changes_from_vm6_1.txt

JPEG2000实现的源码

                        CHANGES IN VM7.0 FROM VM6.1
                       ============================

Markers
-------
* EOI marker renamed as EOC.
* SOS marker renamed as SOD (code changed to FF93)
* SOP marker changed to include length field.
* ERS marker eliminated (flags moved in COD).
* No real change to SOT marker (the new style was the default in VM6 anyway).
* PPM marker changed to allow continuation of tile-parts across markers.
* POC marker renamed as POD and given extra fields for initial box bounds.
* Component transform info moved from SIZ to COD marker.


Precinct Partition
------------------
* Renamed from "packet partition".
* "No partition" mode eliminated.  There is now always a precinct partition
  with the maximum precinct size corresponding to PPx=PPy=15.


Packet Progressions
-------------------
* Augmented packet sequencer to accommodate the lower bounds for resolution
  and component indices in each bounding box.
* Eliminated tests on box nesting so that all POD parameters are deemed
  legal.


Component Transforms
--------------------
* Implementation modified to allow component transform to be changed
  from tile to tile.


Rate Control
------------
* The scan buffer rate controller presented in WG1N1594 is now available in 
  VM7.0.  This rate controller slides a window along so called "scan 
  elements" and releases bits to the output bitstream at each window 
  position.  Scan elements must be defined by either tiles or precinct 
  locations which span the width of the entire image.  Below are two 
  example command lines for using this new rate controller:

    vm7_compress_32 -i aerial2.ppm -o aerial2.enc -rate 2.0 -Corder PC 
                    -Cpp 64x2048 32x2048 16x2048 8x2048 4x2048 2x2048 
                    -Cscan_buffer_elements 2
    vm7_compress_32 -i aerial2.ppm -o aerial2.enc -rate 2.0 -Ftiles 64 2048 
                    -Cscan_buffer_elements 2

  In both above examples above , the scan buffer simulates the possible 
  performance if only two scan elements are held in the buffer.  The VM also 
  reports the smallest possible memory required to hold two scan elements in 
  the buffer.  Memory used for storage of the compressed codewords (see the 
  "COMPRESSED CODEWORD MEMORY" field printed with `-mem') actually varies 
  from that reported with `-Cscan_buffer_elements' to the output screen, but 
  the decompressed performance should be equivalent to an efficiently 
  implemented version of the scan buffer rate controller.  Efforts have been 
  made, however, to actually reduce the amount of memory used in the VM.  
  Use of this new rate controller should limit hte "COMPRESSED CODEWORD 
  MEMORY" memory significantly.

  The scan buffer rate controller has been implemented inside the 
  ebcot_encoder.c, ebcot_send_bits.c and dst_codeword_heap.c files.  Changes 
  inside ebcot_encoder.c simply initialize various units used for this rate 
  controller.  Changes inside ebcot_send_bits.c are in the new routines 
  set_buffer_threshold() and scan_element_filled() as well as the VM6.1 
  routine ebcot_send_bit_stream().  To hopefully help readability, the VM6.1 
  global rate control is unified with the scan buffer rate controller.


Changes to j2g_fiddle
---------------------
* Added a -make_plt option


Bug Fixes
---------
* Modified image I/O to correctly interpret CR/LF pairs for PGM files on PC 
  platforms
* Corrected convolution kernels in DWT component mixer
* Corrected tag tree references to empty coder blocks
* Corrected progressive visual weighting
* Fixed matrix multiplication in linear component transform
* Fixed speed-up functions for decoder