgb(y, u, v, rgb);
float r = rgb[0], g = rgb[1], b = rgb[2];
if(0 <= r && r <= 1 &&
0 <= g && g <= 1 &&
0 <= b && b <= 1 &&
(r > minComponent || g > minComponent || b > minComponent) && // don't want all dark components
(r < maxComponent || g < maxComponent || b < maxComponent)) // don't want all light components
return new float[]{y, u, v};
}
}
private static float sqrdist(float[] a, float[] b) {
float sum = 0;
for(int i = 0; i < a.length; i++) {
float diff = a[i] - b[i];
sum += diff * diff;
}
return sum;
}
private static double worstFit(Color[] colors) {
float worst = 8888;
float[] a = new float[3], b = new float[3];
for(int i = 1; i < colors.length; i++) {
colors[i].getColorComponents(a);
for(int j = 0; j < i; j++) {
colors[j].getColorComponents(b);
float dist = sqrdist(a, b);
if(dist < worst) {
worst = dist;
}
}
}
return Math.sqrt(worst);
}
private static float[] randYUVBetterThan(float bestDistSqrd, float minComponent, float maxComponent, float[][] in) {
for(int attempt = 1; attempt < 100 * in.length; attempt++) {
float[] candidate = randYUVinRGBRange(minComponent, maxComponent);
boolean good = true;
for(int i = 0; i < in.length; i++)
if(sqrdist(candidate, in[i]) < bestDistSqrd)
good = false;
if(good)
return candidate;
}
return null; // after a bunch of passes, couldn't find a candidate that beat the best.
}
/**
* Simple example program.
*/
public static void main(String[] args) {
final int ncolors = 10;
Color[] colors = generateVisuallyDistinctColors(ncolors, .8f, .3f);
for(int i = 0; i < colors.length; i++) {
System.out.println(colors[i].toString());
}
System.out.println("Worst fit color = " + worstFit(colors));
}
}
Here's a solution to managed your "distinct" issue, which is entirely overblown:
Create a unit sphere and drop points on it with repelling charges. Run a particle system until they no longer move (or the delta is "small enough"). At this point, each of the points are as far away from each other as possible. Convert (x, y, z) to rgb.
I mention it because for certain classes of problems, this type of solution can work better than brute force.
I originally saw this approach here for tesselating a sphere.
Again, the most obvious solutions of traversing HSL space or RGB space will probably work just fine.
Like Uri Cohen's answer, but is a generator instead. Will start by using colors far apart. Deterministic.
Sample, left colors first:
#!/usr/bin/env python3.5
from typing import Iterable, Tuple
import colorsys
import itertools
from fractions import Fraction
from pprint import pprint
def zenos_dichotomy() -> Iterable[Fraction]:
"""
http://en.wikipedia.org/wiki/1/2_%2B_1/4_%2B_1/8_%2B_1/16_%2B_%C2%B7_%C2%B7_%C2%B7
"""
for k in itertools.count():
yield Fraction(1,2**k)
def fracs() -> Iterable[Fraction]:
"""
[Fraction(0, 1), Fraction(1, 2), Fraction(1, 4), Fraction(3, 4), Fraction(1, 8), Fraction(3, 8), Fraction(5, 8), Fraction(7, 8), Fraction(1, 16), Fraction(3, 16), ...]
[0.0, 0.5, 0.25, 0.75, 0.125, 0.375, 0.625, 0.875, 0.0625, 0.1875, ...]
"""
yield Fraction(0)
for k in zenos_dichotomy():
i = k.denominator # [1,2,4,8,16,...]
for j in range(1,i,2):
yield Fraction(j,i)
# can be used for the v in hsv to map linear values 0..1 to something that looks equidistant
# bias = lambda x: (math.sqrt(x/3)/Fraction(2,3)+Fraction(1,3))/Fraction(6,5)
HSVTuple = Tuple[Fraction, Fraction, Fraction]
RGBTuple = Tuple[float, float, float]
def hue_to_tones(h: Fraction) -> Iterable[HSVTuple]:
for s in [Fraction(6,10)]: # optionally use range
for v in [Fraction(8,10),Fraction(5,10)]: # could use range too
yield (h, s, v) # use bias for v here if you use range
def hsv_to_rgb(x: HSVTuple) -> RGBTuple:
return colorsys.hsv_to_rgb(*map(float, x))
flatten = itertools.chain.from_iterable
def hsvs() -> Iterable[HSVTuple]:
return flatten(map(hue_to_tones, fracs()))
def rgbs() -> Iterable[RGBTuple]:
return map(hsv_to_rgb, hsvs())
def rgb_to_css(x: RGBTuple) -> str:
uint8tuple = map(lambda y: int(y*255), x)
return "rgb({},{},{})".format(*uint8tuple)
def css_colors() -> Iterable[str]:
return map(rgb_to_css, rgbs())
if __name__ == "__main__":
# sample 100 colors in css format
sample_colors = list(itertools.islice(css_colors(), 100))
pprint(sample_colors)
I have written a package for R called qualpalr that is designed specifically for this purpose. I recommend you look at the vignette to find out how it works, but I will try to summarize the main points.
qualpalr takes a specification of colors in the HSL color space (which was described previously in this thread), projects it to the DIN99d color space (which is perceptually uniform) and find the n that maximize the minimum distance between any oif them.
# Create a palette of 4 colors of hues from 0 to 360, saturations between
# 0.1 and 0.5, and lightness from 0.6 to 0.85
pal <- qualpal(n = 4, list(h = c(0, 360), s = c(0.1, 0.5), l = c(0.6, 0.85)))
# Look at the colors in hex format
pal$hex
#> [1] "#6F75CE" "#CC6B76" "#CAC16A" "#76D0D0"
# Create a palette using one of the predefined color subspaces
pal2 <- qualpal(n = 4, colorspace = "pretty")
# Distance matrix of the DIN99d color differences
pal2$de_DIN99d
#> #69A3CC #6ECC6E #CA6BC4
#> 6ECC6E 22
#> CA6BC4 21 30
#> CD976B 24 21 21
plot(pal2)
Here's a solution to managed your "distinct" issue, which is entirely overblown:
Create a unit sphere and drop points on it with repelling charges. Run a particle system until they no longer move (or the delta is "small enough"). At this point, each of the points are as far away from each other as possible. Convert (x, y, z) to rgb.
I mention it because for certain classes of problems, this type of solution can work better than brute force.
I originally saw this approach here for tesselating a sphere.
Again, the most obvious solutions of traversing HSL space or RGB space will probably work just fine.
HSL color model may be well suited for "sorting" colors, but if you are looking for visually distinct colors you definitively need Lab color model instead.
CIELAB was designed to be perceptually uniform with respect to human color vision, meaning that the same amount of numerical change in these values corresponds to about the same amount of visually perceived change.
Once you know that, finding the optimal subset of N colors from a wide range of colors is still a (NP) hard problem, kind of similar to the Travelling salesman problem and all the solutions using k-mean algorithms or something won't really help.
That said, if N is not too big and if you start with a limited set of colors, you will easily find a very good subset of distincts colors according to a Lab distance with a simple random function.
I've coded such a tool for my own usage (you can find it here: https://mokole.com/palette.html), here is what I got for N=7:
It's all javascript so feel free to take a look on the source of the page and adapt it for your own needs.
Here's an idea. Imagine an HSV cylinder
Define the upper and lower limits you want for the Brightness and Saturation. This defines a square cross section ring within the space.
Now, scatter N points randomly within this space.
Then apply an iterative repulsion algorithm on them, either for a fixed number of iterations, or until the points stabilise.
Now you should have N points representing N colours that are about as different as possible within the colour space you're interested in.
Hugo
This questions appears in quite a few SO discussions:
- Algorithm For Generating Unique Colors
- Generate unique colours
- Generate distinctly different RGB colors in graphs
- How to generate n different colors for any natural number n?
Different solutions are proposed, but none are optimal. Luckily, science comes to the rescue
Arbitrary N
- Colour displays for categorical images (free download)
- A WEB SERVICE TO PERSONALISE MAP COLOURING (free download, a webservice solution should be available by next month)
- An Algorithm for the Selection of High-Contrast Color Sets (the authors offer a free C++ implementation)
- High-contrast sets of colors (The first algorithm for the problem)
The last 2 will be free via most university libraries / proxies.
N is finite and relatively small
In this case, one could go for a list solution. A very interesting article in the subject is freely available:
There are several color lists to consider:
- Boynton's list of 11 colors that are almost never confused (available in the first paper of the previous section)
- Kelly's 22 colors of maximum contrast (available in the paper above)
I also ran into this Palette by an MIT student. Lastly, The following links may be useful in converting between different color systems / coordinates (some colors in the articles are not specified in RGB, for instance):
- http://chem8.org/uch/space-55036-do-blog-id-5333.html
- https://metacpan.org/pod/Color::Library::Dictionary::NBS_ISCC
- Color Theory: How to convert Munsell HVC to RGB/HSB/HSL
For Kelly's and Boynton's list, I've already made the conversion to RGB (with the exception of white and black, which should be obvious). Some C# code:
public static ReadOnlyCollection<Color> KellysMaxContrastSet
{
get { return _kellysMaxContrastSet.AsReadOnly(); }
}
private static readonly List<Color> _kellysMaxContrastSet = new List<Color>
{
UIntToColor(0xFFFFB300), //Vivid Yellow
UIntToColor(0xFF803E75), //Strong Purple
UIntToColor(0xFFFF6800), //Vivid Orange
UIntToColor(0xFFA6BDD7), //Very Light Blue
UIntToColor(0xFFC10020), //Vivid Red
UIntToColor(0xFFCEA262), //Grayish Yellow
UIntToColor(0xFF817066), //Medium Gray
//The following will not be good for people with defective color vision
UIntToColor(0xFF007D34), //Vivid Green
UIntToColor(0xFFF6768E), //Strong Purplish Pink
UIntToColor(0xFF00538A), //Strong Blue
UIntToColor(0xFFFF7A5C), //Strong Yellowish Pink
UIntToColor(0xFF53377A), //Strong Violet
UIntToColor(0xFFFF8E00), //Vivid Orange Yellow
UIntToColor(0xFFB32851), //Strong Purplish Red
UIntToColor(0xFFF4C800), //Vivid Greenish Yellow
UIntToColor(0xFF7F180D), //Strong Reddish Brown
UIntToColor(0xFF93AA00), //Vivid Yellowish Green
UIntToColor(0xFF593315), //Deep Yellowish Brown
UIntToColor(0xFFF13A13), //Vivid Reddish Orange
UIntToColor(0xFF232C16), //Dark Olive Green
};
public static ReadOnlyCollection<Color> BoyntonOptimized
{
get { return _boyntonOptimized.AsReadOnly(); }
}
private static readonly List<Color> _boyntonOptimized = new List<Color>
{
Color.FromArgb(0, 0, 255), //Blue
Color.FromArgb(255, 0, 0), //Red
Color.FromArgb(0, 255, 0), //Green
Color.FromArgb(255, 255, 0), //Yellow
Color.FromArgb(255, 0, 255), //Magenta
Color.FromArgb(255, 128, 128), //Pink
Color.FromArgb(128, 128, 128), //Gray
Color.FromArgb(128, 0, 0), //Brown
Color.FromArgb(255, 128, 0), //Orange
};
static public Color UIntToColor(uint color)
{
var a = (byte)(color >> 24);
var r = (byte)(color >> 16);
var g = (byte)(color >> 8);
var b = (byte)(color >> 0);
return Color.FromArgb(a, r, g, b);
}
And here are the RGB values in hex and 8-bit-per-channel representations:
kelly_colors_hex = [
0xFFB300, # Vivid Yellow
0x803E75, # Strong Purple
0xFF6800, # Vivid Orange
0xA6BDD7, # Very Light Blue
0xC10020, # Vivid Red
0xCEA262, # Grayish Yellow
0x817066, # Medium Gray
# The following don't work well for people with defective color vision
0x007D34, # Vivid Green
0xF6768E, # Strong Purplish Pink
0x00538A, # Strong Blue
0xFF7A5C, # Strong Yellowish Pink
0x53377A, # Strong Violet
0xFF8E00, # Vivid Orange Yellow
0xB32851, # Strong Purplish Red
0xF4C800, # Vivid Greenish Yellow
0x7F180D, # Strong Reddish Brown
0x93AA00, # Vivid Yellowish Green
0x593315, # Deep Yellowish Brown
0xF13A13, # Vivid Reddish Orange
0x232C16, # Dark Olive Green
]
kelly_colors = dict(vivid_yellow=(255, 179, 0),
strong_purple=(128, 62, 117),
vivid_orange=(255, 104, 0),
very_light_blue=(166, 189, 215),
vivid_red=(193, 0, 32),
grayish_yellow=(206, 162, 98),
medium_gray=(129, 112, 102),
# these aren't good for people with defective color vision:
vivid_green=(0, 125, 52),
strong_purplish_pink=(246, 118, 142),
strong_blue=(0, 83, 138),
strong_yellowish_pink=(255, 122, 92),
strong_violet=(83, 55, 122),
vivid_orange_yellow=(255, 142, 0),
strong_purplish_red=(179, 40, 81),
vivid_greenish_yellow=(244, 200, 0),
strong_reddish_brown=(127, 24, 13),
vivid_yellowish_green=(147, 170, 0),
deep_yellowish_brown=(89, 51, 21),
vivid_reddish_orange=(241, 58, 19),
dark_olive_green=(35, 44, 22))
For all you Java developers, here are the JavaFX colors:
// Don't forget to import javafx.scene.paint.Color;
private static final Color[] KELLY_COLORS = {
Color.web("0xFFB300"), // Vivid Yellow
Color.web("0x803E75"), // Strong Purple
Color.web("0xFF6800"), // Vivid Orange
Color.web("0xA6BDD7"), // Very Light Blue
Color.web("0xC10020"), // Vivid Red
Color.web("0xCEA262"), // Grayish Yellow
Color.web("0x817066"), // Medium Gray
Color.web("0x007D34"), // Vivid Green
Color.web("0xF6768E"), // Strong Purplish Pink
Color.web("0x00538A"), // Strong Blue
Color.web("0xFF7A5C"), // Strong Yellowish Pink
Color.web("0x53377A"), // Strong Violet
Color.web("0xFF8E00"), // Vivid Orange Yellow
Color.web("0xB32851"), // Strong Purplish Red
Color.web("0xF4C800"), // Vivid Greenish Yellow
Color.web("0x7F180D"), // Strong Reddish Brown
Color.web("0x93AA00"), // Vivid Yellowish Green
Color.web("0x593315"), // Deep Yellowish Brown
Color.web("0xF13A13"), // Vivid Reddish Orange
Color.web("0x232C16"), // Dark Olive Green
};
the following is the unsorted kelly colors according to the order above.
the following is the sorted kelly colors according to hues (note that some yellows are not very contrasting)
Here's an idea. Imagine an HSV cylinder
Define the upper and lower limits you want for the Brightness and Saturation. This defines a square cross section ring within the space.
Now, scatter N points randomly within this space.
Then apply an iterative repulsion algorithm on them, either for a fixed number of iterations, or until the points stabilise.
Now you should have N points representing N colours that are about as different as possible within the colour space you're interested in.
Hugo
Source: Stackoverflow.com