JasPer: Codecs

JasPer 4.2.1 |

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Codecs

The JasPer software supports several popular codecs. In some cases, the support is native (i.e., built-in to the JasPer library itself). In other cases, the support is provided indirectly through external software dependencies. The sections that follow describe the various codecs supported by JasPer in more detail.

# Common Codec Functionality

Some encoder and decoder options are generic in the sense that they are essentially independent of the codec. Typically, encoders and decoders tend to ignore unrecognized options (although a warning might be issued for such options). So, it is not usually problematic to specify an unsupported option to an encoder or a decoder.

## Generic Encoder Options

The following generic options are supported by some encoders:

- `debug=$level`. Set the debug level to `$level`.

## Generic Decoder Options

The following generic options are supported by some decoders:

- `debug=$level`. Set the debug level to `$level`.
- `max_samples=$n`. Set the maximum number of samples that are permitted in an image to be decoded to `$n`. (An 100 100 RGB-color image has 30000 samples.)

# BMP Codec

One of the most popular image formats on the Microsoft Windows platform is Microsoft's BMP format. The BMP codec in JasPer was written without the benefit of the BMP format specification from Microsoft. This means that the BMP support will inevitably not work correctly for all valid BMP files.

## Encoder Options

The BMP encoder does not support any special options.

## Decoder Options

The BMP decoder supports the following generic options:

- `max_samples`

The BMP decoder does not support any options beyond generic ones.

# HEIC Codec

The HEIC format is quite popular on Apple platforms. The support is experimental and likely has numerous bugs.

## Encoder Options

The HEIC encoder supports the following special options:

- `version`. Print the version of the Libheif library being used without performing any decoding.
- `quality $quality`. Set the quality factor to `$quality`.

## Decoder Options

The HEIC decoder supports the following special options:

- `version`. Print the version of the Libheif library being used without performing any decoding.

The HEIC decoder does not support any generic options.

# JP2 Codec

One of the two image formats specified in the JPEG-2000 Part-1 standard (i.e., ISO/IEC 15444-1 [iso15444p1]) is the so called "JP2" format.

## Encoder Options

The JP2 encoder supports all of the same options as the JPC encoder.

## Decoder Options

The JP2 decoder supports all of the same options as the JPC decoder.

# JPC Codec

One of the two image formats specified in the JPEG-2000 Part-1 standard (i.e., ISO/IEC 15444-1 [iso15444p1]) is the so called JPEG-2000 code stream format. The JPC codec in JasPer implements this format.

The design of the JPEG-2000 codec implementation was driven by several key concerns: execution speed, memory usage, robustness, portability, modularity, maintainability, and extensibility. In some cases, however, during the design process, modularity, portability, and understandability of the code were weighed more heavily than execution speed and memory usage. Code understandability and portability were critical considerations since this software was intended to be used as a reference implementation of the JPEG-2000 Part-1 codec in the JPEG-2000 Part-5 standard [iso15444p5].

Since the JPEG-2000 standard does not specify any means for encoding color space information in a JPEG-2000 code stream, the decoder must make certain assumptions about the color space of an image. If accurate color representation is important, the JPEG-2000 code stream format should not be employed. The JPEG-2000 JP2 format should be used instead.

## Encoder Options

The JPC encoder supports the following special options:

- `debug=$level`. Set the debug level to `$level`.
- `imgareatlx=$x`. Set the x-coordinate of the top-left corner of the image area to `$x`.
- `imgareatly=$y`. Set the y-coordinate of the top-left corner of the image area to `$y`.
- `tilegrdtlx=$x`. Set the x-coordinate of the top-left corner of the tiling grid to `$x`.
- `tilegrdtly=$y`. Set the y-coordinate of the top-left corner of the tiling grid to `$y`.
- `tilewidth=$w`. Set the nominal tile width to `$w`.
- `tileheight=$h`. Set the nominal tile height to `$h`.
- `prcwidth=$w`. Set the precinct width to `$w`. The argument `$w` must be an integer power of two. The default value is 32768.
- `prcheight=$h`. Set the precinct height to `$h`. The argument `$h` must be an integer power of two. The default value is 32768.
- `cblkwidth=$w`. Set the nominal code block width to `$w`. The argument `$w` must be an integer power of two. The default value is 64.
- `cblkheight=$h`. Set the nominal code block height to `$h`. The argument `$h` must be an integer power of two. The default value is 64.

- `mode=$m`. Set the coding mode to `$m`. The argument `$m` must have one of the following values:

  - `int`. integer mode
  - `real`. real mode

  If lossless coding is desired, the integer mode must be used. By default, the integer mode is employed. The choice of mode also determines which multicomponent and wavelet transforms (if any) are employed.

- `rate=$r`. Specify the target rate. The argument `$r` is a positive real number. Since a rate of one corresponds to no compression, one should never need to explicitly specify a rate greater than one. By default, the target rate is considered to be infinite.
- `ilyrrates=` [, , , ]. Specify the rates for any intermediate layers. The argument to this option is a comma separated list of rates. Each rate is a positive real number. The rates must increase monotonically. The last rate in the list should be less than or equal to the overall rate (as specified with the `rate` option).

- `prg=$p`. Set the progression order to `$p`. The argument `$p` must have one of the following values:

  - `lrcp`. layer-resolution-component-position (LRCP) progressive (i.e., rate scalable)
  - `rlcp`. resolution-layer-component-position (RLCP) progressive (i.e., resolution scalable)
  - `rpcl`. resolution-position-component-layer (RPCL) progressive
  - `pcrl`. position-component-resolution-layer (PCRL) progressive
  - `cprl`. component-position-resolution-layer (CPRL) progressive

  By default, LRCP progressive ordering is employed. Note that the RPCL and PCRL progressions are not valid for all possible image geometries. (See [iso15444p1] for more details.)

- `nomct`. Disallow the use of any multicomponent transform.
- `numrlvls=$n`. Set the number of resolution levels to `$n`. The argument `$n` must be an integer that is greater than or equal to one. The default value is 6.
- `sop`. Generate SOP marker segments.
- `eph`. Generate EPH marker segments.
- `lazy`. Enable lazy coding mode (a.k.a. arithmetic coding bypass).
- `termall`. Terminate all coding passes.
- `segsym`. Use segmentation symbols.
- `vcausal`. Use vertically stripe causal contexts.
- `pterm`. Use predictable termination.
- `resetprob`. Reset the probability models after each coding pass.
- `numgbits=$n`. Set the number of guard bits to `$n`.
- `_jp2overhead=$n`. This is for internal use only. It allows the rate to compensate for the overhead of a container format in which the JPEG-2000 code stream is embedded.

## Decoder Options

The JPC decoder supports the following special options:

- `maxlyrs=$n`. Set the maximum number of layers to decode to `$n`.
- `maxpkts=$n`. Set the maximum number of packets to decode to `$n`.

The following generic options are supported:

- `debug`
- `max_samples`

## Rate Specification

All rates are specified in terms of compression factors (i.e., as reciprocals of compression ratio) and not as actual bit rates! Although image coding folks frequently use the number of bits per pixel to specify rate, this quantity is often inconvenient to use when dealing with images that have differing sample precisions. Furthermore, the number of bits per pixel is not well defined for multicomponent images with distinct subsampling factors. The compression factor, however, is independent of sample precision and well defined for all types of images. For these reasons, JasPer uses the compression factor and not the number of bits per pixel to specify rates.

# JPG Codec

For lossy coding, one of the most popular image formats is specified in the JPEG standard (i.e., ISO/IEC 10918-1 [iso10918p1]). In JasPer, the JPG codec implements this format.

The JPEG support in JasPer requires the JPEG library from the Independent JPEG Group (IJG). For legal reasons, the IJG JPEG library source code is not included with JasPer. The source code for this library can be downloaded from the IJG web site (i.e., http://www.ijg.org).

## Encoder Options

The JPG encoder supports the following special options:

- `quality=$q`. Set the quality factor to `$q`. This is used in order to indirectly control the bit rate for lossy coding.

The JPG encoder does not support any generic options.

## Decoder Options

The JPG decoder supports the following special options:

- `version`. Indicate that the decoder should only print the version of the IJG JPEG library in use. (No decoding is performed.)

The JPG decoder supports the following generic options:

- `max_samples`

# PGX Codec

The JPEG-2000 Verification Model software employs a non-standard format called PGX. In JasPer, this format is handled by the PGX codec. The PGX format can only handle single components images, and consequently, is of limited use.

## Encoder Options

The PGX encoder does not support any generic or special options.

## Decoder Options

The PGX decoder supports the following special options:

- `allow_trunc`. Allow decoding of truncated bitstreams (i.e., reaching EOF during decoding is not deemed to be an error).

The PGX decoder supports the following generic options:

- `max_samples`

# MIF Codec

The MIF format is not a standard format. This format was invented solely for the purpose of testing the JasPer software. The support for the MIF format is experimental. It is intended to be used for advanced testing of the JasPer JPEG-2000 codec implementation. It is strongly recommended that this codec not be enabled/used in code running in environments where security is a concern.

## Encoder Options

The MIF encoder does not support any generic or special options.

## Decoder Options

The MIF decoder does not support any generic or special options.

# PNM Codec

On UNIX platforms, the Portable Pixmap/Graymap/Bitmap (PNM) format is quite popular for coding image data. The PNM codec in JasPer supports this format. In JasPer, the support for the PNM/PGM/PPM format is complete. Therefore, the use of this format is favored over the BMP format. A (nonstandard) extension has also been added to the support for the PNM format so that it can handle images with signed sample values.

## Encoder Options

The PNM encoder supports the following special options:

- `text`. Use a non-raw (i.e., non-binary) flavor of PNM format.

## Decoder Options

The PNM decoder supports the following special options:

- `allow_trunc`. Allow decoding of truncated bitstreams (i.e., reaching EOF during decoding is not deemed to be an error).

The PNM decoder supports the following generic options:

- `max_samples`

# RAS Codec

One popular image format on Sun workstations is the Sun Rasterfile format. The RAS codec in JasPer implements this format.

## Encoder Options

The encoder does not support any special options.

## Decoder Options

The RAS decoder supports the following special options:

- `allow_trunc`. Allow decoding of truncated bitstreams (i.e., reaching EOF during decoding is not deemed to be an error).

The RAS decoder supports the following generic options:

- `max_samples`