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Count Number of Unique Colors in an Image

, 19 Feb 2011 CPOL
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Determine the number of unique colors in an image
Got an image? Want to know how many unique colors are contained in it?

public static int CountImageColors(string fileName)
    int count = 0;
    HashSet<Color> colors = new HashSet<Color>();
    Bitmap bmp = null;
    if (File.Exists(fileName))
            bmp = new Bitmap(fileName);
            if (bmp != null)
                for (int y = 0; y < bmp.Size.Height; y++)
                    for (int x = 0; x < bmp.Size.Width; x++)
                        colors.Add(bmp.GetPixel(x, y));
                count = colors.Count;
    return count;

Since a HashSet doesn't allow duplicate keys, we use the color as the key, and any duplicates will not be added. This will result in a collection of unique colors, and we use the Count property to get that value.

Keep in mind the example provided above is merely illustrating the technique of using a HashSet to keep track of unique colors, and is not necessarily a reasonable implementation of the method itself in your case.

It may not be the fastest way to do it, but it works great, and the general technique illustrated above can be used in pretty much any language that supports a HashSet-style collection.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

John Simmons / outlaw programmer
Software Developer (Senior)
United States United States
I've been paid as a programmer since 1982 with experience in Pascal, and C++ (both self-taught), and began writing Windows programs in 1991 using Visual C++ and MFC. In the 2nd half of 2007, I started writing C# Windows Forms and ASP.Net applications, and have since done WPF, Silverlight, WCF, web services, and Windows services.

My weakest point is that my moments of clarity are too brief to hold a meaningful conversation that requires more than 30 seconds to complete. Thankfully, grunts of agreement are all that is required to conduct most discussions without committing to any particular belief system.

Comments and Discussions

QuestionI need to find the total no of dot images in the image file of the same color.How it can be possible..?? PinmemberMember 1055062226-Jan-14 22:46 
AnswerRe: I need to find the total no of dot images in the image file of the same color.How it can be possible..?? PinmemberJohn Simmons / outlaw programmer21-Jul-14 5:36 
QuestionColor Counting - GetPixel is slow, so... Pinmemberjma24007-Mar-13 21:00 
AnswerRe: Color Counting - GetPixel is slow, so... PinmemberJohn Simmons / outlaw programmer8-Mar-13 0:34 
GeneralRe: My mistake then - I didn't dive into the docs for HashSet de... PinadminChris Maunder21-Feb-11 16:11 
GeneralReason for my vote of 5 I like this concept and solution, th... PinmemberDrABELL22-Feb-11 17:49 
GeneralReason for my vote of 5 I like it PinmvpRaviRanjankr19-Feb-11 9:28 
GeneralMost images are going to have many, many repeated colours so... PinadminChris Maunder18-Feb-11 18:49 
GeneralRe: I don't understand your question. The try/catch was for the ... PinmvpJohn Simmons / outlaw programmer19-Feb-11 1:56 
GeneralReason for my vote of 5 Thanks for Sharing. Pinmemberlinuxjr18-Feb-11 10:46 
GeneralReason for my vote of 5 Sweet. PinmentorHans Dietrich18-Feb-11 7:51 

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