PSET3: Resize vertical fseek issue?

Question

So I have looked through every response to this and I still for the life of me can't figure out what I am doing wrong here. The output is repeating each line twice all the way down. I have tried moving the vertical "repeat n times" loop around to no avail. Any insights would be greatly appreciated. Thanks!

    // scales a BMP file by n

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

    #include "bmp.h"

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

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

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

    int n = atoi(argv[1]);

    if(n <0 || n>100)
    {
        fprintf(stderr, "Usage: n must be a value from 1 to 100\n");
        return 1;
    }

    // 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;
    }

    // create outfile's BITMAPFILEHEADER
    BITMAPFILEHEADER oBf = bf;
    BITMAPINFOHEADER oBi = bi;

    //Assign outfile BITMAPINFOHEADER values for width and hieght
    oBi.biWidth *= n;
    oBi.biHeight *= n;

    // assigning values to infile and outfile padding
    int iPadding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    int oPadding = (4 - ((oBi.biWidth) * sizeof(RGBTRIPLE)) % 4) % 4;

  //Change Outfile BITMAPINFOHEADER to new dimensions

    bi.biSizeImage = ((sizeof(RGBTRIPLE)*bi.biWidth)+oPadding)*abs(bi.biHeight);

    int headers = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

    //change outfile Image size in BITMAPFILEHEADER
    oBf.bfSize = headers + bi.biSizeImage;


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

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


    // iterate over infile's scanlines
    for (int j = 0, iHeight = abs(bi.biHeight); j < iHeight; j++)
    {
        for(int i = 0; i < n; i++)
        {
            //Sets inptr to the beginning of each scanline
            fseek(inptr, headers + ((bi.biWidth * sizeof(RGBTRIPLE) + iPadding)*i), SEEK_SET);

            //iterates through each pixel in a scanline
            for(int x = 0; x < bi.biWidth; x++)
            {
                // temporary storage
                RGBTRIPLE triple;

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

                for(int t = 0; t < n; t++)
                {
                // write RGB triple to outfile n times
                fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr); 
                }
            }

            //writes outfile padding in each row
            for (int k = 0; k < oPadding; k++)
            {
                fputc(0x00, outptr);
            }
        }

    }


    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}

Answer 1

Something's wrong with your for loops. You want them to run per line/pixel of the input image, yet you use i < biHeight*n (which makes no sense, i * n < biHeight would) and j < bi.biWidth (the output width).

Some fix would be to either use input size in those two (e.g. saving them in some other variable), or multiplying/dividing by n in the right way.