I have some code I’m trying to write for a CS class. On resizing BMP files. I’ve already done horizontal resizing, but I just can’t figure out the vertical resizing code. I’m also not sure what to look for when debugging it, or else I would’ve done it myself.

This is my code:

Code:

/**
 * resize.c
 *
 * Computer Science 50
 * Problem Set 4
 *
 * Resizes the file whose name is passed to it as a commandline argument.
 */
       
#include <stdio.h>
#include <stdlib.h>

#include "bmp.h"

int main(int argc, char* argv[])
{
    // ensure proper usage
    if (argc != 4)
    {
        printf("Usage: ./resize n infile outfilen");
        return 1;
    }

    // remember filenames
    char* infile = argv[2];
    char* outfile = argv[3];
    
    // change n into a number so we can use it as such
    int n = atoi(argv[1]);
    
    if (n <= 0 || n > 100)
    {
        printf("The value provided as the size must be that of a positive integer less than or equal to 100!n");
        return 2;
    }

    // open input file 
    FILE* inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        printf("Could not open %s.n", infile);
        return 3;
    }

    // open output file
    FILE* outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.n", outfile);
        return 4;
    }

    // read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 || 
        bi.biBitCount != 24 || bi.biCompression != 0)
    {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.n");
        return 4;
    }

    //remember the old padding
    int padding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    
    // remember dimensions of resized image
    int img_height = bi.biHeight;
    int img_width = bi.biWidth;
    
    // calculate dimensions of resized image
    bi.biHeight *= n;
    bi.biWidth *= n;
    int resized_height = bi.biHeight;
    int resized_width = bi.biWidth;
    
    // determine scanline padding
    int new_padding = (4 - (resized_width * sizeof(RGBTRIPLE)) % 4) % 4;
    
    // update header info
    bi.biSizeImage = ((resized_width * sizeof(RGBTRIPLE) + new_padding) * abs(resized_height));
    bf.bfSize = bi.biSizeImage + (sizeof(BITMAPINFOHEADER) + sizeof(BITMAPFILEHEADER));

    // write outfile's BITMAPFILEHEADER
    fwrite(&bf, sizeof(BITMAPFILEHEADER), 1, outptr);

    // write outfile's BITMAPINFOHEADER
    fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr);

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(img_height); i < biHeight; i++)
    {
        // iterate over pixels in scanline
        for (int j = 0; j < img_width; j++)
        {
            // temporary storage
            RGBTRIPLE triple;
            
            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
            
            for (int index = 0; index <= n - 1; index++)
            {
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
            }
        }
        
        // iterate over images' widths and heights to resize vertically
        for (int l = 0; l < img_width; l++)
        {
            // storage for the triples data
            RGBTRIPLE triple;
            
            // read from the infile and store in triple
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
            
            for (int m = 0; m < biHeight; m++)
            {
                for (int index = 0; index <= n - 1; index++)
                {
                    fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
                    fseek(inptr, -padding, SEEK_CUR);
                    for (int k1 = 0; k1 < new_padding; k1++)
                    {
                        fputc(0x00, outptr);
                    }
                }
            }
            fseek(inptr, -padding, SEEK_CUR);
        }
        //fseek(inptr, -padding, SEEK_CUR);
    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // that's all folks
    return 0;
}

I have some pseudocode that I’m trying to follow for the vertical resize, but I guess I have some confusion and I can’t understand how to implement it correctly. Here’s the psuedocode (I got it from a walkthrough from the course, so it’s not exactly mine):

Code:


for each row
    for n - 1 times
        write pixels and padding to outfile
        send infile cursor back
    write pixels and padding to outfile
    skip over infile padding