Console BMP Viewer V2 - Python Programming Exercise

import sys

def read_bmp_header(filename):
    """
    Reads the BMP header and validates that it is a 72x24 BMP file.
    Returns the start of image data and the dimensions.
    """
    try:
        with open(filename, "rb") as file:
            # Check the BMP signature (first two bytes should be 'BM')
            signature = file.read(2)
            if signature != b'BM':
                raise ValueError("The file is not a valid BMP file.")
            
            # Skip to width and height in the BMP header
            file.seek(18)
            width = int.from_bytes(file.read(4), byteorder='little')
            height = int.from_bytes(file.read(4), byteorder='little')

            if width != 72 or height != 24:
                raise ValueError("This program only supports BMP files with dimensions 72x24.")

            # Skip to the start of image data
            file.seek(10)
            start_of_data = int.from_bytes(file.read(4), byteorder='little')

            return start_of_data, width, height

    except FileNotFoundError:
        print(f"Error: File '{filename}' not found.")
        sys.exit(1)
    except Exception as e:
        print(f"Error: {str(e)}")
        sys.exit(1)


def read_bmp_data(filename, start_of_data, width, height):
    """
    Reads the image data from the BMP file starting at the specified position.
    Returns the pixel data as a 2D list.
    """
    try:
        with open(filename, "rb") as file:
            # Go to the start of the image data
            file.seek(start_of_data)
            
            # Read the image data into a 2D list
            pixels = []
            row_size = (width + 3) // 4 * 4  # Each row must be a multiple of 4 bytes
            for _ in range(height):
                row = list(file.read(row_size))[:width]
                pixels.append(row)

            return pixels[::-1]  # BMP stores rows bottom-to-top

    except Exception as e:
        print(f"Error while reading image data: {str(e)}")
        sys.exit(1)


def display_bmp_image(pixels):
    """
    Displays the BMP image on the console using 'X' for 255 and ' ' for other values.
    """
    for row in pixels:
        line = ''.join('X' if pixel == 255 else ' ' for pixel in row)
        print(line)


if __name__ == "__main__":
    # Ensure the filename is provided as a command-line argument
    if len(sys.argv) != 2:
        print("Usage: python bmp_viewer.py ")
        sys.exit(1)

    filename = sys.argv[1]

    # Read BMP header
    start_of_data, width, height = read_bmp_header(filename)

    # Read BMP image data
    pixels = read_bmp_data(filename, start_of_data, width, height)

    # Display the image
    display_bmp_image(pixels)

 Output

Execution:

python bmp_viewer.py example.bmp

Output:

X X X X X X 
 X X X X X X
X X X X X X 
 X X X X X X
...