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C# Tutorial: How To Convert a Color Image To Grayscale

Andraz Krzisnik
C# Tutorial: How To Convert a Color Image To...

Grayscale images are basically black and white images. These images are good for programming, since their pixels hold only intensity values or in other words, shades of gray, they use up less information to process (8-bits per pixel, to be exact).

How many shades of grey are there?

For an 8-bit grayscale image, a pixel can have 255 different shades of gray, including black and white. A shade of gray is created when RGB values of a pixel are the same. These values will go from 0 to 255, which means if a pixel holds 0 for red, green and blue, it will show as a complete black color. And if it will hold 255 for all three, you will see it as a white color. Everything in between will appear as a shade of gray.

Converting a image to grayscale

When converting an image, intensity of a pixel is fractioned for each color (Red, Green, Blue). All fractioned numbers sum up to 1 (more or less).

Equation for converting pixel colors to grayscale follows

?(?,?) = 0,21 ∙ ?(?,?) + 0,71 ∙ ?(?,?) + 0,071 ∙ ?(?,?)

When calculated, RGB values in a pixel are replaced by Y.

The largest fraction is assigned to green color, because human eyes are most sensitive to green and the lowest fraction is assigned to blue color, because … you guessed it, human vision is least sensitive to blue.

GetPixel Method

for (int i = 0; i < bmp.Width; i++)
    for (int j = 0; j < bmp.Height; j++)
         Color c = bmp.GetPixel(i, j);

         //Apply conversion equation
         byte gray = (byte)(.21 * c.R + .71 * c.G + .071 * c.B);

         //Set the color of this pixel
         bmp.SetPixel(i, j, Color.FromArgb(gray, gray, gray));

LockBits Method

//Lock bitmap's bits to system memory
Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, bmp.PixelFormat);

//Scan for the first line
IntPtr ptr = bmpData.Scan0;

//Declare an array in which your RGB values will be stored
int bytes = Math.Abs(bmpData.Stride) * bmp.Height;
byte[] rgbValues = new byte[bytes];

//Copy RGB values in that array
Marshal.Copy(ptr, rgbValues, 0, bytes);

for (int i = 0; i < rgbValues.Length; i += 3)
    //Set RGB values in a Array where all RGB values are stored
    byte gray = (byte)(rgbValues[i] * .21 + rgbValues[i + 1] * .71 + rgbValues[i + 2] * .071);
    rgbValues[i] = rgbValues[i + 1] = rgbValues[i + 2] = gray;

//Copy changed RGB values back to bitmap
Marshal.Copy(rgbValues, 0, ptr, bytes);

//Unlock the bits

The GetPixel method converts pixel colors directly in a bitmap form, which takes longer time to convert an entire image as it would take with LockBits method, which only changes array values.

Speaking of downsides, LockBits on the other hand needs a filter for each image format, because some formats (.PNG for example) will store 4 values instead of 3 (they include alpha channel for transparency).

In this particular program there is no filter applied, so images will have to be of format that carry only RGB values, like .JPG.

Complete project is available here (made with Visual Studio 2015)

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