What is this

This repository contains sample application code for Android uses jpegtran via JNI. This repository uses libjpeg-turbo instead of IJG code.

What is Jpegtran

Jpegtran is a command line program which can exetute the following operations to JPEG image without loss.

• Crop
• Rotate (90,180,270 degrees)
• Flip (horizontal,vertical)
• To Grayscale

It is a part of the IJG(Independet JPEG Group) and libjpeg-turbo code.

File list

• This folder
  In Android Studio, you can import projects by specifying this folder.
• app/src/main/cpp/libjpeg-turbo-2.1.3
  This folder contains libjpeg-turbo code. Some files are modified and removed for this app.
• app/src/main/cpp
  This folder contains ".mk " files which are a kind of makefile for ndk-build. The files were created with reference to DeviceFarmer/android-libjpeg-turbo
• ChangesIJG.md
  This file describes the changes from the original IJG code.
• License.md
  License terms for this codes
• APIUsage.md
  How to use the API. Please read this file when importing this library into your project.

How to build

Import this folder from Android studio, and build.
NDK (Native Development Kit) is required to build. Any version can be used, maybe.
SDK version 29 or later can be used.

Application usage

1. Tap [OPEN] button. The file selector will come up, and select file.
   JPEG file properties will be shown under the button, [SAVE] button will appear right of [OPEN].
2. Tap [SAVE] button, File selector will come up, and specify output file.
   A grayscaled and flipped JPEG image will be output.
   To change the operation, change line 214 of MainActivity.java.

Performance

The following table is ratio of operation time vs IJG code while basic jpegtran operation in my experimence.

Index	Decode	Encode	Total
64bit ARM	82%	54%	66%
32bit ARM	107%	59%	85%

The huffman decoder is not optimized by SIMD intrinsic, so improvement is very small or no.