Combining Images with a C Program using ImageMagick: Two and Four Image Use Cases

In our previous blog post, we explored how to combine images with spacers using ImageMagick commands. Now, we will take this a step further and create a C program that automates this process, taking image filenames as command-line parameters. This program will handle two or four images, compute the necessary spacer height, and produce an output image.

Requirements

To follow along with this tutorial, you’ll need:

• A working installation of ImageMagick.
• GCC (GNU Compiler Collection) to compile the C program.

Step-by-Step Guide

Writing the C Program

We’ll create a C program that:

1. Takes the image filenames as command-line parameters.
2. Computes the height of the first image.
3. Creates a spacer image with the computed height.
4. Combines the images with the spacer in between.

Full C Program

Below is the complete C program to achieve this functionality:

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

void create_spacer_image(int height) {
    char command[256];
    snprintf(command, sizeof(command), "convert -size 20x%d xc:none spacer.png", height);
    system(command);
}

void combine_images(int image_count, char *image_files[]) {
    char command[1024];
    strcpy(command, "convert +append");

    for (int i = 0; i < image_count; i++) {
        strcat(command, " ");
        strcat(command, image_files[i]);
        if (i < image_count - 1) {
            strcat(command, " spacer.png");
        }
    }

    strcat(command, " output.jpg");
    system(command);
}

int get_image_height(char *image_file) {
    char command[256];
    snprintf(command, sizeof(command), "identify -format \"%%h\" %s > height.txt", image_file);
    system(command);

    FILE *file = fopen("height.txt", "r");
    int height;
    fscanf(file, "%d", &height);
    fclose(file);
    system("rm height.txt");

    return height;
}

int main(int argc, char *argv[]) {
    if (argc != 3 && argc != 5) {
        fprintf(stderr, "Usage: %s <image1> <image2> [image3] [image4]\n", argv[0]);
        return 1;
    }

    int height = get_image_height(argv[1]);
    create_spacer_image(height);

    if (argc == 3) {
        char *images[] = {argv[1], argv[2]};
        combine_images(2, images);
    } else if (argc == 5) {
        char *images[] = {argv[1], argv[2], argv[3], argv[4]};
        combine_images(4, images);
    }

    return 0;
}

Compiling and Running the Program

1. Compile the Program:

   gcc -o combine_images combine_images.c

2. Run the Program for Two Images:

   ./combine_images image1.jpg image2.jpg

3. Run the Program for Four Images:

   ./combine_images image1.jpg image2.jpg image3.jpg image4.jpg

How the Program Works

1. Getting the Height of the First Image:
   The program uses the identify command from ImageMagick to get the height of the first image and stores it in a temporary file, height.txt. The height is then read from this file and used to create the spacer image.
2. Creating the Spacer Image:
   The program uses the convert command from ImageMagick to create a spacer image of width 20 pixels and the computed height.
3. Combining Images:
   The program constructs a command to append the images horizontally with spacers in between using the convert +append command from ImageMagick.

Summary

By integrating ImageMagick commands into a C program, we’ve automated the process of combining images with spacers in between. This program can handle both two and four images, making it versatile for different use cases. The key steps involve computing the height of the images, creating a spacer of the same height, and then using ImageMagick to combine the images. This approach not only simplifies the process but also makes it easily repeatable and adaptable for other image manipulation tasks.