Working with BMP - loading and saving - c++

Question

With friends we're trying to write app to work with BMP files and we're going to make it as simple as it could be for us, because we're just starting to learn C and C++. Copying was going good with new real size of lines but now I wanted to add grayscale effect and got another problem: the right side of the picture is moved to the left - check out pictures. What's causing this problem?
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <unistd.h>
using namespace std;
void ReadBMP()
{
FILE* f = fopen("test2.bmp", "rb");
FILE* w = fopen("zapis.bmp", "wb");
if(f == NULL)
throw "Argument Exception";
unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f);
fwrite(info, sizeof(unsigned char), 54, w);
int width = *(int*)&info[18];
int height = *(int*)&info[22];
cout << endl;
cout << "Width: " << width << endl;
cout << "Height: " << height << endl;
int realwidth = 3*width+(4 - ((3*width)%4))%4;
int volume = height * realwidth;
unsigned char* data = new unsigned char[volume];
fwrite(info, sizeof(unsigned char), 54, w);
fread(data, sizeof(unsigned char), volume, f);
unsigned char color = 0;
for(int i = 0; i < volume; i+=3)
{
color = 0;
color+=data[i]*0.114;
color+=data[i+1]*0.587;
color+=data[i+2]*0.299;
data[i] = color;
data[i+1] = color;
data[i+2] = color;
}
fwrite(data, sizeof(unsigned char), volume, w);
fclose(f);
fclose(w);
delete(data);
}
int main()
{
ReadBMP();
return 0;
}
Input image
Output image

Answer 1

Your formula for the size of the image data is wrong. First you need to find the pitch, by multiplying the width by the bytes per pixel (3 for a 24-bit image), and then rounding up to the nearest multiple of 4. Then multiply the pitch by the height;
int byte_width = width * 3;
int pitch = byte_width + (4 - byte_width % 4) % 4;
int volume = pitch * height;

Answer 2

unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f);
// fwrite(info, sizeof(unsigned char), 54, w); --- comment this line !!!!!!!
int width = *(int*)&info[18];
int height = *(int*)&info[22];
You're writing header to file twice unnecessarily.

Answer 3

As I see in my own code (that was written about 20 years ago) each line of the image is complemented by 0 or more empty bytes to align the start byte. It seems, your calculate wrong alignment.
Just copy & paste here:
unsigned short paddingSize;
unsigned short bitsPerLine = width * bitsPerPixel;
if(1 == bitsPerPixel || 4 == bitsPerPixel)
{
if(bitsPerLine % 8)
bitsPerLine += 8;
paddingSize = (bitsPerLine/8) % 2;
}
else if(8 == bitsPerPixel)
paddingSize = 0x0003 & ~((bitsPerLine/8) % 4 - 1);
else
paddingSize = (bitsPerLine/8) % 2;
Real size of each line is calculatedSize + paddingSize where calculatedSize is exact size of line in bytes i.e. ceil(bitsPerLine/8) or (bitsPerLine + 7)/8 ic C/C++.
What I can say about the code is it's debugged and it works. But I don't remember why all these checks here.