It seems I need some help with resizing the BMP image when the zoom factor is less than 1. You can see the most crucial part of my code below. The variable f in the code is the zoom factor. It seems logical to me but it works improperly.
- this is the image I need to resize (to make it even smaller).
- and this is the result picture which doesn't look properly. I think I failed in uploading that here, but it looks like a small green square without any white center at all.
Moreover, I tried to resize one more image - this beautiful smiley:
And the result was rather unexpected: 
This makes me think that there's a problem with the for-cycles, though it seems completely logical to me.
And this is how the BMP is organized.
// BMP-related data types based on Microsoft's own
#include <stdint.h>
// aliases for C/C++ primitive data types
// https://msdn.microsoft.com/en-us/library/cc230309.aspx
typedef uint8_t BYTE;
typedef uint32_t DWORD;
typedef int32_t LONG;
typedef uint16_t WORD;
// information about the type, size, and layout of a file
// 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;
// information about the dimensions and color format
// 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;
// relative intensities of red, green, and blue
// https://msdn.microsoft.com/en-us/library/dd162939(v=vs.85).aspx
typedef struct
{
BYTE rgbtBlue;
BYTE rgbtGreen;
BYTE rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;
Does anybody know how to fix the code below?
// Copies a BMP file
#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 n infile outfile\n");
return 1;
}
// remember filenames
float f=atof(argv[1]);
int n=f;
if(f<=0||f>=1){
fprintf(stderr, "f, the resize factor, must be between 0 and 1.\n");
return 1;
}
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;
}
// 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;
}
BITMAPFILEHEADER bf_resize = bf;
BITMAPINFOHEADER bi_resize = bi;
bi_resize.biWidth = bi.biWidth * f;
bi_resize.biHeight = bi.biHeight * f;
int padding_resize = (4 - (bi_resize.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
bi_resize.biSizeImage = (bi_resize.biWidth * sizeof(RGBTRIPLE) + padding_resize) * abs(bi_resize.biHeight);
bf_resize.bfSize = bi_resize.biSizeImage + sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER);
int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
int fn = f * 10;
int x = 10 / fn;
int diff = x;
diff--;
int w = 0;
int h = 0;
// write outfile's BITMAPFILEHEADER
fwrite(&bf_resize, sizeof(BITMAPFILEHEADER), 1, outptr);
// write outfile's BITMAPINFOHEADER
fwrite(&bi_resize, sizeof(BITMAPINFOHEADER), 1, outptr);
// iterate over infile's scanlines
for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)
{
if(h==0){
// iterate over pixels in scanline
for (int j = 0; j < bi.biWidth; j++)
{
// temporary storage
RGBTRIPLE triple;
// read RGB triple from infile
fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
if(w==0){
// write RGB triple to outfile
fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
w++;
}
else{
w++;
}
if(w==diff){
w=0;
}
}
}
h++;
if(h==diff){
h=0;
}
// skip over padding, if any
fseek(inptr, padding, SEEK_CUR);
// then add it back (to demonstrate how)
for (int k = 0; k < padding; k++)
{
fputc(0x00, outptr);
}
}
// close infile
fclose(inptr);
// close outfile
fclose(outptr);
// success
return 0;
}
