How to Create a Monochrome BMP Image from a Boolean Matrix in Pure C/C ?

Creating a Monochromatic BMP Image in Pure C/C

Bitmap image files are widely used for storing digital images. This article addresses the need to write a boolean matrix into a monochrome BMP file, where pixels are set to white if the corresponding matrix element is true.

Understanding the BMP Header

The BMP file format has a specific header structure that contains essential information about the image. The header consists of two parts: the file header and the information header.

The File Header

• Size (14 bytes): The size of the file in bytes.
• Identifier (2 bytes): The signature "BM" to identify the file as a BMP.
• Reserved (4 bytes): Reserved for future use (set to zero).
• Offset to the image data (4 bytes): The offset in bytes from the beginning of the file to the start of the pixel data.

The Information Header

• Size (40 bytes): The size of the information header.
• Width (4 bytes): The width of the image in pixels.
• Height (4 bytes): The height of the image in pixels.
• Planes (2 bytes): Always set to 1.
• Bit Count (2 bytes): 1 bit per pixel for monochrome images.
• Compression (4 bytes): 0 for uncompressed.
• Size of Image Data (4 bytes): The size of the pixel data in bytes.
• XPelsPerMeter and YPelsPerMeter (4 bytes each): Horizontal and vertical resolution of the image in pixels per meter.
• Color Palette (4 bytes): The number of colors in the palette (set to 0 for monochrome images).
• Important Colors (4 bytes): The number of important colors in the palette (set to 0 for monochrome images).

Creating the Monochrome BMP

To create the monochrome BMP:

1. Calculate the file size and offset to the image data.
2. Initialize the file header and information header with appropriate values.
3. Create a 2D array of booleans representing the matrix.
4. Convert the boolean values to pixel values (0 for false and 255 for true).
5. Write the file header to the file.
6. Write the information header to the file.
7. Iterate through the pixel data and write it to the file.
8. Pad each row to a multiple of 4 bytes to ensure alignment.
9. Close the file.

Example Code

The following C/C code demonstrates the creation of a monochrome BMP from a boolean matrix:

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *f;
    unsigned char *img = NULL;
    int filesize = 54 + 3*w*h;  //w is your image width, h is image height, both int

    img = (unsigned char *)malloc(3*w*h);
    memset(img,0,3*w*h);

    for(int i=0; i<w; i++)
    {
        for(int j=0; j<h; j++)
        {
            bool = matrix[i][j]
            x=i; y=(h-1)-j;
            r = bool? 255 : 0;
            g = bool? 255 : 0;
            b = bool? 255 : 0;
            img[(x+y*w)*3+2] = (unsigned char)(r);
            img[(x+y*w)*3+1] = (unsigned char)(g);
            img[(x+y*w)*3+0] = (unsigned char)(b);
        }
    }

    unsigned char bmpfileheader[14] = {'B','M', 0,0,0,0, 0,0, 0,0, 54,0,0,0};
    unsigned char bmpinfoheader[40] = {40,0,0,0, 0,0,0,0, 0,0,0,0, 1,0, 24,0};
    unsigned char bmppad[3] = {0,0,0};

    bmpfileheader[ 2] = (unsigned char)(filesize    );
    bmpfileheader[ 3] = (unsigned char)(filesize>> 8);
    bmpfileheader[ 4] = (unsigned char)(filesize>>16);
    bmpfileheader[ 5] = (unsigned char)(filesize>>24);

    bmpinfoheader[ 4] = (unsigned char)(       w    );
    bmpinfoheader[ 5] = (unsigned char)(       w>> 8);
    bmpinfoheader[ 6] = (unsigned char)(       w>>16);
    bmpinfoheader[ 7] = (unsigned char)(       w>>24);
    bmpinfoheader[ 8] = (unsigned char)(       h    );
    bmpinfoheader[ 9] = (unsigned char)(       h>> 8);
    bmpinfoheader[10] = (unsigned char)(       h>>16);
    bmpinfoheader[11] = (unsigned char)(       h>>24);

    f = fopen("img.bmp","wb");
    fwrite(bmpfileheader,1,14,f);
    fwrite(bmpinfoheader,1,40,f);
    for(int i=0; i<h; i++)
    {
        fwrite(img+(w*(h-i-1)*3),3,w,f);
        fwrite(bmppad,1,(4-(w*3)%4)%4,f);
    }

    free(img);
    fclose(f);

    return 0;
}

This code takes a boolean matrix matrix and creates a monochrome BMP image with a specified width (w) and height (h). It converts the boolean values to pixel values and writes the BMP file to the disk.

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