Determining percentage of colors in a bitmap using C# without getPixel

To keep things simple, let's suppose we have a black and white bitmap image.
I will not use getPixel method here, Bitmap.getPixel() is a very expensive method, and it alone can turn your program into a lazy snail.

Please note that I am using a Format24bpprgb image format here, 24BPPRGB can be defined as:
Each pixel in the image is represented by 24 bits. First 8 bits represent red color, next 8 bits represent green color, and last 8 bits represent blue color in a bitmap.

Another popular format is 32BPPARGB, in this format the first 8 bits represent "Alpha" or the opacity of pixel, next 3 bytes or 24 bits describe the RGB components same like 24BPPRGB.

Given below is the C# language source code for checking pixel colors without Bitmap.getPixel() method.



public void 

calculateBlackAndWhitePixels

(string strFileName, ref int nBlackCount, ref int nWhiteCount,  ref int nTotalPixels)

{

Image image = null;

Bitmap bmpCrop = null;

BitmapData bmpData = null;

byte[] imgData = null;

int n = 0;

      

try

{

image = new Bitmap(strFileName);

bmpCrop = new Bitmap(image);

             

nBlackPercent = 0;

nWhitePercent = 0;

             

for (int h = 0; h < bmpCrop.Height; h++)// scan through all rows of image

{
       // This part is a very fast replacement of getPixel() method

bmpData = bmpCrop.LockBits(new Rectangle(0, h, bmpCrop.Width, 1),

System.Drawing.Imaging.ImageLockMode.ReadOnly, image.PixelFormat);

imgData = new byte[bmpData.Stride];

System.Runtime.InteropServices.Marshal.Copy(bmpData.Scan0, imgData, 0

, imgData.Length);

bmpCrop.UnlockBits(bmpData);

for (n = 0; n < imgData.Length - 3; n += 3)// scanning width of image

{

if ((int)imgData[n] == 0 && (int)imgData[n + 1] == 0 && (int)imgData[n + 2] == 0)// Color black is #000000

{

 nBlackCount++;

}

else

{

 nWhiteCount++;

}

}

nTotalPixels += n;

}

             

string.Format("{0} pixels are black, {1} are white(or non black at least).", nBlackCount, nWhiteCount);

}

catch(Exception e)

{

MessageBox.Show(ex.Message);

}     

}

Now that you have the number of total pixels inside the image, the white pixels, and the black pixels calculating the percentages is trivial, do it as a home work :)

Note:
The code is copied from a working software, their might be a small typo but over all it is supposed to work.
Please feel free to ask questions and post comments.

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