hacking

JPEG-MPEG編解碼技術書集的代碼

************************************************************************
LAME API

For a general outline of the code, see the file API.  
Also, main.c is a simple front end to libmp3lame.a

The guts of the code are called from lame_encode_buffer().

lame_encode_buffer() handles buffering, resampling, filtering, and
then calls lame_encode_frame() for each frame:

lame_encode_frame():
   l3psycho_anal()        compute masking thresholds
   mdct_sub()             compute MDCT coefficients
   iteration_loop()       choose scalefactors (via iteration)
                          which determine noise shapping, and 
                          choose best huffman tables for lossless compression

   III_formatBitstream    format the bitstream and store in bit_buffer
   copy_buffer()          make a copy of bit_buffer, to return to calling program
   write_buffer()         optionally output bit_buffer to a file
   empty_buffer()	  mark bit_buffer as empty





************************************************************************
We are in the process of switching the whole code to FLOAT or FLOAT8,
which ever is faster (defined in machine.h).  
Avoid using float or double, and instead use:

FLOAT    4 byte floating point.  
FLOAT8   8 byte floating point (only when necessary)


************************************************************************
Initialization:   LAME can be used as a library, and there are two
kinds of static data:  data that has to be initialized only once
when the library is called, and data that has to be initialized
every time you start encoding a new sample.

These routines should be coded something like:

#include "globalflags"
void subroutine(void)
{
  static int firstcall=1;
  if (firstcall) {
    firstcall=0;
    /* code that needs to be done only once when routine is loaded */
  }
  if (gf.frameNum==0) {
    /* code that needs to be done before encoding each file/stream */
  }
}

The frameNum==0 code only works for routines which are only called
once per frame.  More general code would be:


#include "globalflags"
void subroutine(void)
{
  static int init=0;
  if (gf.frameNum==0 && !init) {
    /* code that needs to be done before encoding each file/stream */
    init=1;
  }
  if (gf.frameNum>0) init=0;
}





************************************************************************
Global flags

For better or worse, there are many global flags in lame, 
in the 'gf' structure.  

The rules for adding a global flag are:

1. declair in struct gf in lame.h
2. default values must be set in lame_init()
3. include "globalflags.h" in routines that need access to these flags.