My implementation of CS50x Pset4 - Resize

resize.c

/****************************************************************************
 * resize.c
 *
 * HarvardX - CS50
 * Problem Set 4
 *
 * Resize a BMP image by a factor of n
 ***************************************************************************/

#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 factor infile outfile\n");
        return 1;
    }

    // remember filenames
    char *infile = argv[2];
    char *outfile = argv[3];
    
    int factor = atoi(argv[1]);
    
    // check to make sure the factor is an integer between 1 and 100
    if (factor < 1 || factor > 100)
    {
        fprintf(stderr, "The factor must be a number between 1 and 100.\n");
        return 1;
    }

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

    // 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, bfResize;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);
    bfResize = bf;

    // read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi, biResize;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);
    biResize = bi;

    // 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 the dimensions for the resized image
    biResize.biWidth = bi.biWidth * factor;
    biResize.biHeight = bi.biHeight * factor;
    
    // determine the padding for the original image and the resized image
    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    int resizePadding = (4 - (biResize.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
    
    // determine the size of the resized image
    biResize.biSizeImage = ((sizeof(RGBTRIPLE) * biResize.biWidth) + resizePadding) * abs(biResize.biHeight);
    bfResize.bfSize = biResize.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

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

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

    // iterate over infile's scanlines
    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
    {
        // write each line factor times
	for (int j = 0; j < factor; j++)
	{
	    // iterate over pixels in scanline
	    for (int k = 0; k < bi.biWidth; k++)
	    {
	        // temporary storage
		RGBTRIPLE triple;

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

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

	    // skip over padding in infile
	    fseek(inptr, padding, SEEK_CUR);

	    // then add it to outfile
	    for (int m = 0; m < resizePadding; m++)
	    {
	        fputc(0x00, outptr);
	    }

            // return to start of scanline
	    fseek(inptr, -(bi.biWidth * sizeof(RGBTRIPLE) + padding), SEEK_CUR);
	}
	fseek(inptr, bi.biWidth * sizeof(RGBTRIPLE) + padding, SEEK_CUR);
    }
    

    // close infile
    fclose(inptr);

    // close outfile
    fclose(outptr);

    // success
    return 0;
}


/****************************************************************************
 * bmp.h
 *
 * BMP-related data types based on Microsoft's own.
 ***************************************************************************/

#include <stdint.h>

/**
 * Common Data Types 
 *
 * The data types in this section are essentially aliases for C/C++ 
 * primitive data types.
 *
 * Adapted from https://msdn.microsoft.com/en-us/library/cc230309.aspx.
 * See http://en.wikipedia.org/wiki/Stdint.h for more on stdint.h.
 */
typedef uint8_t  BYTE;
typedef uint32_t DWORD;
typedef int32_t  LONG;
typedef uint16_t WORD;

/**
 * BITMAPFILEHEADER
 *
 * The BITMAPFILEHEADER structure contains information about the type, size,
 * and layout of a file that contains a DIB [device-independent bitmap].
 *
 * Adapted from https://msdn.microsoft.com/en-us/library/dd183374(v=vs.85).aspx.
 */
typedef struct 
{ 
    WORD bfType; 
    DWORD bfSize; 
    WORD bfReserved1; 
    WORD bfReserved2; 
    DWORD bfOffBits; 
} __attribute__((__packed__)) 
BITMAPFILEHEADER; 

/**
 * BITMAPINFOHEADER
 *
 * The BITMAPINFOHEADER structure contains information about the 
 * dimensions and color format of a DIB [device-independent bitmap].
 *
 * Adapted from https://msdn.microsoft.com/en-us/library/dd183376(v=vs.85).aspx.
 */
typedef struct
{
    DWORD biSize; 
    LONG biWidth; 
    LONG biHeight; 
    WORD biPlanes; 
    WORD biBitCount; 
    DWORD biCompression; 
    DWORD biSizeImage; 
    LONG biXPelsPerMeter; 
    LONG biYPelsPerMeter; 
    DWORD biClrUsed; 
    DWORD biClrImportant; 
} __attribute__((__packed__))
BITMAPINFOHEADER; 

/**
 * RGBTRIPLE
 *
 * This structure describes a color consisting of relative intensities of
 * red, green, and blue.
 *
 * Adapted from https://msdn.microsoft.com/en-us/library/dd162939(v=vs.85).aspx.
 */
typedef struct
{
    BYTE rgbtBlue;
    BYTE rgbtGreen;
    BYTE rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;