ps4 resize error

Question

I'd been starting at this for hours and can't figure out what I'm doing wrong. To scale for the horizontal I put fwrite() in a for loop repeating n times for each pixel. To scale vertically I put the whole code in another for loop in where it goes back to the start of the old scanline, re-reading and re-rewriting it.

Here's my code:

#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 outfile");
        return 1;
    }

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

    int n = atoi(argv[1]);

    if (n <= 0 && n>= ((2^32)-1))
    {
        printf("n should be less than 2^32-1");
        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;
    }

    //old
    int oldWidth = bi.biWidth;
    int oldHeight = abs(bi.biHeight);
    int oldPadding =  (4 - (oldWidth * sizeof(RGBTRIPLE)) % 4) % 4;  //padding = 3

    //new
    bi.biWidth *= n;
    bi.biHeight *= n;

    int padding =  (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;  //padding = 3
    bi.biSizeImage = ((sizeof(RGBTRIPLE) * 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 < oldHeight; i++)  //oldHeight = 3
    {  
        for(int a=0; a<n;a++)               //resizing vertically
        {
            // iterate over pixels in scanline
            for (int j = 0; j < oldWidth; j++)  //bi.biWidth = 6
            {
                // temporary storage
                RGBTRIPLE triple;

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

                // write RGB triple to outfile (repeated n times for each pixel)
                for(int r=0; r< n ; r++)
                    fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);

             }

                // skip over padding, if any
             fseek(inptr, oldPadding, SEEK_CUR);  //skipping padding of infile (3)


          // then add it back (to demonstrate how)
            for (int k = 0; k < padding; k++)    //new padding = 2
            {
                fputc(0x00, outptr);
            }
        fseek(inptr, -(oldWidth+oldPadding), SEEK_CUR);  //going back 

        }


    }
    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // that's all folks
    return 0;
}

Output:

~/workspace/pset4/bmp/ $ ./resize 2 small.bmp t.bmp

Any help would be appreciated.

Answer 1

I see three issues with your code. The good news is that you're pretty much on target.

Two of the problems are related to the backwards fseek near the end of the for loops. The fseek backwards executes after every execution. This shouldn't be done after n lines have been written to the output file. As written, it goes back one time too many for each line in the input file. I'll leave it to you to figure out how to implement the change. ;-)

Next, the fseek isn't going back the correct amount. Here's the big hint. What units are used to measure oldWidth and what units are oldpadding? Bytes or pixels?

Finally, a problem that wouldn't be caught by check50. The program spec says that the scale factor goes from 1 to 100, not 2^32. Further, the ^ operator may mean exponent in traditional math, but in programming, it is a bit shift operator, something totally different.

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