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USING THE IJG JPEG LIBRARY Copyright (C) 1994-2009, Thomas G. Lane, Guido Vollbeding. This file is part of the Independent JPEG Group's software. For conditions of distribution and use, see the accompanying README file. This file describes how to use the IJG JPEG library within an application program. Read it if you want to write a program that uses the library. The file example.c provides heavily commented skeleton code for calling the JPEG library. Also see jpeglib.h (the include file to be used by application programs) for full details about data structures and function parameter lists. The library source code, of course, is the ultimate reference. Note that there have been *major* changes from the application interface presented by IJG version 4 and earlier versions. The old design had several inherent limitations, and it had accumulated a lot of cruft as we added features while trying to minimize application-interface changes. We have sacrificed backward compatibility in the version 5 rewrite, but we think the improvements justify this. TABLE OF CONTENTS ----------------- Overview: Functions provided by the library Outline of typical usage Basic library usage: Data formats Compression details Decompression details Mechanics of usage: include files, linking, etc Advanced features: Compression parameter selection Decompression parameter selection Special color spaces Error handling Compressed data handling (source and destination managers) I/O suspension Progressive JPEG support Buffered-image mode Abbreviated datastreams and multiple images Special markers Raw (downsampled) image data Really raw data: DCT coefficients Progress monitoring Memory management Memory usage Library compile-time options Portability considerations Notes for MS-DOS implementors You should read at least the overview and basic usage sections before trying to program with the library. The sections on advanced features can be read if and when you need them. OVERVIEW ======== Functions provided by the library --------------------------------- The IJG JPEG library provides C code to read and write JPEG-compressed image files. The surrounding application program receives or supplies image data a scanline at a time, using a straightforward uncompressed image format. All details of color conversion and other preprocessing/postprocessing can be handled by the library. The library includes a substantial amount of code that is not covered by the JPEG standard but is necessary for typical applications of JPEG. These functions preprocess the image before JPEG compression or postprocess it after decompression. They include colorspace conversion, downsampling/upsampling, and color quantization. The application indirectly selects use of this code by specifying the format in which it wishes to supply or receive image data. For example, if colormapped output is requested, then the decompression library automatically invokes color quantization. A wide range of quality vs. speed tradeoffs are possible in JPEG processing, and even more so in decompression postprocessing. The decompression library provides multiple implementations that cover most of the useful tradeoffs, ranging from very-high-quality down to fast-preview operation. On the compression side we have generally not provided low-quality choices, since compression is normally less time-critical. It should be understood that the low-quality modes may not meet the JPEG standard's accuracy requirements; nonetheless, they are useful for viewers. A word about functions *not* provided by the library. We handle a subset of the ISO JPEG standard; most baseline, extended-sequential, and progressive JPEG processes are supported. (Our subset includes all features now in common use.) Unsupported ISO options include: * Hierarchical storage * Lossless JPEG * DNL marker * Nonintegral subsampling ratios We support both 8- and 12-bit data precision, but this is a compile-time choice rather than a run-time choice; hence it is difficult to use both precisions in a single application. By itself, the library handles only interchange JPEG datastreams --- in particular the widely used JFIF file format. The library can be used by surrounding code to process interchange or abbreviated JPEG datastreams that are embedded in more complex file formats. (For example, this library is used by the free LIBTIFF library to support JPEG compression in TIFF.) Outline of typical usage ------------------------ The rough outline of a JPEG compression operation is: Allocate and initialize a JPEG compression object Specify the destination for the compressed data (eg, a file) Set parameters for compression, including image size & colorspace jpeg_start_compress(...); while (scan lines remain to be written) jpeg_write_scanlines(...); jpeg_finish_compress(...); Release the JPEG compression object A JPEG compression object holds parameters and working state for the JPEG library. We make creation/destruction of the object separate from starting or finishing compression of an image; the same object can be re-used for a series of image compression operations. This makes it easy to re-use the same parameter settings for a sequence of images. Re-use of a JPEG object also has important implications for processing abbreviated JPEG datastreams, as discussed later. The image data to be compressed is supplied to jpeg_write_scanlines() from in-memory buffers. If the application is doing file-to-file compression, reading image data from the source file is the application's responsibility. The library emits compressed data by calling a "data destination manager", which typically will write the data into a file; but the application can provide its own destination manager to do something else. Similarly, the rough outline of a JPEG decompression operation is: Allocate and initialize a JPEG decompression object Specify the source of the compressed data (eg, a file) Call jpeg_read_header() to obtain image info Set parameters for decompression jpeg_start_decompress(...); while (scan lines remain to be read) jpeg_read_scanlines(...); jpeg_finish_decompress(...); Release the JPEG decompression object This is comparable to the compression outline except that reading the datastream header is a separate step. This is helpful because information about the image's size, colorspace, etc is available when the application selects decompression parameters. For example, the application can choose an output scaling ratio that will fit the image into the available screen size. The decompression library obtains compressed data by calling a data source manager, which typically will read the data from a file; but other behaviors can be obtained with a custom source manager. Decompressed data is delivered into in-memory buffers passed to jpeg_read_scanlines(). It is possible to abort an incomplete compression or decompression operation by calling jpeg_abort(); or, if you do not need to retain the JPEG object, simply release it by calling jpeg_destroy(). JPEG compression and decompression objects are two separate struct types. However, they share some common fields, and certain routines such as jpeg_destroy() can work on either type of object. The JPEG library has no static variables: all state is in the compression or decompression object. Therefore it is possible to process multiple compression and decompression operations concurrently, using multiple JPEG objects. Both compression and decompression can be done in an incremental memory-to- memory fashion, if suitable source/destination managers are used. See the section on "I/O suspension" for more details. BASIC LIBRARY USAGE =================== Data formats ------------ Before diving into procedural details, it is helpful to understand the image data format that the JPEG library expects or returns. The standard input image format is a rectangular array