Question

I was trying to create simple program that would perform some trivial operation with given image.
For some reason, it works only for the first time (when the app is launched):

//pseudo code

Bitmap im=Bitmap.FromFile("D:\\x.BMP");
Color [,] ColorArray=new [im.Width,im.Height];
private override voide OnPaint(EventArgs e)
{

  for(int X=0;X<im.Width;X++)
  {
      for(int Y=0;Y<im.Height;Y++)
      {
        ColorArray[X,Y]=im.GetPixel[X,Y];
      }

  }
  for(int X=0;X<im.Width;X++)
  {
      for(int Y=0;Y<im.Height;Y++)
      {
        Color c=ColorArray[X,Y];
        ...
        //some code that adds 100 to R,G,B
        im.SetPixel(X,Y,c);
      }

  }


  e.Graphics.DrawImage(im);
}

Answer 1

[EDIT]

I have modified my sample code. With the previous version, it could be hard to see the difference from one iteration to the next (especially with the bmp I was using for testing). This one allows you to change color gradually by pressing 1, 2, or 3 on the Num Pad. As others have mentioned, SetPixel/GetPixel is slow, so each KeyUp event does take some time to run.

This works fine for me.

Create a new Windows Forms project. Drop this code in. Make sure you have “x.bmp” available. Each time you press 1, 2, or 3 on the number pad (you might need turn on Num Lock), the colors of the bitmap change slightly:

  public partial class Form1 : Form
  {
    Bitmap bm;
    Color [,] colors;

    public Form1()
    {
      bm = (Bitmap)Bitmap.FromFile("x.bmp");
      colors = new Color[bm.Width, bm.Height];

      InitializeComponent();
    }

    private void Form1_Paint(object sender, PaintEventArgs e)
    {
      e.Graphics.DrawImage(bm,new Point(0,0));
    }

    private void Form1_KeyUp(object sender, KeyEventArgs e)
    {
      //Change the colors a little bit on each KeyUp.
      //On NumPad1, change red.
      //On NumPad2, change green.
      //On NumPad3, change blue.
      //Mod with 256 to ensure a value between 0 and 255.
      int rInc = 0;
      int gInc = 0;
      int bInc = 0;

      switch (e.KeyCode)
      {
        case Keys.NumPad1:
          rInc = 20;
          break;
        case Keys.NumPad2:
          gInc = 20;
          break;
        case Keys.NumPad3:
          bInc = 20;
          break;
        default:
          break;
      }

      for (int x = 0; x < bm.Width; x++)
      {
        for (int y = 0; y < bm.Height; y++)
        {
          colors[x, y] = bm.GetPixel(x, y);
        }
      }

      for (int x = 0; x < bm.Width; x++)
      {
        for (int y = 0; y < bm.Height; y++)
        {
          Color c = colors[x, y];

          int r = (c.R + rInc) % 256;
          int g = (c.G + gInc) % 256;
          int b = (c.B + bInc) % 256;

          c = Color.FromArgb(255, r, g, b);
          bm.SetPixel(x, y, c);
        }
      }

      Invalidate();
    }

  }

It’s not very sophisticated, but it does what you asked with real code that closely matches your pseudocode.