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Image Transformation: Grayscale to Color

, 6 May 2009
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Algorithm for easy transformation images

Introduction  

This is an easy algorithm of image transformation from grayscale image to color. It's far from ideal, but it is very simple.

Background

Usually we calculate gray color as:

Gray  = Green * 0.59 + Blue * 0.30 + Red * 0.11;

Each color with structure [Gray, Gray, Gray] has a set of colors:

G = [Gray, Gray, Gray]
G -> P = {C}, for each C from P:  Green * 0.59 + Blue * 0.30 + Red * 0.11 = Gray.

First of all, we create a set of control points. Control point is equivalent of "gray" color and "full" color. When we have control points set, we can approximate any "gray" color to "full" color. If "gray" color is located between two control points C1 and C2, then the color approximates as:

K = (Gray - C1<sub>Gray</sub>)/(C2<sub>Gray </sub>- C1<sub>Gray</sub>)
Red = C1<sub>Red </sub>+ K*(C2<sub>Red </sub>- C1<sub>Red</sub>)
Green = C1<sub>Green </sub>+ K*(C2<sub>Green </sub>- C1<sub>Green</sub>)
Blue = C1<sub>Blue </sub>+ K*(C2.<sub>Blue </sub>- C1<sub>Blue</sub>)

Now we has "full color" equivalent for each "gray" color.

scheme_2.jpg

Figure 1. Transformation schema.

Using the Code  

ControlPoint is a structure for storing data about control points.

public struct ControlPoint
{
    private int level;
     public int Level
    {
        get { return level; }
        set { level = value; }
    }
    private Color color;
     public Color Color
    {
        get { return color; }
        set { color = value; }
    }
     public ControlPoint(int level, Color color)
    {
        this.color = color;
        this.level = level;
    }
     public override string ToString()
    {
        return "Level: " + level.ToString() + "; Color: " + color.ToString();
    }
} 

PointsComparer is a class for comparing two control points:

public class PointsComparer: IComparer<ControlPoint>
{
    #region IComparer<ControlPoint> Members
     public int Compare(ControlPoint x, ControlPoint y)
    {
        if (x.Level > y.Level)
        {
            return 1;
        }
        if (x.Level < y.Level)
        {
            return -1;
        }
        return 0;
    }
     #endregion
} 

ColorBuilder is a class for building a color diagram. It returns an array of 256 colors - our diagram.

public static Color[] GetColorDiagram(List<ControlPoint> points)
{
    Color[] colors = new Color[256];
    points.Sort(new PointsComparer());
     for (int i = 0; i < 256; i++)
    {
        ControlPoint leftColor = new ControlPoint
				(0, GetNearestLeftColor(points[0].Color)); 
        ControlPoint rightColor = new ControlPoint
			(255, GetNearestRigthColor(points[points.Count - 1].Color));
        if (i < points[0].Level)
        {
            rightColor = points[0];
        }
        if (i > points[points.Count - 1].Level)
        {
            leftColor = points[points.Count - 1];
        }
        else
        {
            for (int j = 0; j < points.Count - 1; j++)
            {
                if ((points[j].Level <= i) & (points[j + 1].Level > i))
                {
                    leftColor = points[j];
                    rightColor = points[j + 1];
                }
            }
        }
        if ((rightColor.Level - leftColor.Level) != 0)
        {
            double koef = (double)(i - leftColor.Level) / 
			(double)(rightColor.Level - leftColor.Level);
            int r = leftColor.Color.R + (int)(koef * 
			(rightColor.Color.R - leftColor.Color.R));
            int g = leftColor.Color.G + (int)(koef * 
			(rightColor.Color.G - leftColor.Color.G));
            int b = leftColor.Color.B + (int)(koef * 
			(rightColor.Color.B - leftColor.Color.B));
            colors[i] = Color.FromArgb(r, g, b);
        }
        else
        {
            colors[i] = leftColor.Color;
        }
     }
     return colors;
} 

Now we can get the color image:

colorBitmap = new Bitmap(sourceBitmap);
for (int i = 0; i < sourceBitmap.Width; i++)
{
    for (int j = 0; j < sourceBitmap.Height; j++)
    {
        int level = sourceBitmap.GetPixel(i, j).B;
        colorBitmap.SetPixel(i, j, colors[level]);
    }
}
pbxColorImage.Image = colorBitmap;

ColorDialog.JPG

Figure 2. Simple color dialog.

GrayColor.JPG

Figure 3. Grayscale image, color image and color diagram.

Points of Interest  

The algorithms is very easy, and has many defects. But it is a good example for beginners in image processing. 

Update Information

  1. New formula for grayscale level calculation
  2. Bug with empty list was removed

License

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

About the Author

Maxim_Barsuk
Software Developer
Russian Federation Russian Federation
Hello! My name is Maxim Subbotin.
 
Now I work in sphere of web-development. I'm interesting researches in SEO field.
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Comments and Discussions

 
Questionabout algorithm PinmemberNorhaat18-Dec-11 13:44 
Generalgrayscale to color on HSI model Pinmemberthanh_rio27-Jun-09 0:28 
GeneralRe: grayscale to color on HSI model PinmemberMaxim_Barsuk28-Jun-09 19:16 
QuestionCouldnt you make it better??? Pinmembergonzospy1-May-09 6:26 
AnswerRe: Couldnt you make it better??? PinmemberMaxim_Barsuk3-May-09 19:45 
GeneralRe: Couldnt you make it better??? Pinmembergonzospy5-May-09 18:25 
GeneralRe: Couldnt you make it better??? [modified] PinmemberMaxim_Barsuk6-May-09 19:08 
GeneralКруто PinmemberLaserson26-Jan-09 20:44 
GeneralRe: Круто PinmemberMaxim_Barsuk27-Jan-09 4:53 
GeneralSomething to go off of.... PinmemberSkully102226-Jan-09 4:34 
GeneralRe: Something to go off of.... PinmemberMaxim_Barsuk26-Jan-09 6:10 
GeneralRe: Something to go off of.... PinmemberMicroImaging3-Jun-09 7:13 
GeneralMy vote of 2 PinmemberMadZookeeper26-Jan-09 2:37 
Questionwhat about color to grayscale? Pinmemberbluish24-Jan-09 8:26 
AnswerRe: what about color to grayscale? PinmemberMaxim_Barsuk25-Jan-09 19:24 
GeneralLuminace from RGB Pinmemberterrymohre25-Nov-08 3:22 
GeneralRe: Luminace from RGB PinmemberMaxim_Barsuk25-Nov-08 5:55 
GeneralMy vote of 1 PinmemberChristian Wikander25-Nov-08 1:59 
QuestionWhy? PinmemberChristian Wikander25-Nov-08 1:58 
GeneralBugs PinmemberAlexander Yakovlev21-Nov-08 1:30 
GeneralRe: Bugs PinmemberMaxim_Barsuk21-Nov-08 5:39 
NewsRe: Bugs Pinmemberthanh_rio4-Jun-09 6:55 
GeneralRe: Bugs PinmemberMaxim_Barsuk4-Jun-09 19:08 

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