从二进制数据还原MP3文件

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?)