2

I was trying to implement the rewrite method. the code compiles without error but I always get a weird result bitmap.

Here is the code I wrote:

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

#include "bmp.h"

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

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

    // remember scaling factor
    int n = atoi(argv[1]);

    //ensure the scaling factor is within the limits
    if (n < 1 || n > 100)
    {
        fprintf(stderr, "n must be a positive integer less than  or equal to 100\n");
        return 2;
    }

    // open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        fprintf(stderr, "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 5;
    }

    // remember the width and height of the infile as well as its padding
    int width_in = bi.biWidth;
    int height_in = bi.biHeight;
    int padding_in = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    //changing the header properties to match the scaling
    bi.biWidth *= n;
    bi.biHeight *= n;
    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    bi.biSizeImage = ((3 * bi.biWidth) + padding) * abs(bi.biHeight);
    bf.bfSize = bi.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

    // 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; i < height_in; i++)
    {
        // declare an array for storing the entire row to be written
        RGBTRIPLE row[bi.biWidth];

        // iterate over pixels in scanline
        for (int j = 0; j < width_in; j++)
        {
            // temporary storage
            RGBTRIPLE triple;

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

            // writting the pixel n times in the array
            for (int x = 0; x < n; x++)
            {
                row[x + (n * j)] = triple;
            }

        }

        for (int j = 0; j < n; j++)
        {
            fwrite(row, sizeof(row), 1, outptr);
            for (int k = 0; k < padding; k++)
            {
                fputc(0x00, outptr);
            }
        }

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

    }

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}
1

The error was due to bi.biHeight being negative. So to get it running just changed the declaration of height_in as follows

int height_in = abs(bi.biHeight); 

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