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Image Transformation: Grayscale to Color

, 6 May 2009 CPOL
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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)

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About the Author

Maxim_Barsuk
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Hello! My name is Maxim Subbotin.

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Comments and Discussions

 
Questionabout algorithm Pin
Norhaat18-Dec-11 14:44
memberNorhaat18-Dec-11 14:44 
Generalgrayscale to color on HSI model Pin
thanh_rio27-Jun-09 1:28
memberthanh_rio27-Jun-09 1:28 
GeneralRe: grayscale to color on HSI model Pin
Maxim_Barsuk28-Jun-09 20:16
memberMaxim_Barsuk28-Jun-09 20:16 
QuestionCouldnt you make it better??? Pin
gonzospy1-May-09 7:26
membergonzospy1-May-09 7:26 
AnswerRe: Couldnt you make it better??? Pin
Maxim_Barsuk3-May-09 20:45
memberMaxim_Barsuk3-May-09 20:45 
GeneralRe: Couldnt you make it better??? Pin
gonzospy5-May-09 19:25
membergonzospy5-May-09 19:25 
GeneralRe: Couldnt you make it better??? [modified] Pin
Maxim_Barsuk6-May-09 20:08
memberMaxim_Barsuk6-May-09 20:08 
GeneralКруто Pin
Laserson26-Jan-09 21:44
memberLaserson26-Jan-09 21:44 
GeneralRe: Круто Pin
Maxim_Barsuk27-Jan-09 5:53
memberMaxim_Barsuk27-Jan-09 5:53 
GeneralSomething to go off of.... Pin
Skully102226-Jan-09 5:34
memberSkully102226-Jan-09 5:34 
GeneralRe: Something to go off of.... Pin
Maxim_Barsuk26-Jan-09 7:10
memberMaxim_Barsuk26-Jan-09 7:10 
GeneralRe: Something to go off of.... Pin
MicroImaging3-Jun-09 8:13
memberMicroImaging3-Jun-09 8:13 
GeneralMy vote of 2 Pin
MadZookeeper26-Jan-09 3:37
memberMadZookeeper26-Jan-09 3:37 
Questionwhat about color to grayscale? Pin
bluish24-Jan-09 9:26
memberbluish24-Jan-09 9:26 
AnswerRe: what about color to grayscale? Pin
Maxim_Barsuk25-Jan-09 20:24
memberMaxim_Barsuk25-Jan-09 20:24 
GeneralLuminace from RGB Pin
terrymohre25-Nov-08 4:22
memberterrymohre25-Nov-08 4:22 
GeneralRe: Luminace from RGB Pin
Maxim_Barsuk25-Nov-08 6:55
memberMaxim_Barsuk25-Nov-08 6:55 
GeneralMy vote of 1 Pin
Christian Wikander25-Nov-08 2:59
memberChristian Wikander25-Nov-08 2:59 
QuestionWhy? Pin
Christian Wikander25-Nov-08 2:58
memberChristian Wikander25-Nov-08 2:58 
GeneralBugs Pin
Alexander Yakovlev21-Nov-08 2:30
memberAlexander Yakovlev21-Nov-08 2:30 
GeneralRe: Bugs Pin
Maxim_Barsuk21-Nov-08 6:39
memberMaxim_Barsuk21-Nov-08 6:39 
NewsRe: Bugs Pin
thanh_rio4-Jun-09 7:55
memberthanh_rio4-Jun-09 7:55 
GeneralRe: Bugs Pin
Maxim_Barsuk4-Jun-09 20:08
memberMaxim_Barsuk4-Jun-09 20:08 

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