Restore MP3 file from binary data - c++

Question

My taks is to restore an mp3 file, wich is coded bit-per-bit in a PNG file. I got the right bits from the PNG RGB data (per pixel) in a vector. I'm using C++.
I have to go through the png file and read the RGB data of a pixel: then I have 3 decimal values. From binary representation of the decimal values, I need the least smallest local value. The 11 pixels shows on 33 bits the length of the mp3. Then i decode all of the binary data from the pixels, and put in a vector;
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <vector>
#include <math.h>
#include <iostream>
#include <fstream>
#define PNG_DEBUG 3
#include <png.h>
void abort_(const char * s, ...)
{
va_list args;
va_start(args, s);
vfprintf(stderr, s, args);
fprintf(stderr, "\n");
va_end(args);
abort();
}
void itob(short n, std::vector<int> &bin)
{
int d = n;
if (n > 1)
{
d = n % 2;
itob(n / 2, bin);
}
bin.push_back(d);
}
void btoi(unsigned int& n, std::vector<int> bin)
{
n = 0;
int k = 32;
for(int i = 0; i < bin.size() ; i++){
if(bin[i] == 1){
long int num = pow(2,k);
n += num;
}
k--;
}
}
int x, y;
int width, height;
png_byte color_type;
png_byte bit_depth;
png_structp png_ptr;
png_infop info_ptr;
int number_of_passes;
png_bytep * row_pointers;
void read_png_file()
{
unsigned char header[8]; // 8 is the maximum size that can be checked
/* open file and test for it being a png */
FILE *fp = fopen("image.png", "rb");
if (!fp)
abort_("[read_png_file] File %s could not be opened for reading", "image.png");
fread(header, 1, 8, fp);
if (png_sig_cmp(header, 0, 8))
abort_("[read_png_file] File %s is not recognized as a PNG file", "image.png");
/* initialize stuff */
png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
if (!png_ptr)
abort_("[read_png_file] png_create_read_struct failed");
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
abort_("[read_png_file] png_create_info_struct failed");
png_init_io(png_ptr, fp);
png_set_sig_bytes(png_ptr, 8);
png_read_info(png_ptr, info_ptr);
width = png_get_image_width(png_ptr, info_ptr);
height = png_get_image_height(png_ptr, info_ptr);
color_type = png_get_color_type(png_ptr, info_ptr);
bit_depth = png_get_bit_depth(png_ptr, info_ptr);
number_of_passes = png_set_interlace_handling(png_ptr);
png_read_update_info(png_ptr, info_ptr);
row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);
for (y=0; y<height; y++)
row_pointers[y] = (png_byte*) malloc(png_get_rowbytes(png_ptr,info_ptr));
png_read_image(png_ptr, row_pointers);
fclose(fp);
}
void process_file(void)
{
if (png_get_color_type(png_ptr, info_ptr) == PNG_COLOR_TYPE_RGBA)
abort_("[process_file] input file is PNG_COLOR_TYPE_RGB but must be PNG_COLOR_TYPE_RGB "
"(lacks the alpha channel)");
if (png_get_color_type(png_ptr, info_ptr) != PNG_COLOR_TYPE_RGB)
abort_("[process_file] color_type of input file must be PNG_COLOR_TYPE_RGB (%d) (is %d)",
PNG_COLOR_TYPE_RGBA, png_get_color_type(png_ptr, info_ptr));
printf("width: %d\nheight: %d\n", width, height);
int mHeader = 33; unsigned int mSize = 0;
std::vector<int> mSizeByBites;
for (y=0; y<height; y++) {
png_byte* row = row_pointers[y];
for (x=0; x<width; x++) {
png_byte* ptr = &(row[x*3]);
if(mHeader == 0){ break; }
mHeader-=3;
std::vector<int> b;
itob(ptr[0], b);
mSizeByBites.push_back(b[b.size()-1]);
b.clear();
itob(ptr[1], b);
mSizeByBites.push_back(b[b.size()-1]);
b.clear();
itob(ptr[2], b);
mSizeByBites.push_back(b[b.size()-1]);
b.clear();
}
if(mHeader == 0){ break; }
}
for(int i =0; i<mSizeByBites.size(); i++){
printf("%d", mSizeByBites[i]);
}
btoi(mSize, mSizeByBites);
printf(" = %i\n", mSize);
std::vector<int> mDataBaBites;
for (y=0; y<height; y++) {
png_byte* row = row_pointers[y];
for (x=0; x<width; x++) {
if(mSize <= 0){ break; }
png_byte* ptr = &(row[x*3]);
std::vector<int> b;
itob(ptr[0], b);
mDataBaBites.push_back(b[b.size()-1]);
b.clear();
mSize--;
if(mSize <= 0){ break; }
itob(ptr[1], b);
mDataBaBites.push_back(b[b.size()-1]);
b.clear();
mSize--;
if(mSize <= 0){ break; }
itob(ptr[2], b);
mDataBaBites.push_back(b[b.size()-1]);
b.clear();
mSize--;
if(mSize <= 0){ break; }
printf("%i\n", mSize);
}
if(mSize<=0){ break; }
}
std::ofstream output("result.mp3", std::ios::out | std::ios::binary);
printf("[D] Writing to file start: %li\n", mDataBaBites.size());
output.write( (char*)(&mDataBaBites[0]), mDataBaBites.size() );
output.close();
}
int main(int argc, char **argv)
{
read_png_file();
process_file();
return 0;
}
Now I have no clue, how to write it in a file, wich i can play as an mp3. I tried to convert the bits to hexa.
What is the correct format of an mp3 file? How can I write the bits in the correct format?

Answer 1

Try this:
#include <fstream> //For std::min
std::ofstream mp3File( "restored.mp3", std::ios::out | std::ios::binary );
//Assuming rgbData is a char* with the mp3 data,
//and rgbDataSize is its size in bytes
mp3File.write( rgbData, rgbDataSize );
mp3File.close();
Update: When we (programmers) say "binary representation" we almost always mean bytes, not bits. From your description of the decoding process, I gather you should compare the 3 RGB components for each pixel and keep the minimum as the decoded byte. To do that:
#include <algorithm>
//...
std::vector<char> mDataBaBites;
for (y=0; y<height; y++) {
png_byte* row = row_pointers[y];
for (x=0; x<width; x++) {
png_byte red = row[x*3];
png_byte green = row[x*3 + 1];
png_byte blue = row[x*3 + 2];
png_byte minByte = std::min( std::min(red,green), blue );
mDataBaBites.push_back( minByte );
mSize -= 3;
}
if(mSize<=0){ break; }
}
std::ofstream output("result.mp3", std::ios::out | std::ios::binary);
printf("[D] Writing to file start: %li\n", mDataBaBites.size());
output.write( (char*)(&mDataBaBites[0]), mDataBaBites.size() );
output.close();
Update 2:
std::ofstream output("result.mp3", std::ios::out | std::ios::binary);
printf("[D] Writing to file start: %li\n", mDataBaBites.size());
for( int i=0; i<mDataBaBites.size(); i+=8 ){
char decodedByte = 0;
for( int j=0; j<8; j++ )
decodedByte |= (mDataBaBites[i+j] << j);
output.write( (char*)(&mDataBaBites[0]), 1 );
}
output.close();
If this doesn't work either, you might want to clarify the decoding process definition (which is its source? is there some formal definition?)