C program for course work that resizes a BMP file

resize.c

// Resizes a BMP file



#include <stdio.h>

#include <stdlib.h>

#include <string.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 *nString = argv[1];

    int n = atoi(nString);

    char *infile = argv[2];

    char *outfile = argv[3];



    // validate user input

    // check if n is between 0 and 100

    int err = 0;

    if ( n < 0 || n > 100)

    {

        fprintf(stderr, "Usage: n must be a postive integer less than or equal to 100\n");

        err++;

    }

    // check if infile is a valid .bmp

    // find the extension

    int ret = 1;

    char *ext = strrchr(infile, '.');

    if (!ext)

    {

        fprintf(stderr, "Usage: infile must be the name of a BMP to be resized\n");

        err++;

    }

    else

    {

        ret = strncmp(ext, ".bmp", 3);

        if (ret != 0)

        {

            fprintf(stderr, "Usage: infile must be the name of a BMP to be resized\n");

            err++;

        }

    }

    // check if outfile is a valid .bmp

    // find the extension

    ext = strrchr(outfile, '.');



    if (!ext)

    {

        fprintf(stderr, "Usage: outfile must be the name of a BMP to be resized\n");

        err++;

    }

    else

    {

        ret = strncmp(ext, ".bmp", 3);

        if (ret != 0)

        {

            fprintf(stderr, "Usage: outfile must be the name of a BMP to be resized\n");

            err++;

        }

    }

    // if any of the 3 arguments don't validate, exit program

    if (err > 0)

    {

        return 1;

    }



    // open input file

    FILE *inptr = fopen(infile, "r");

    if (inptr == NULL)

    {

        fprintf(stderr, "Could not open %s.\n", infile);

        return 2;

    }



    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;

    }



    // determine padding for infile scanlines

    int inPadding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;



    // preserve infile bi.biWidth and bi.biHeight

    int inBiWidth = bi.biWidth;

    int inBiHeight = bi.biHeight;

    int posInBiHeight = abs(inBiHeight);



    // update width and height in file headers for outfile

    bi.biWidth = bi.biWidth * n;

    bi.biHeight = bi.biHeight * n;

    // fprintf(stderr,"inBiWidth = %i\n", inBiWidth);



    // define variables used in equations for updating biSizeImage and bfSize

    int infileW = bi.biWidth;

    int infileH = bi.biHeight;



    // convert to positive integers for use in equations

    int posBiWidth = abs(infileW);

    int posBiHeight= abs(infileH);



    // determine padding for outfile scanlines

    int outPadding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;



    // update biSizeImage and bfSize in file headers for outfile

    // convert to positive integers for use in equations

    int posOutPadding = outPadding;

    posOutPadding = abs(posOutPadding);



    // update biSizeImage

    int biSizeImageInt = 3 * (posBiWidth * posBiHeight) + (posOutPadding * posBiHeight);

    bi.biSizeImage = biSizeImageInt;



    // update bfSize

    int bfSizeInt = 54 + biSizeImageInt;

    bf.bfSize = bfSizeInt;



    // 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 < posInBiHeight; i++)

    {

        int repeat = -1;

        do {



        // iterate over pixels in scanline

        for (int j = 0; j < inBiWidth; j++)

        {

            // temporary storage

            RGBTRIPLE triple;



            // read RGB triple from infile

            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);



            // scale pixels horizontally by n

            for (int l = 0; l < n; l++)

            {

                fwrite(&triple.rgbtBlue, sizeof(triple.rgbtBlue), 1, outptr);

                fwrite(&triple.rgbtGreen, sizeof(triple.rgbtGreen), 1, outptr);

                fwrite(&triple.rgbtRed, sizeof(triple.rgbtRed), 1, outptr);

            }

        }



        // add padding to outfile if needed

        for (int k = 0; k < outPadding; k++)

        {

            // printf("0x00 ");

            fputc(0x00, outptr);

        }



        // scale vertically by n

        repeat ++;

        if (repeat != n - 1)

        {

            // send infile cursor back to the beginning of the row

            fseek(inptr, 0 - (inBiWidth * 3), SEEK_CUR);

        }

        }

        while (repeat != n - 1);



        // skip over padding, if any

        fseek(inptr, inPadding, SEEK_CUR);

    }



    // close infile

    fclose(inptr);



    // close outfile

    fclose(outptr);



    // success

    return 0;

}