Question

Anyone know whats this problem?

I got this warning Field xxx is never assigned to, and will always have its default value null on private static Quantizer quantit;

I dont know what to do to fix, cause when I try to use quantit.Quantize() debug says: “Object reference not set to an instance of an object.” and point to au = quantit.Quantize();

The code:

public class Quantization : System.Windows.Forms.Form
{ 
    private static Quantizer quantit;

    private Button btnLoad;
    private PictureBox imgPhoto;

    public Quantization()
    {

        btnLoad = new Button();
        btnLoad.Text = "&Load";
        btnLoad.Left = 10;
        btnLoad.Top = 10;
        btnLoad.Click += new System.EventHandler(this.OnLoadClick);

        imgPhoto = new PictureBox();
        imgPhoto.BorderStyle = System.Windows.Forms.BorderStyle.Fixed3D;
        imgPhoto.Width = this.Width / 2;
        imgPhoto.Height = this.Height / 2;
        imgPhoto.Left = (this.Width - imgPhoto.Width) / 2;
        imgPhoto.Top = (this.Height - imgPhoto.Height) / 2;
        imgPhoto.SizeMode = PictureBoxSizeMode.StretchImage;

        this.Controls.Add(btnLoad);
        this.Controls.Add(imgPhoto);  
    }

    protected void OnLoadClick(object sender, System.EventArgs e)
    {
      OpenFileDialog dlg = new OpenFileDialog();

      dlg.Title = "Open Image";
      dlg.Filter = "jpg files (*.jpg)|*.jpg|All files (*.*)|*.*" ;

      if (dlg.ShowDialog() == DialogResult.OK)
      {
          Bitmap au;

          //Image bmp = Image.FromFile("D:\\Documents and Settings\\kiosk.suprisul\\My Documents\\foto1.jpg");

          au = quantit.Quantize();
          imgPhoto.Image = au;
          //imgPhoto.Image = bmp;
          //imgPhoto.Image = au;
          //new Bitmap(dlg.OpenFile());
      }

      dlg.Dispose();
    }
    [STAThread]
    static void Main(string[] args)
    {

        //Image bmp;

        //bmp = Image.FromFile("teste.jpg");
        //PaintEventArgs e;
        //teste2.Quantize(bmp);


        Application.Run(new Quantization());

        /*
        System.Console.WriteLine("Hello, World!");
        System.Console.ReadLine();*/

    }
}

The class:

namespace ImageManipulation
{
    public unsafe abstract class Quantizer
    {
        /// <summary>
        /// Construct the quantizer
        /// </summary>
        /// <param name="singlePass">If true, the quantization only needs to loop through the source pixels once</param>
        /// <remarks>
        /// If you construct this class with a true value for singlePass, then the code will, when quantizing your image,
        /// only call the 'QuantizeImage' function. If two passes are required, the code will call 'InitialQuantizeImage'
        /// and then 'QuantizeImage'.
        /// </remarks>
        public Quantizer(bool singlePass)
        {
            _singlePass = singlePass;
        }

        /// <summary>
        /// Quantize an image and return the resulting output bitmap
        /// </summary>
        /// <param name="source">The image to quantize</param>
        /// <returns>A quantized version of the image</returns>
        public Bitmap Quantize()//Image source)
        {
            Image source = Image.FromFile("C:\\Users\\crashboy\\Downloads\\image009.jpg");
            // Get the size of the source image
            int height = source.Height;
            int width = source.Width;

            // And construct a rectangle from these dimensions
            Rectangle bounds = new Rectangle(0, 0, width, height);

            // First off take a 32bpp copy of the image
            Bitmap copy = new Bitmap(width, height, PixelFormat.Format32bppArgb);

            // And construct an 8bpp version
            Bitmap output = new Bitmap(width, height, PixelFormat.Format8bppIndexed);

            // Now lock the bitmap into memory
            using (Graphics g = Graphics.FromImage(copy))
            {
                g.PageUnit = GraphicsUnit.Pixel;

                // Draw the source image onto the copy bitmap,
                // which will effect a widening as appropriate.
                g.DrawImage(source, bounds);
            }

            // Define a pointer to the bitmap data
            BitmapData sourceData = null;

            try
            {
                // Get the source image bits and lock into memory
                sourceData = copy.LockBits(bounds, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

                // Call the FirstPass function if not a single pass algorithm.
                // For something like an octree quantizer, this will run through
                // all image pixels, build a data structure, and create a palette.
                if (!_singlePass)
                    FirstPass(sourceData, width, height);

                // Then set the color palette on the output bitmap. I'm passing in the current palette 
                // as there's no way to construct a new, empty palette.
                output.Palette = this.GetPalette(output.Palette);

                // Then call the second pass which actually does the conversion
                SecondPass(sourceData, output, width, height, bounds);
            }
            finally
            {
                // Ensure that the bits are unlocked
                copy.UnlockBits(sourceData);
            }

            // Last but not least, return the output bitmap
            return output;

        }

        /// <summary>
        /// Execute the first pass through the pixels in the image
        /// </summary>
        /// <param name="sourceData">The source data</param>
        /// <param name="width">The width in pixels of the image</param>
        /// <param name="height">The height in pixels of the image</param>
        protected virtual void FirstPass(BitmapData sourceData, int width, int height)
        {
            // Define the source data pointers. The source row is a byte to
            // keep addition of the stride value easier (as this is in bytes)
            byte* pSourceRow = (byte*)sourceData.Scan0.ToPointer();
            Int32* pSourcePixel;

            // Loop through each row
            for (int row = 0; row < height; row++)
            {
                // Set the source pixel to the first pixel in this row
                pSourcePixel = (Int32*)pSourceRow;

                // And loop through each column
                for (int col = 0; col < width; col++, pSourcePixel++)
                    // Now I have the pixel, call the FirstPassQuantize function...
                    InitialQuantizePixel((Color32*)pSourcePixel);

                // Add the stride to the source row
                pSourceRow += sourceData.Stride;
            }
        }

        /// <summary>
        /// Execute a second pass through the bitmap
        /// </summary>
        /// <param name="sourceData">The source bitmap, locked into memory</param>
        /// <param name="output">The output bitmap</param>
        /// <param name="width">The width in pixels of the image</param>
        /// <param name="height">The height in pixels of the image</param>
        /// <param name="bounds">The bounding rectangle</param>
        protected virtual void SecondPass(BitmapData sourceData, Bitmap output, int width, int height, Rectangle bounds)
        {
            BitmapData outputData = null;

            try
            {
                // Lock the output bitmap into memory
                outputData = output.LockBits(bounds, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);

                // Define the source data pointers. The source row is a byte to
                // keep addition of the stride value easier (as this is in bytes)
                byte* pSourceRow = (byte*)sourceData.Scan0.ToPointer();
                Int32* pSourcePixel = (Int32*)pSourceRow;
                Int32* pPreviousPixel = pSourcePixel;

                // Now define the destination data pointers
                byte* pDestinationRow = (byte*)outputData.Scan0.ToPointer();
                byte* pDestinationPixel = pDestinationRow;

                // And convert the first pixel, so that I have values going into the loop
                byte pixelValue = QuantizePixel((Color32*)pSourcePixel);

                // Assign the value of the first pixel
                *pDestinationPixel = pixelValue;

                // Loop through each row
                for (int row = 0; row < height; row++)
                {
                    // Set the source pixel to the first pixel in this row
                    pSourcePixel = (Int32*)pSourceRow;

                    // And set the destination pixel pointer to the first pixel in the row
                    pDestinationPixel = pDestinationRow;

                    // Loop through each pixel on this scan line
                    for (int col = 0; col < width; col++, pSourcePixel++, pDestinationPixel++)
                    {
                        // Check if this is the same as the last pixel. If so use that value
                        // rather than calculating it again. This is an inexpensive optimisation.
                        if (*pPreviousPixel != *pSourcePixel)
                        {
                            // Quantize the pixel
                            pixelValue = QuantizePixel((Color32*)pSourcePixel);

                            // And setup the previous pointer
                            pPreviousPixel = pSourcePixel;
                        }

                        // And set the pixel in the output
                        *pDestinationPixel = pixelValue;
                    }

                    // Add the stride to the source row
                    pSourceRow += sourceData.Stride;

                    // And to the destination row
                    pDestinationRow += outputData.Stride;
                }
            }
            finally
            {
                // Ensure that I unlock the output bits
                output.UnlockBits(outputData);
            }
        }

        /// <summary>
        /// Override this to process the pixel in the first pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <remarks>
        /// This function need only be overridden if your quantize algorithm needs two passes,
        /// such as an Octree quantizer.
        /// </remarks>
        protected virtual void InitialQuantizePixel(Color32* pixel)
        {

        }

        /// <summary>
        /// Override this to process the pixel in the second pass of the algorithm
        /// </summary>
        /// <param name="pixel">The pixel to quantize</param>
        /// <returns>The quantized value</returns>
        protected abstract byte QuantizePixel(Color32* pixel);

        /// <summary>
        /// Retrieve the palette for the quantized image
        /// </summary>
        /// <param name="original">Any old palette, this is overrwritten</param>
        /// <returns>The new color palette</returns>
        protected abstract ColorPalette GetPalette(ColorPalette original);

        /// <summary>
        /// Flag used to indicate whether a single pass or two passes are needed for quantization.
        /// </summary>
        private bool _singlePass;

        /// <summary>
        /// Struct that defines a 32 bpp colour
        /// </summary>
        /// <remarks>
        /// This struct is used to read data from a 32 bits per pixel image
        /// in memory, and is ordered in this manner as this is the way that
        /// the data is layed out in memory
        /// </remarks>
        [StructLayout(LayoutKind.Explicit)]
        public struct Color32
        {
            /// <summary>
            /// Holds the blue component of the colour
            /// </summary>
            [FieldOffset(0)]
            public byte Blue;
            /// <summary>
            /// Holds the green component of the colour
            /// </summary>
            [FieldOffset(1)]
            public byte Green;
            /// <summary>
            /// Holds the red component of the colour
            /// </summary>
            [FieldOffset(2)]
            public byte Red;
            /// <summary>
            /// Holds the alpha component of the colour
            /// </summary>
            [FieldOffset(3)]
            public byte Alpha;

            /// <summary>
            /// Permits the color32 to be treated as an int32
            /// </summary>
            [FieldOffset(0)]
            public int ARGB;

            /// <summary>
            /// Return the color for this Color32 object
            /// </summary>
            public Color Color
            {
                get { return Color.FromArgb(Alpha, Red, Green, Blue); }
            }
        }
    }
}

Answer 1

The compiler is warning you that quantit is never initialized and will always be null.

You probably should initialize it with an instance of a class that derives from ImageManipulation.Quantizer (you cannot instantiate Quantizer itself because it’s an abstract class):

private static Quantizer quantit = new QuantizerImplementation();