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Overview

This document describes changing the IJG code. The IJG code is in the following folder.
app/src/main/cpp
Base version of the IJG code is 9c. (Not latest)

The changes were made from the following three points.

  • Remove unused functions and files.
  • Modify interfaces for Android.
  • Add extension functions.

Almost modified parts are commented with keyword ajpegtran. Execute grep to check.

Remove unused

Remove unused files

The following files are removed.

  • makefile, cofig files and project files for another systems
  • files for unused functions
    image reader,image writer, DCT, iDCT, command line interface...

Remove unused functions

Modified jmorecfg.h for removing unused functions. Some '#define' are commented out.

Modify interfaces for Android.

Add entry function for Android.

Added ajpegtran.c based on jpegtran.c, a command line interface. This file contains functions can be called from Android.

Add .mk file

Added Android.mk, a kind of makefile for th native build environment, ndkBuild.

Modify error interface

In the original implementation, when an error occurs, exit() function is called and program terminates. This implementation uses the longjmp() and setjmp() APIs. When an error occurs, longjmp() is called, and back to root function which called setjmp(). And error message is set to global variable. Refer jerror.c and jerror.c to check implementation.

Modify file I/O interface

In the original implementation, file pathes are pathed to interface function and access with fopen(), fread() and fwrite() functions. Ih this implementation, file descriptors are pathed, and access with read() and write() functions. Because in Android system, user application can't obtain the file path. Refer last part of jinclude.h and ajpegtran.c to check implementation.

Add extension functions

Pixelize

The pixelize function is implemented to do_pixelize() function in transupp.c. The function is based on do_flatten() function. do_flatten() clears all DCT coefficent to fill block with gray color. do_pixelize() clears AC coefficients only, this causes 8x8 pixlization.

Offset and Monochrome

The offset and monochrome functions are implemented in ajpegtran.c. Refer toMonochrome() and brightnessControl() function.

Clear EXIF tags

Clear orientation information, thumbnail and GEOTAGs functions are implimented to scan_exif_parameters_for_clear() in transupp.c. This implementation fills these tags with zeros, instead of front-packing.