How to process image pixel-by-pixel with WinAPI in a FAST WAY?

Yep, Get/SetPixel functions are very slow. Use Bitmap.LockBits() / UnlockBits() instead. It returns the raw bit data for you to manipulate.

From the msdn reference:

private void LockUnlockBitsExample(PaintEventArgs e){    // Create a new bitmap.    Bitmap bmp = new Bitmap("c:\\fakePhoto.jpg");    // Lock the bitmap's bits.      Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);    System.Drawing.Imaging.BitmapData bmpData =         bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,        bmp.PixelFormat);    // Get the address of the first line.   IntPtr ptr = bmpData.Scan0;    // Declare an array to hold the bytes of the bitmap.    // This code is specific to a bitmap with 24 bits per pixels.    int bytes = bmp.Width * bmp.Height * 3;    byte[] rgbValues = new byte[bytes];    // Copy the RGB values into the array.    System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);    // Set every red value to 255.      for (int counter = 2; counter < rgbValues.Length; counter+=3)        rgbValues[counter] = 255;    // Copy the RGB values back to the bitmap    System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, bytes);    // Unlock the bits.    bmp.UnlockBits(bmpData);    // Draw the modified image.    e.Graphics.DrawImage(bmp, 0, 150);}