I am implementing resize.c. The program compiles but I cannot download the larger image. It says that it is in invalid format?

Question

https://sandbox.cs50.net/checks/480ca0b5599747bf966ef0c139584d60


/**
 * resize.c
 *
 * Computer Science 50
 * Problem Set 4
 *
 * Magnifies a BMP piece by piece in accordance to the inputted value by the user.
 */

#include <stdio.h>
#include <stdlib.h>
#include<string.h>
#include<math.h>
#include<ctype.h>
#include "bmp.h"

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

    // remember filenames
    char* infile = argv[2];
    char* outfile = argv[3];

    //checking if n is a number
    for(int i = 0, l = strlen(argv[1]); i < l; i++)
        {
            if(!isdigit(argv[1][i]))
                {
                    printf("n should be an integer");
                    return 2;
                }
        }

   //remember resizing factor;
   int n =atoi(argv[1]);
   if((n>100) || (n<0))
    {
        printf("n should be less than 100 and greater than 0");
        return 2;
    }

    // open input file 
    FILE* inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        printf("Could not open %s.\n", infile);
        return 2;
    }

    // open output file
    FILE* outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 3;
    }

    // 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;
    }
    //calculating width of original file to ultimately calculate padding
    int orig_Width = bi.biWidth;

    //calculate padding of the infile
    int orig_Padding = (4 - (orig_Width * sizeof(RGBTRIPLE)) % 4) % 4;

    //update header file's info
    bi.biWidth*=n;
    bi.biHeight*=n;

    // determine padding for scanlines
    int padding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    //Calculate biSize = (biWidth * biHeight * 3bytesper pixel) + padding per line
    bi.biSize = (bi.biWidth * abs(bi.biHeight) * 3) + padding * abs(bi.biHeight);
    //bf.bfOffBits contains the size from start till the RGB is reached(i.e. 54)
    bf.bfSize = bi.biSize + bf.bfOffBits;

    // 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(bi.biHeight); i < biHeight; i++)
    {
        // iterate over pixels in scanline
        for (int j = 0; j < bi.biWidth; j++)
        {
            // temporary storage
            RGBTRIPLE triple;

            // read RGB triple from infile
            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

            // write RGB triple to outfile after resizing it horizontally
            for(int k = 0; k < n; k++)
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
        }

        // skip over padding, if any
        fseek(inptr, padding, SEEK_CUR);

        // then add it back (to demonstrate how)
        for (int k = 0; k < padding; k++)
        {
            fputc(0x00, outptr);
        }
    //resize vertically by using re-copy method. if i % n becomes 0, then continue to the next row
    long offset = orig_Width*sizeof(RGBTRIPLE) + orig_Padding;
    if((i+1)%n!=0)
        fseek(inptr, -offset, SEEK_CUR);
    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // that's all folks
    return 0;
}

Answer 1

You have a problem in your header. Take a close look at which header fields you are changing, and then review which ones you should be changing. Keep in mind that some fields have very similar names. You might also want to google the microsoft bitmap header specifications and review the field descriptions.

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