Reading and Writing to BMP file - c++

Question

I'm new to C++ and programming. I have the following flawed code to read from a BMP file and Write to another BMP file. I did not want to use any external libraries.
I have an 800kb 24bit bmp file. mybmp.bmp. Will try and upload it to dropbox.
`#include <iostream>
#include <conio.h>
#include <fstream>
#include<stdio.h>
#include<stdlib.h>
using namespace std;
unsigned char* editing(char* filename)
{
int i;
int j;
FILE* mybmpfilespointer;
mybmpfilespointer = fopen(filename, "rb");
unsigned char headerinfo[54];
fread(headerinfo, sizeof(unsigned char), 54, mybmpfilespointer); // read the 54-byte header size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );
// extract image height and width from header
int width = *(int*)&headerinfo[18];
int height = *(int*)&headerinfo[22];
int size = 3 * width * height;
unsigned char* imagesdata = new unsigned char[size]; // allocate 3 bytes per pixel
fread(imagesdata, sizeof(unsigned char), size, mybmpfilespointer); // read the rest of the imagesdata at once
// display image height and width from header
cout << " width:" << width << endl;
cout << " height:" << height << endl;
ofstream arrayfile("bmpofstream.bmp"); // File Creation
for(int a = 0; a < 53; a++) //bgr to rgb
{
arrayfile << headerinfo[a];
}
for(int k=0; k<size; k++)
{
arrayfile<<imagesdata[k]<<endl; //Outputs array to file
}
arrayfile.close();
delete[] mybmpfilespointer;
delete[] imagesdata;
fclose(mybmpfilespointer);
return imagesdata;
return headerinfo;
}
int main()
{
FILE* mybmpfilespointer = fopen("mybmp.bmp", "rb");
if (mybmpfilespointer)
{
editing("mybmp.bmp");
}
else
{
cout << "Cant Read File";
}
}`
As you can see I read from mybmp.bmp which is 819680bytes
and write to bmpofstream.bmp as it is.
But somehow the resulting file is exactly 3x times the size of the mybmp around 2460826bytes.
I read header from mybmp file as headerinfo.
and
data from mybmp as imagesdata.
When I write to bmpofstream.bmp these arrays it is a messed up bmp file.
1) I'm guessing the increase in filesize is related to reading individual pixels and writing them 3 times or something but couldnt figure out. Why do you think this would be?
2) Once I figure out how to read and write this file as it is, I wanted to modify it. So I might as well ask this now:
I wanted to modify this image so that I can increase the value of each pixel by 50, so this would end up in a darker image. Can I do this directly as:
for(j = 0; j < size; j++)
{
imagesdata[j]=imagesdata[j]+50;
}
thank you.

Answer 1

I suggest taking a look at some existing library, even if you want to code yourself you may learn a lot. See for example libbmp source code,
https://code.google.com/p/libbmp/

Answer 2

try this
#include <iostream>
#include <conio.h>
#include <fstream>
#include <vector>
#include <iterator>
void editing( char* filename )
{
std::ifstream input( filename, std::ios::binary );
if( !input.is_open() )
{
std::cout << "Can`t open file" << std::endl;
return;
}
// copies all data into buffer
std::vector<char> buffer( ( std::istreambuf_iterator<char>( input ) ), ( std::istreambuf_iterator<char>() ) );
input.close();
std::vector<char> headerinfo;
{
auto it = std::next( buffer.begin(), 54 );
std::move( buffer.begin(), it, std::back_inserter( headerinfo ) );
buffer.erase( buffer.begin(), it );
}
// extract image height and width from header
int width = *reinterpret_cast<int*>( (char*)headerinfo.data() + 18 );
int height = *reinterpret_cast<int*>( (char*)headerinfo.data() + 22 );
int size = 3 * width * height;
std::vector<char> imagesdata;
{
auto it = std::next( buffer.begin(), size );
std::move( buffer.begin(), it, std::back_inserter( imagesdata ) );
buffer.erase( buffer.begin(), it );
}
// display image height and width from header
std::cout << " width:" << width << std::endl;
std::cout << " height:" << height << std::endl;
// paste your code here
// ...
std::ofstream arrayfile( "bmpofstream.bmp" ); // File Creation
std::ostream_iterator<char> output_iterator( arrayfile );
std::copy( headerinfo.begin(), headerinfo.end(), output_iterator ); // write header to file
std::copy( imagesdata.begin(), imagesdata.end(), output_iterator ); // write image data to file
}
int main(int argc, char**argv)
{
editing( argv[1] );
}