Reading a 16 Color 32x32 bitmap

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Introduction

This is my first post. I wrote this code when I was learning binary file I/O in C#. This code is not accurate because it displays an inverted image of a bitmap file. I hope this article will help someone learn C#. This code is not optimised, it is just for beginners learning to work with files. 

Background

Bitmap file: This file contains the pixel data we want to read and display. Here is structure of a bitmap file specified in MSDN:

Structure	                         Corresponding bytes	   
BITMAPFILEHEADER	                     0x00 0x0D	   
BITMAPINFOHEADER	                     0x0E 0x35	   
RGBQUAD array	                         0x36 0x75	   
Color-index array	                     0x76 0x275

Color data is stored from the 0x76 byte.

BIT Manipulation

In bitmap 1 byte stores two colors, i.e., first 4 bits contain one color and the last 4 contain another color.

Now to extract to the color from the byte. We have to shift the byte. To extract left 4 bits from the byte, just right shift it by 4 bits.

Now we get the required value from the byte. To extract the right 4 bits from the byte, we should left shift it by 4 bits.

This byte does not give the desired value so we again right shift the 4 bits.

Now we get the required value in the byte.

Code

First open Visual Studio and create a new Windows app in C#. In the form, add a picturebox and a button.

Now double click and write the following code:

private void button1_Click(object sender, EventArgs e)
{
    Color bitcolor; // create new color type variable which is used to store color 
                    // return by our getcolor method

    Bitmap image = new Bitmap(32, 32);//this creates a new bitmap image with 32x32 pixels

    // now opening our bitmap file.
    FileStream file = new FileStream("face.bmp", FileMode.Open, FileAccess.Read);

    //now seek file pointer to 0x76 byte because color index starts from here.
    file.Seek(0x76, 0);


    byte bhigh, blow; //bhigh will store the left 4 bits and blow will store right 4 bits.

    int tmp; // this is the temp variable which will be used to store byte return by
            // file.ReadByte() method

    /*  now for the loop since it is a 32x32 bitmap there are 32 row & 32 column of pixels
     * here first loop with i is for row & second loop j is for column
     * we start by reading first row then second , third so on till 31 row
     * * 
     * */ 

    for (int i = 0; i < 32; i++) // set row
    {
        for (int j = 0; j < 32 ; j += 2) //now read first row and all its columns
                                        // since 1 byte contains two colors we are setting 
                                        // 2 pixels at a time 
        {
            tmp = file.ReadByte();      // get first byte from color
            bhigh = (byte)(tmp >> 4); //now get left for bits by shifting int to right by 4 bits
            blow = (byte)(tmp << 4); // get right 4 bits by first left  shift 4
            blow = (byte)(blow >> 4); // now right shift by 4
           
            bitcolor = getcolor(bhigh); // get color representing left 4 bits
            //=== setting the pixels of bitmap
            image.SetPixel(j, i, bitcolor);    // e.g. first pixel (1,0)   
            bitcolor = getcolor(blow);  // get color representing right 4 bits
            //=== setting the pixels of bitmap
            image.SetPixel(j+1 ,i, bitcolor);  // e.g. second pixel (2,0)
        }
    }
    //now set the picture box with our image
    this.pictureBox1.Image = image;
}

Now the method which will help us to retrieve our color:

public Color getcolor(byte a) // this is the method which will return our 16 color 
{                            // configure your own color
    switch (a)
    {
        case 0: return Color.Black;
            break;
        case 1: return Color.DarkRed;
            break;
        case 2: return Color.DarkGreen;
            break;
        case 3: return Color.Yellow;
            break;
        case 4: return Color.Blue;
            break;
        case 5: return Color.Cyan;
            break;
        case 6: return Color.DarkGray;
            break;
        case 7: return Color.LightGray;
            break;
        case 8: return Color.LightGreen;
            break;
        case 9: return Color.LightPink;
            break;
        case 10: return Color.HotPink;
            break;
        case 11: return Color.Silver;
            break;
        case 12: return Color.Gold;
            break;
        case 13: return Color.Turquoise;
            break;
        case 14: return Color.Violet;
            break;
        case 15: return Color.White;
            break;
        default: return Color.Coral; 
           // default is not required
    }
}

Bugs

I don't know why the image shows upside down. If you know, please tell me.