JP2 for Android

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An open-source JPEG-2000 image encoder/decoder for Android based on OpenJPEG v2.4.0.

Set up

Add dependency to your build.gradle:

implementation 'com.gemalto.jp2:jp2-android:1.0.3'

Basic Usage

Decoding an image:

Bitmap bmp = new JP2Decoder(jp2data).decode();
imgView.setImageBitmap(bmp);

Encoding an image:

Bitmap bmp = ...;
//lossless encode
byte[] jp2data = new JP2Encoder(bmp).encode();
//lossy encode (target PSNR = 50dB)
byte[] jp2data = new JP2Encoder(bmp)
                     .setVisualQuality(50)
                     .encode();

Advanced Usage

Multiple Resolutions

A single JPEG-2000 image can contain multiple resolutions. The final resolution is always equal to <image_width> x <image_height>; each additional resolution reduces the width and height by the factor of two. If you don't need the full resolution, you can save memory and decoding time by decoding at a lower resolution.

Encoding

Default number of resolutions is 6, but you can specify your own value:

byte[] jp2data = new JP2Encoder(bmp)
                     .setNumResolutions(3)
                     .encode();

The number of resolutions must be between 1 and 32 and both image width and height must be greater or equal to 2^(numResolutions - 1).

Decoding

You can obtain the number of resolutions (as well as some other information about the image) by calling the readHeader() method:

Header header = new JP2Decoder(jp2data).readHeader();
int numResolutions = header.numResolutions;

If you don't need the full resolution image, you can skip one or more resolutions during the decoding process.

Bitmap reducedBmp = new JP2Decoder(jp2data)
                            .setSkipResolutions(2)
                            .decode();

Multiple Quality Layers

Multiple layers can be encoded in a JPEG-2000 image, each having a different visual quality. If you don't need maximum visual quality, you can save decoding time by skipping the higher-quality layers.

Encoding

Quality layers can be specified in two ways: as a list of compression ratios or visual qualities. The compression ratios are specified as factors of compression, i.e. 20 means the size will be 20 times less than the raw uncompressed size. Compression ratio 1 means lossless compression. Example:

byte[] jp2data = new JP2Encoder(bmp)
                     //specify 3 quality layers with compression ratios 1:50, 1:20, and lossless.
                     .setCompressionRatio(50, 10, 1)
                     .encode();

Visual quality is specified as PSNR values in dB. Usable values are roughly between 20 (very aggressive compression) and 70 (almost lossless). Special value 0 indicates lossless compression. Example:

byte[] jp2data = new JP2Encoder(bmp)
                     //specify 3 quality layers with PSNR 30dB, 50dB, and lossless.
                     .setVisualQuality(30, 50, 0)
                     .encode();

setCompressionRatio() and setVisualQuality() can not be used at the same time.

Decoding

You can obtain the number of available quality layers by calling the readHeader() method:

Header header = new JP2Decoder(jp2data).readHeader();
int numQualityLayers = header.numQualityLayers;

If you don't need a maximum quality image, you can trade some visual quality for a shorter decoding time by not decoding all the quality layers.

Bitmap lowQualityBmp = new JP2Decoder(jp2data)
                              .setLayersToDecode(2)
                              .decode();

File Format

JP2Encoder supports two output formats:

• JP2 - standard JPEG-2000 file format (encapsulating a JPEG-2000 codestream)
• J2K - unencapsulated JPEG-2000 codestream

JP2 is the default output format, but it can be changed:

byte[] j2kdata = new JP2Encoder(bmp)
                     .setOutputFormat(FORMAT_J2K)
                     .encode();