@julusian/jpeg-turbo

USING THE IJG JPEG LIBRARY This file was part of the Independent JPEG Group's software: Copyright (C) 1994-2013, Thomas G. Lane, Guido Vollbeding. libjpeg-turbo Modifications: Copyright (C) 2010, 2014-2018, 2020, D. R. Commander. Copyright (C) 2015, Google, Inc. For conditions of distribution and use, see the accompanying README.ijg 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.txt 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 Partial image decompression 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 ICC profiles Raw (downsampled) image data Really raw data: DCT coefficients Progress monitoring Memory management Memory usage Library compile-time options Portability considerations 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 of pixels, with each pixel having the same number of "component" or "sample" values (color channels). You must specify how many components there are and the colorspace interpretation of the components. Most applications will use RGB data (three components per pixel) or grayscale data (one component per pixel). PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE. A remarkable number of people manage to miss this, only to find that their programs don't work with grayscale JPEG files. There is no provision for colormapped input. JPEG files are always full-color or full grayscale (or sometimes another colorspace such as CMYK). You can feed in a colormapped image by expanding it to full-color format. However JPEG often doesn't work very well with source data that has been colormapped, because of dithering noise. This is discussed in more detail in the JPEG FAQ and the other references mentioned in the README.ijg file. Pixels are stored by scanlines, with each scanline running from left to right. The component values for each pixel are adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an array of data type JSAMPLE --- which is typically "unsigned char", unless you've changed jmorecfg.h. (You can also change the RGB pixel layout, say to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in that file before doing so.) A 2-D array of pixels is formed by making a list of pointers to the starts of scanlines; so the scanlines need not be physically adjacent in memory. Even if you process just one scanline at a time, you must make a one-element pointer array to conform to this structure. Pointers to JSAMPLE rows are of type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY. The library accepts or supplies one or more complete scanlines per call. It is not possible to process part of a row at a time. Scanlines are always processed top-to-bottom. You can process an entire image in one call if you have it all in memory, but usually it's simplest to process one scanline at a time. For best results, source data values should have the precision specified by BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress data that's only 6 bits/channel, you should left-justify each value in a byte before passing it to the compressor. If you need to compress data that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12. (See "Library compile-time options", later.) The data format returned by the decompressor is the same in all details, except that colormapped output is supported. (Again, a JPEG file is never colormapped. But you can ask the decompressor to perform on-the-fly color quantization to deliver colormapped output.) If you request colormapped output then the returned data array contains a single JSAMPLE per pixel; its value is an index into a color map. The color map is represented as a 2-D JSAMPARRAY in which each row holds the values of one color component, that is, colormap[i][j] is the value of the i'th color component for pixel value (map index) j. Note that since the colormap indexes are stored in JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE (ie, at most 256 colors for an 8-bit JPEG library). Compression details ------------------- Here we revisit the JPEG compression outline given in the overview. 1. Allocate and initialize a JPEG compression object. A JPEG compression object is a "struct jpeg_compress_struct". (It also has a bunch of subsidiary structures which are allocated via malloc(), but the application doesn't control those directly.) This struct can be just a local variable in the calling routine, if a single routine is going to execute the whole JPEG compression sequence. Otherwise it can be static or allocated from malloc(). You will also need a structure representing a JPEG error handler. The part of this that the library cares about is a "struct jpeg_error_mgr". If you are providing your own error handler, you'll typically want to embed the jpeg_error_mgr struct in a larger structure; this is discussed later under "Error handling". For now we'll assume you are just using the default error handler. The default error handler will print JPEG error/warning messages on stderr, and it will call exit() if a fatal error occurs. You must initialize the error handler structure, store a pointer to it into the JPEG object's "err" field, and then call jpeg_create_compress() to initialize the rest of the JPEG object. Typical code for this step, if you are using the default error handler, is struct jpeg_compress_struct cinfo; struct jpeg_error_mgr jerr; ... cinfo.err = jpeg_std_error(&jerr); jpeg_create_compress(&cinfo); jpeg_create_compress allocates a small amount of memory, so it could fail if you are out of memory. In that case it will exit via the error handler; that's why the error handler must be initialized first. 2. Specify the destination for the compressed data (eg, a file). As previously mentioned, the JPEG library delivers compressed data to a "data destination" module. The library includes one data destination module which knows how to write to a stdio stream. You can use your own destination module if you want to do something else, as discussed later. If you use the standard destination module, you must open the target stdio stream beforehand. Typical code for this step looks like: FILE *outfile; ... if ((outfile = fopen(filename, "wb")) == NULL) { fprintf(stderr, "can't open %s\n", filename); exit(1); } jpeg_stdio_dest(&cinfo, outfile); where the last line invokes the standard destination module. WARNING: it is critical that the binary compressed data be delivered to the output file unchanged. On non-Unix systems the stdio library may perform newline translation or otherwise corrupt binary data. To suppress this behavior, you may need to use a "b" option to fopen (as shown above), or use setmode() or another routine to put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that has been found to work on many systems. You can select the data destination after setting other parameters (step 3), if that's more convenient. You may not change the destination between calling jpeg_start_compress() and jpeg_finish_compress(). 3. Set parameters for compression, including image size & colorspace. You must supply information about the source image by setting the following fields in the JPEG object (cinfo structure): image_width Width of image, in pixels image_height Height of image, in pixels input_components Number of color channels (samples per pixel) in_color_space Color space of source image The image dimensions are, hopefully, obvious. JPEG supports image dimensions of 1 to 64K pixels in either direction. The input color space is typically RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special color spaces", later, for more info.) The in_color_space field must be assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or JCS_GRAYSCALE. JPEG has a large number of compression parameters that determine how the image is encoded. Most applications don't need or want to know about all these parameters. You can set all the parameters to reasonable defaults by calling jpeg_set_defaults(); then, if there are particular values you want to change, you can do so after that. The "Compression parameter selection" section tells about all the parameters. You must set in_color_space correctly before calling jpeg_set_defaults(), because the defaults depend on the source image colorspace. However the other three source image parameters need not be valid until you call jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more than once, if that happens to be convenient. Typical code for a 24-bit RGB source image is cinfo.image_width = Width; /* image width and height, in pixels */ cinfo.image_height = Height; cinfo.input_components = 3; /* # of color components per pixel */ cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ jpeg_set_defaults(&cinfo); /* Make optional parameter settings here */ 4. jpeg_start_compress(...); After you have established the data destination and set all the necessary source image info and other parameters, call jpeg_start_compress() to begin a compression cycle. This will initialize internal state, allocate working storage, and emit the first few bytes of the JPEG datastream header. Typical code: jpeg_start_compress(&cinfo, TRUE); The "TRUE" parameter ensures that a complete JPEG interchange datastream will be written. This is appropriate in most cases. If you think you might want to use an abbreviated datastream, read the section on abbreviated datastreams, below. Once you have called jpeg_start_compress(), you may not alter any JPEG parameters or other fields of the JPEG object until you have completed the compression cycle. 5. while (scan lines remain to be written) jpeg_write_scanlines(...); Now write all the required image data by calling jpeg_write_scanlines() one or more times. You can pass one or more scanlines in each call, up to the total image height. In most applications it is convenient to pass just one or a few scanlines at a time. The expected format for the passed data is discussed under "Data formats", above. Image data should be written in top-to-bottom scanline order. Rec. ITU-T T.81 | ISO/IEC 10918-1 says, "Applications determine which edges of a source image are defined as top, bottom, left, and right." However, if you want your files to be compatible with everyone else's, then top-to-bottom order must be used. If the source data must be read in bottom-to-top order, then you can use the JPEG library's virtual array mechanism to invert the data efficiently. Examples of this can be found in the sample application cjpeg. The library maintains a count of the number of scanlines written so far in the next_scanline field of the JPEG object. Usually you can just use this variable as the loop counter, so that the loop test looks like "while (cinfo.next_scanline < cinfo.image_height)". Code for this step depends heavily on the way that you store the source data. example.txt shows the following code for the case of a full-size 2-D source array containing 3-byte RGB pixels: JSAMPROW row_pointer[1]; /* pointer to a single row */ int row_stride; /* physical row width in buffer */ row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ while (cinfo.next_scanline < cinfo.image_height) { row_pointer[0] = &image_buffer[cinfo.next_scanline * row_stride]; jpeg_write_scanlines(&cinfo, row_pointer, 1); } jpeg_write_scanlines() returns the number of scanlines actually written. This will normally be equal to the number passed in, so you can usually ignore the return value. It is different in just two cases: * If you try to write more scanlines than the declared image height, the additional scanlines are ignored. * If you use a suspending data destination manager, output buffer overrun will cause the compressor to return before accepting all the passed lines. This feature is discussed under "I/O suspension", below. The normal stdio destination manager will NOT cause this to happen. In any case, the return value is the same as the change in the value of next_scanline. 6. jpeg_finish_compress(...); After all the image data has been written, call jpeg_finish_compress() to complete the compression cycle. This step is ESSENTIAL to ensure that the last bufferload of data is written to the data destination. jpeg_finish_compress() also releases working memory associated with the JPEG object. Typical code: jpeg_finish_compress(&cinfo); If using the stdio destination manager, don't forget to close the output stdio stream (if necessary) afterwards. If you have requested a multi-pass operating mode, such as Huffman code optimization, jpeg_finish_compress() will perform the additional passes using data buffered by the first pass. In this case jpeg_finish_compress() may take quite a while to complete. With the default compression parameters, this will not happen. It is an error to call jpeg_finish_compress() before writing the necessary total number of scanlines. If you wish to abort compression, call jpeg_abort() as discussed below. After completing a compression cycle, you may dispose of the JPEG object as discussed next, or you may use it to compress another image. In that case return to step 2, 3, or 4 as appropriate. If you do not change the destination manager, the new datastream will be written to the same target. If you do not change any JPEG parameters, the new datastream will be written with the same parameters as before. Note that you can change the input image dimensions freely between cycles, but if you change the input colorspace, you should call jpeg_set_defaults() to adjust for the new colorspace; and then you'll need to repeat all of step 3. 7. Release the JPEG compression object. When you are done with a JPEG compression object, destroy it by calling jpeg_destroy_compress(). This will free all subsidiary memory (regardless of the previous state of the object). Or you can call jpeg_destroy(), which works for either compression or decompression objects --- this may be more convenient if you are sharing code between compression and decompression cases. (Actually, these routines are equivalent except for the declared type of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy() should be passed a j_common_ptr.) If you allocated the jpeg_compress_struct structure from malloc(), freeing it is your responsibility --- jpeg_destroy() won't. Ditto for the error handler structure. Typical code: jpeg_destroy_compress(&cinfo); 8. Aborting. If you decide to abort a compression cycle before finishing, you can clean up in either of two ways: * If you don't need the JPEG object any more, just call jpeg_destroy_compress() or jpeg_destroy() to release memory. This is legitimate at any point after calling jpeg_create_compress() --- in fact, it's safe even if jpeg_create_compress() fails. * If you want to re-use the JPEG object, call jpeg_abort_compress(), or call jpeg_abort() which works on both compression and decompression objects. This will return the object to an idle state, releasing any working memory. jpeg_abort() is allowed at any time after successful object creation. Note that cleaning up the data destination, if required, is your responsibility; neither of these routines will call term_destination(). (See "Compressed data handling", below, for more about that.) jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG object that has reported an error by calling error_exit (see "Error handling" for more info). The internal state of such an object is likely to be out of whack. Either of these two routines will return the object to a known state. Decompression details --------------------- Here we revisit the JPEG decompression outline given in the overview. 1. Allocate and initialize a JPEG decompression object. This is just like initialization for compression, as discussed above, except that the object is a "struct jpeg_decompress_struct" and you call jpeg_create_decompress(). Error handling is exactly the same. Typical code: struct jpeg_decompress_struct cinfo; struct jpeg_error_mgr jerr; ... cinfo.err = jpeg_std_error(&jerr); jpeg_create_decompress(&cinfo); (Both here and in the IJG code, we usually use variable name "cinfo" for both compression and decompression objects.) 2. Specify the source of the compressed data (eg, a file). As previously mentioned, the JPEG library reads compressed data from a "data source" module. The library includes one data source module which knows how to read from a stdio stream. You can use your own source module if you want to do something else, as discussed later. If you use the standard source module, you must open the source stdio stream beforehand. Typical code for this step looks like: FILE *infile; ... if ((infile = fopen(filename, "rb")) == NULL) { fprintf(stderr, "can't open %s\n", filename); exit(1); } jpeg_stdio_src(&cinfo, infile); where the last line invokes the standard source module. WARNING: it is critical that the binary compressed data be read unchanged. On non-Unix systems the stdio library may perform newline translation or otherwise corrupt binary data. To suppress this behavior, you may need to use a "b" option to fopen (as shown above), or use setmode() or another routine to put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that has been found to work on many systems. You may not change the data source between calling jpeg_read_header() and jpeg_finish_decompress(). If you wish to read a series of JPEG images from a single source file, you should repeat the jpeg_read_header() to jpeg_finish_decompress() sequence without reinitializing either the JPEG object or the data source module; this prevents buffered input data from being discarded. 3. Call jpeg_read_header() to obtain image info. Typical code for this step is just jpeg_read_header(&cinfo, TRUE); This will read the source datastream header markers, up to the beginning of the compressed data proper. On return, the image dimensions and other info have been stored in the JPEG object. The application may wish to consult this information before selecting decompression parameters. More complex code is necessary if * A suspending data source is used --- in that case jpeg_read_header() may return before it has read all the header data. See "I/O suspension", below. The normal stdio source manager will NOT cause this to happen. * Abbreviated JPEG files are to be processed --- see the section on abbreviated datastreams. Standard applications that deal only in interchange JPEG files need not be concerned with this case either. It is permissible to stop at this point if you just wanted to find out the image dimensions and other header info for a JPEG file. In that case, call jpeg_destroy() when you are done with the JPEG object, or call jpeg_abort() to return it to an idle state before selecting a new data source and reading another header. 4. Set parameters for decompression. jpeg_read_header() sets appropriate default decompression parameters based on the properties of the image (in particular, its colorspace). However, you may well want to alter these defaults before beginning the decompression. For example, the default is to produce full color output from a color file. If you want colormapped output you must ask for it. Other options allow the returned image to be scaled and allow various speed/quality tradeoffs to be selected. "Decompression parameter selection", below, gives details. If the defaults are appropriate, nothing need be done at this step. Note that all default values are set by each call to jpeg_read_header(). If you reuse a decompression object, you cannot expect your parameter settings to be preserved across cycles, as you can for compression. You must set desired parameter values each time. 5. jpeg_start_decompress(...); Once the parameter values are satisfactory, call jpeg_start_decompress() to begin decompression. This will initialize internal state, allocate working memory, and prepare for returning data. Typical code is just jpeg_start_decompress(&cinfo); If you have requested a multi-pass operating mode, such as 2-pass color quantization, jpeg_start_decompress() will do everything needed before data output can begin. In this case jpeg_start_decompress() may take quite a while to complete. With a single-scan (non progressive) JPEG file and default decompression parameters, this will not happen; jpeg_start_decompress() will return quickly. After this call, the final output image dimensions, including any requested scaling, are available in the JPEG object; so is the selected colormap, if colormapped output has been requested. Useful fields include output_width image width and height, as scaled output_height out_color_components # of color components in out_color_space output_components # of color components returned per pixel colormap the selected colormap, if any actual_number_of_colors number of entries in colormap output_components is 1 (a colormap index) when quantizing colors; otherwise it equals out_color_components. It is the number of JSAMPLE values that will be emitted per pixel in the output arrays. Typically you will need to allocate data buffers to hold the incoming image. You will need output_width * output_components JSAMPLEs per scanline in your output buffer, and a total of output_height scanlines will be returned. Note: if you are using the JPEG library's internal memory manager to allocate data buffers (as djpeg does), then the manager's protocol requires that you request large buffers *before* calling jpeg_start_decompress(). This is a little tricky since the output_XXX fields are not normally valid then. You can make them valid by calling jpeg_calc_output_dimensions() after setting the relevant parameters (scaling, output color space, and quantization flag). 6. while (scan lines remain to be read) jpeg_read_scanlines(...); Now you can read the decompressed image data by calling jpeg_read_scanlines() one or more times. At each call, you pass in the maximum number of scanlines to be read (ie, the height of your working buffer); jpeg_read_scanlines() will return up to that many lines. The return value is the number of lines actually read. The format of the returned data is discussed under "Data formats", above. Don't forget that grayscale and color JPEGs will return different data formats! Image data is returned in top-to-bottom scanline order. If you must write out the image in bottom-to-top order, you can use the JPEG library's virtual array mechanism to invert the data efficiently. Examples of this can be found in the sample application djpeg. The library maintains a count of the number of scanlines returned so far in the output_scanline field of the JPEG object. Usually you can just use this variable as the loop counter, so that the loop test looks like "while (cinfo.output_scanline < cinfo.output_height)". (Note that the test should NOT be against image_height, unless you never use scaling. The image_height field is the height of the original unscaled image.) The return value always equals the change in the value of output_scanline. If you don't use a suspending data source, it is safe to assume that jpeg_read_scanlines() reads at least one scanline per call, until the bottom of the image has been reached. If you use a buffer larger than one scanline, it is NOT safe to assume that jpeg_read_scanlines() fills it. (The current implementation returns only a few scanlines per call, no matter how large a buffer you pass.) So you must always provide a loop that calls jpeg_read_scanlines() repeatedly until the whole image has been read. 7. jpeg_finish_decompress(...); After all the image data has been read, call jpeg_finish_decompress() to complete the decompression cycle. This causes working memory associated with the JPEG object to be released. Typical code: jpeg_finish_decompress(&cinfo); If using the stdio source manager, don't forget to close the source stdio stream if necessary. It is an error to call jpeg_finish_decompress() before reading the correct total number of scanlines. If you wish to abort decompression, call jpeg_abort() as discussed below. After completing a decompression cycle, you may dispose of the JPEG object as discussed next, or you may use it to decompress another image. In that case return to step 2 or 3 as appropriate. If you do not change the source manager, the next image will be read from the same source. 8. Release the JPEG decompression object. When you are done with a JPEG decompression object, destroy it by calling jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of destroying compression objects applies here too. Typical code: jpeg_destroy_decompress(&cinfo); 9. Aborting. You can abort a decompression cycle by calling jpeg_destroy_decompress() or jpeg_destroy() if you don't need the JPEG object any more, or jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object. The previous discussion of aborting compression cycles applies here too. Partial image decompression --------------------------- Partial image decompression is convenient for performance-critical applications that wish to view only a portion of a large JPEG image without decompressing the whole thing. It it also useful in memory-constrained environments (such as on mobile devices.) This library provides the following functions to support partial image decompression: 1. Skipping rows when decompressing jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines); This function provides application programmers with the ability to skip over multiple rows in the JPEG image. Suspending data sources are not supported by this function. Calling jpeg_skip_scanlines() with a suspending data source will result in undefined behavior. Two-pass color quantization is also not supported by this function. Calling jpeg_skip_scanlines() with two-pass color quantization enabled will result in an error. jpeg_skip_scanlines() will not allow skipping past the bottom of the image. If the value of num_lines is large enough to skip past the bottom of the image, then the function will skip to the end of the image instead. If the value of num_lines is valid, then jpeg_skip_scanlines() will always skip all of the input rows requested. There is no need to inspect the return value of the function in that case. Best results will be achieved by calling jpeg_skip_scanlines() for large chunks of rows. The function should be viewed as a way to quickly jump to a particular vertical offset in the JPEG image in order to decode a subset of the image. Used in this manner, it will provide significant performance improvements. Calling jpeg_skip_scanlines() for small values of num_lines has several potential drawbacks: 1) JPEG decompression occurs in blocks, so if jpeg_skip_scanlines() is called from the middle of a decompression block, then it is likely that much of the decompression work has already been done for the first couple of rows that need to be skipped. 2) When this function returns, it must leave the decompressor in a state such that it is ready to read the next line. This may involve decompressing a block that must be partially skipped. These issues are especially tricky for cases in which upsampling requires context rows. In the worst case, jpeg_skip_scanlines() will perform similarly to jpeg_read_scanlines() (since it will actually call jpeg_read_scanlines().) 2. Decompressing partial scanlines jpeg_crop_scanline (j_decompress_ptr cinfo, JDIMENSION *xoffset, JDIMENSION *width) This function provides application programmers with the ability to decompress only a portion of each row in the JPEG image. It must be called after jpeg_start_decompress() and before any calls to jpeg_read_scanlines() or jpeg_skip_scanlines(). If xoffset and width do not form a valid subset of the image row, then this function will generate an error. Note that if the output image is scaled, then xoffset and width are relative to the scaled image dimensions. xoffset and width are passed by reference because xoffset must fall on an iMCU boundary. If it doesn't, then it will be moved left to the nearest iMCU boundary, and width will be increased accordingly. If the calling program does not like the adjusted values of xoffset and width, then it can call jpeg_crop_scanline() again with new values (for instance, if it wants to move xoffset to the nearest iMCU boundary to the right instead of to the left.) After calling this function, cinfo->output_width will be set to the adjusted width. This value should be used when allocating an output buffer to pass to jpeg_read_scanlines(). The output image from a partial-width decompression will be identical to the corresponding image region from a full decode, with one exception: The "fancy" (smooth) h2v2 (4:2:0) and h2v1 (4:2:2) upsampling algorithms fill in the missing chroma components by averaging the chroma components from neighboring pixels, except on the right and left edges of the image (where there are no neighboring pixels.) When performing a partial-width decompression, these "fancy" upsampling algorithms may treat the left and right edges of the partial image region as if they are the left and right edges of the image, meaning that the upsampling algorithm may be simplified. The result is that the pixels on the left or right edge of the partial image may not be exactly identical to the corresponding pixels in the original image. Mechanics of usage: include files, linking, etc ----------------------------------------------- Applications using the JPEG library should include the header file jpeglib.h to obtain declarations of data types and routines. Before including jpeglib.h, include system headers that define at least the typedefs FILE and size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on older Unix systems, you may need <sys/types.h> to define size_t. If the application needs to refer to individual JPEG library error codes, also include jerror.h to define those symbols. jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are installing the JPEG header files in a system directory, you will want to install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h. The most convenient way to include the JPEG code into your executable program is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix machines) and reference it at your link step. If you use only half of the library (only compression or only decompression), only that much code will be included from the library, unless your linker is hopelessly brain-damaged. The supplied makefiles build libjpeg.a automatically (see install.txt). While you can build the JPEG library as a shared library if the whim strikes you, we don't really recommend it. The trouble with shared libraries is that at some point you'll probably try to substitute a new version of the library without recompiling the calling applications. That generally doesn't work because the parameter struct declarations usually change with each new version. In other words, the library's API is *not* guaranteed binary compatible across versions; we only try to ensure source-code compatibility. (In hindsight, it might have been smarter to hide the parameter structs from applications and introduce a ton of access functions instead. Too late now, however.) It may be worth pointing out that the core JPEG library does not actually require the stdio library: only the default source/destination managers and error handler need it. You can use the library in a stdio-less environment if you replace those modules and use jmemnobs.c (or another memory manager of your own devising). More info about the minimum system library requirements may be found in jinclude.h. ADVANCED FEATURES ================= Compression parameter selection ------------------------------- This section describes all the optional parameters you can set for JPEG compression, as well as the "helper" routines provided to assist in this task. Proper setting of some parameters requires detailed understanding of the JPEG standard; if you don't know what a parameter is for, it's best not to mess with it! See REFERENCES in the README.ijg file for pointers to more info about JPEG. It's a good idea to call jpeg_set_defaults() first, even if you plan to set all the parameters; that way your code is more likely to work with future JPEG libraries that have additional parameters. For the same reason, we recommend you use a helper routine where one is provided, in preference to twiddling cinfo fields directly. The helper routines are: jpeg_set_defaults (j_compress_ptr cinfo) This routine sets all JPEG parameters to reasonable defaults, using only the input image's color space (field in_color_space, which must already be set in cinfo). Many applications will only need to use this routine and perhaps jpeg_set_quality(). jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace) Sets the JPEG file's colorspace (field jpeg_color_space) as specified, and sets other color-space-dependent parameters appropriately. See "Special color spaces", below, before using this. A large number of parameters, including all per-component parameters, are set by this routine; if you want to twiddle individual parameters you should call jpeg_set_colorspace() before rather than after. jpeg_default_colorspace (j_compress_ptr cinfo) Selects an appropriate JPEG colorspace based on cinfo->in_color_space, and calls jpeg_set_colorspace(). This is actually a subroutine of jpeg_set_defaults(). It's broken out in case you want to change just the colorspace-dependent JPEG parameters. jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline) Constructs JPEG quantization tables appropriate for the indicated quality setting. The quality value is expressed on the 0..100 scale recommended by IJG (cjpeg's "-quality" switch uses this routine). Note that the exact mapping from quality values to tables may change in future IJG releases as more is learned about DCT quantization. If the force_baseline parameter is TRUE, then the quantization table entries are constrained to the range 1..255 for full JPEG baseline compatibility. In the current implementation, this only makes a difference for quality settings below 25, and it effectively prevents very small/low quality files from being generated. The IJG decoder is capable of reading the non-baseline files generated at low quality settings when force_baseline is FALSE, but other decoders may not be. jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor, boolean force_baseline) Same as jpeg_set_quality() except that the generated tables are the sample tables given in Annex K (Clause K.1) of Rec. ITU-T T.81 (1992) | ISO/IEC 10918-1:1994, multiplied by the specified scale factor (which is expressed as a percentage; thus scale_factor = 100 reproduces the spec's tables). Note that larger scale factors give lower quality. This entry point is useful for conforming to the Adobe PostScript DCT conventions, but we do not recommend linear scaling as a user-visible quality scale otherwise. force_baseline again constrains the computed table entries to 1..255. int jpeg_quality_scaling (int quality) Converts a value on the IJG-recommended quality scale to a linear scaling percentage. Note that this routine may change or go away in future releases --- IJG may choose to adopt a scaling method that can't be expressed as a simple scalar multiplier, in which case the premise of this routine collapses. Caveat user. jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline) [libjpeg v7+ API/ABI emulation only] Set default quantization tables with linear q_scale_factor[] values (see below). jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl, const unsigned int *basic_table, int scale_factor, boolean force_baseline) Allows an arbitrary quantization table to be created. which_tbl indicates which table slot to fill. basic_table points to an array of 64 unsigned ints given in normal array order. These values are multiplied by scale_factor/100 and then clamped to the range 1..65535 (or to 1..255 if force_baseline is TRUE). CAUTION: prior to library version 6a, jpeg_add_quant_table expected the basic table to be given in JPEG zigzag order. If you need to write code that works with either older or newer versions of this routine, you must check the library version number. Something like "#if JPEG_LIB_VERSION >= 61" is the right test. jpeg_simple_progression (j_compress_ptr cinfo) Generates a default scan script for writing a progressive-JPEG file. This is the recommended method of creating a progressive file, unless you want to make a custom scan sequence. You must ensure that the JPEG color space is set correctly before calling this routine. Compression parameters (cinfo fields) include: boolean arith_code If TRUE, use arithmetic coding. If FALSE, use Huffman coding. J_DCT_METHOD dct_method Selects the algorithm used for the DCT step. Choices are: JDCT_ISLOW: accurate integer method JDCT_IFAST: less accurate integer method [legacy feature] JDCT_FLOAT: floating-point method [legacy feature] JDCT_DEFAULT: default method (normally JDCT_ISLOW) JDCT_FASTEST: fastest method (normally JDCT_IFAST) When the Independent JPEG Group's software was first released in 1991, the compression time for a 1-megapixel JPEG image on a mainstream PC was measured in minutes. Thus, JDCT_IFAST provided noticeable performance benefits. On modern CPUs running libjpeg-turbo, however, the compression time for a 1-megapixel JPEG image is measured in milliseconds, and thus the performance benefits of JDCT_IFAST are much less noticeable. On modern x86/x86-64 CPUs that support AVX2 instructions, JDCT_IFAST and JDCT_ISLOW have similar performance. On other types of CPUs, JDCT_IFAST is generally about 5-15% faster than JDCT_ISLOW. For quality levels of 90 and below, there should be little or no perceptible quality difference between the two algorithms. For quality levels above 90, however, the difference between JDCT_IFAST and JDCT_ISLOW becomes more pronounced. With quality=97, for instance, JDCT_IFAST incurs generally about a 1-3 dB loss in PSNR relative to JDCT_ISLOW, but this can be larger for some images. Do not use JDCT_IFAST with quality levels above 97. The algorithm often degenerates at quality=98 and above and can actually produce a more lossy image than if lower quality levels had been used. Also, in libjpeg-turbo, JDCT_IFAST is not fully accelerated for quality levels above 97, so it will be slower than JDCT_ISLOW. JDCT_FLOAT does not produce significantly more accurate results than JDCT_ISLOW, and it is much slower. JDCT_FLOAT may also give different results on different machines due to varying roundoff behavior, whereas the integer methods should give the same results on all machines. J_COLOR_SPACE jpeg_color_space int num_components The JPEG color space and corresponding number of components; see "Special color spaces", below, for more info. We recommend using jpeg_set_color_space() if you want to change these. boolean optimize_coding TRUE causes the compressor to compute optimal Huffman coding tables for the image. This requires an extra pass over the data and therefore costs a good deal of space and time. The default is FALSE, which tells the compressor to use the supplied or default Huffman tables. In most cases optimal tables save only a few percent of file size compared to the default tables. Note that when this is TRUE, you need not supply Huffman tables at all, and any you do supply will be overwritten. unsigned int restart_interval int restart_in_rows To emit restart markers in the JPEG file, set one of these nonzero. Set restart_interval to specify the exact interval in MCU blocks. Set restart_in_rows to specify the interval in MCU rows. (If restart_in_rows is not 0, then restart_interval is set after the image width in MCUs is computed.) Defaults are zero (no restarts). One restart marker per MCU row is often a good choice. NOTE: the overhead of restart markers is higher in grayscale JPEG fil