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I'm having some trouble with the resize problem, specifically copying the scanline of interest so it can be duplicated vertically. I've been working on this for awhile and I think I'm on the right track (I hope?), but I can't seem to figure out where I'm going wrong. Any pointers (haha), debugging tips, or general nudges in the right direction would be helpful.

Edit: The program compiles and runs, but the picture is not what I want it to be.

/**
* resize.c
*
* Computer Science 50
* Problem Set 4
* 
* Resizes a bmp image
**/

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

#include "bmp.h"

//prototypes
bool isNumber(char* arr);

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

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

    // ensure n is positive number less than or equal to 100
    if (n < 1 || n > 100)
    {
        printf("n must be greater than 0 and no larger than 100.\n");
        return 2;
    }

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

    // open input file
    FILE* inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        printf("Sorry, %s could not be opened.\n", argv[2]);
        return 3;
    }

    // open output file
    FILE* outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", argv[3]);
        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);
    fseek(inptr, 0, SEEK_SET);

    // 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;
    }

    // create new file header for outfile
    BITMAPFILEHEADER bf_outfile;
    fread(&bf_outfile, sizeof(BITMAPFILEHEADER), 1, inptr);

    // create new info header for outfile
    BITMAPINFOHEADER bi_outfile;
    fread(&bi_outfile, sizeof(BITMAPINFOHEADER), 1, inptr);

    // update outfile width and height
    bi_outfile.biWidth = bi.biWidth * n;
    bi_outfile.biHeight = bi.biHeight * n;

    // determine padding of infile
    int padding_infile = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;

    // determine padding for outfile
    int padding_outfile =  (4 - (bi_outfile.biWidth * sizeof(RGBTRIPLE)) %    4) % 4;

    // update outfile image size
    bi_outfile.biSizeImage = ((bi_outfile.biWidth * sizeof(RGBTRIPLE)) + padding_outfile) * abs(bi_outfile.biHeight);

    // update outfile file header
    bf_outfile.bfSize = bi_outfile.biSizeImage + bf.bfOffBits;

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

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

    // memory to remember outfile's scanline
    RGBTRIPLE* scanline = malloc(bi_outfile.biWidth);

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
        // reset scanline_place
        int scanline_place = 0;

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

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

            // copy triple to new scanline
            int horizontal_copies = 0;
            while (horizontal_copies < n)
            {
                scanline[scanline_place] = triple;
                scanline_place++;
                horizontal_copies++;
            }
        }

        // write new scanlines
        int vertical_copies = 0;
        while (vertical_copies < n)
        {
            fwrite(&scanline, bi_outfile.biWidth * sizeof(RGBTRIPLE), 1, outptr);

            // put in padding, if any
            int padding = 0;
            while (padding < padding_outfile)
            {
                fputc(0x00, outptr);
                padding++;
            }

            vertical_copies++;
        }

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

    // free memory
    free(scanline);  

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // that's all folks
    return 0;
}


bool isNumber(char* arr)
{
    for (int i = 0, n = strlen(arr); i < n; i++)
    {
        if(isalpha(arr[i]))
        {
            return false;
        }
    }
    return true;
}
1

Let's assume that you correctly calculated the output padding. Now, look at this: fputc(0x000000, outptr);

The padding is dimensioned in bytes, not pixels. So why are you writing out 1 pixel * padding?

No guarantees that this is the only problem, but it's a big one.

If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)

2
  • Hi Cliff, thanks so much for taking the time to look at this for me, and also for answering so many questions on this stack exchange in general. Seeing as I had a misconception about what padding is, exactly, as you pointed out, I decided to put my whole program up above in case I made some inaccurate assumptions before. I think I fixed the padding issue, but for some reason this problem is really giving me a hard time. I've been stuck on it for over a week and I just can't seem to make any progress. Is there any other advice you can give me here? – LeviJames Feb 25 '16 at 4:17
  • Actually, nevermind. Your response let me to go back through my code and I found a couple of places where I went wrong. Thanks again for all you do! – LeviJames Feb 25 '16 at 18:58

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