434 lines
17 KiB
C
434 lines
17 KiB
C
/*
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* example.c
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*
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* This file illustrates how to use the IJG code as a subroutine library
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* to read or write JPEG image files. You should look at this code in
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* conjunction with the documentation file libjpeg.txt.
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*
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* This code will not do anything useful as-is, but it may be helpful as a
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* skeleton for constructing routines that call the JPEG library.
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*
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* We present these routines in the same coding style used in the JPEG code
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* (ANSI function definitions, etc); but you are of course free to code your
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* routines in a different style if you prefer.
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*/
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#include <stdio.h>
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/*
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* Include file for users of JPEG library.
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* You will need to have included system headers that define at least
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* the typedefs FILE and size_t before you can include jpeglib.h.
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* (stdio.h is sufficient on ANSI-conforming systems.)
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* You may also wish to include "jerror.h".
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*/
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#include "jpeglib.h"
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/*
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* <setjmp.h> is used for the optional error recovery mechanism shown in
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* the second part of the example.
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*/
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#include <setjmp.h>
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/******************** JPEG COMPRESSION SAMPLE INTERFACE *******************/
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/* This half of the example shows how to feed data into the JPEG compressor.
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* We present a minimal version that does not worry about refinements such
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* as error recovery (the JPEG code will just exit() if it gets an error).
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*/
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/*
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* IMAGE DATA FORMATS:
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*
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* The standard input image format is a rectangular array of pixels, with
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* each pixel having the same number of "component" values (color channels).
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* Each pixel row is an array of JSAMPLEs (which typically are unsigned chars).
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* If you are working with color data, then the color values for each pixel
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* must be adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit
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* RGB color.
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*
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* For this example, we'll assume that this data structure matches the way
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* our application has stored the image in memory, so we can just pass a
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* pointer to our image buffer. In particular, let's say that the image is
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* RGB color and is described by:
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*/
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extern JSAMPLE * image_buffer; /* Points to large array of R,G,B-order data */
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extern int image_height; /* Number of rows in image */
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extern int image_width; /* Number of columns in image */
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/*
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* Sample routine for JPEG compression. We assume that the target file name
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* and a compression quality factor are passed in.
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*/
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GLOBAL(void)
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write_JPEG_file (char * filename, int quality)
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{
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/* This struct contains the JPEG compression parameters and pointers to
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* working space (which is allocated as needed by the JPEG library).
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* It is possible to have several such structures, representing multiple
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* compression/decompression processes, in existence at once. We refer
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* to any one struct (and its associated working data) as a "JPEG object".
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*/
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struct jpeg_compress_struct cinfo;
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/* This struct represents a JPEG error handler. It is declared separately
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* because applications often want to supply a specialized error handler
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* (see the second half of this file for an example). But here we just
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* take the easy way out and use the standard error handler, which will
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* print a message on stderr and call exit() if compression fails.
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* Note that this struct must live as long as the main JPEG parameter
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* struct, to avoid dangling-pointer problems.
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*/
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struct jpeg_error_mgr jerr;
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/* More stuff */
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FILE * outfile; /* target file */
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JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */
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int row_stride; /* physical row width in image buffer */
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/* Step 1: allocate and initialize JPEG compression object */
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/* We have to set up the error handler first, in case the initialization
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* step fails. (Unlikely, but it could happen if you are out of memory.)
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* This routine fills in the contents of struct jerr, and returns jerr's
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* address which we place into the link field in cinfo.
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*/
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cinfo.err = jpeg_std_error(&jerr);
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/* Now we can initialize the JPEG compression object. */
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jpeg_create_compress(&cinfo);
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/* Step 2: specify data destination (eg, a file) */
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/* Note: steps 2 and 3 can be done in either order. */
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/* Here we use the library-supplied code to send compressed data to a
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* stdio stream. You can also write your own code to do something else.
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* VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
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* requires it in order to write binary files.
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*/
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if ((outfile = fopen(filename, "wb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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exit(1);
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}
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jpeg_stdio_dest(&cinfo, outfile);
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/* Step 3: set parameters for compression */
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/* First we supply a description of the input image.
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* Four fields of the cinfo struct must be filled in:
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*/
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cinfo.image_width = image_width; /* image width and height, in pixels */
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cinfo.image_height = image_height;
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cinfo.input_components = 3; /* # of color components per pixel */
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cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
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/* Now use the library's routine to set default compression parameters.
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* (You must set at least cinfo.in_color_space before calling this,
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* since the defaults depend on the source color space.)
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*/
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jpeg_set_defaults(&cinfo);
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/* Now you can set any non-default parameters you wish to.
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* Here we just illustrate the use of quality (quantization table) scaling:
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*/
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jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);
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/* Step 4: Start compressor */
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/* TRUE ensures that we will write a complete interchange-JPEG file.
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* Pass TRUE unless you are very sure of what you're doing.
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*/
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jpeg_start_compress(&cinfo, TRUE);
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/* Step 5: while (scan lines remain to be written) */
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/* jpeg_write_scanlines(...); */
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/* Here we use the library's state variable cinfo.next_scanline as the
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* loop counter, so that we don't have to keep track ourselves.
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* To keep things simple, we pass one scanline per call; you can pass
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* more if you wish, though.
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*/
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row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */
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while (cinfo.next_scanline < cinfo.image_height) {
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/* jpeg_write_scanlines expects an array of pointers to scanlines.
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* Here the array is only one element long, but you could pass
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* more than one scanline at a time if that's more convenient.
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*/
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row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
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(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
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}
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/* Step 6: Finish compression */
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jpeg_finish_compress(&cinfo);
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/* After finish_compress, we can close the output file. */
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fclose(outfile);
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/* Step 7: release JPEG compression object */
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/* This is an important step since it will release a good deal of memory. */
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jpeg_destroy_compress(&cinfo);
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/* And we're done! */
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}
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/*
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* SOME FINE POINTS:
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*
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* In the above loop, we ignored the return value of jpeg_write_scanlines,
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* which is the number of scanlines actually written. We could get away
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* with this because we were only relying on the value of cinfo.next_scanline,
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* which will be incremented correctly. If you maintain additional loop
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* variables then you should be careful to increment them properly.
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* Actually, for output to a stdio stream you needn't worry, because
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* then jpeg_write_scanlines will write all the lines passed (or else exit
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* with a fatal error). Partial writes can only occur if you use a data
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* destination module that can demand suspension of the compressor.
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* (If you don't know what that's for, you don't need it.)
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*
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* If the compressor requires full-image buffers (for entropy-coding
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* optimization or a multi-scan JPEG file), it will create temporary
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* files for anything that doesn't fit within the maximum-memory setting.
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* (Note that temp files are NOT needed if you use the default parameters.)
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* On some systems you may need to set up a signal handler to ensure that
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* temporary files are deleted if the program is interrupted. See libjpeg.txt.
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*
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* Scanlines MUST be supplied in top-to-bottom order if you want your JPEG
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* files to be compatible with everyone else's. If you cannot readily read
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* your data in that order, you'll need an intermediate array to hold the
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* image. See rdtarga.c or rdbmp.c for examples of handling bottom-to-top
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* source data using the JPEG code's internal virtual-array mechanisms.
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*/
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/******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************/
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/* This half of the example shows how to read data from the JPEG decompressor.
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* It's a bit more refined than the above, in that we show:
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* (a) how to modify the JPEG library's standard error-reporting behavior;
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* (b) how to allocate workspace using the library's memory manager.
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*
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* Just to make this example a little different from the first one, we'll
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* assume that we do not intend to put the whole image into an in-memory
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* buffer, but to send it line-by-line someplace else. We need a one-
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* scanline-high JSAMPLE array as a work buffer, and we will let the JPEG
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* memory manager allocate it for us. This approach is actually quite useful
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* because we don't need to remember to deallocate the buffer separately: it
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* will go away automatically when the JPEG object is cleaned up.
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*/
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/*
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* ERROR HANDLING:
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*
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* The JPEG library's standard error handler (jerror.c) is divided into
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* several "methods" which you can override individually. This lets you
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* adjust the behavior without duplicating a lot of code, which you might
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* have to update with each future release.
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*
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* Our example here shows how to override the "error_exit" method so that
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* control is returned to the library's caller when a fatal error occurs,
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* rather than calling exit() as the standard error_exit method does.
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*
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* We use C's setjmp/longjmp facility to return control. This means that the
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* routine which calls the JPEG library must first execute a setjmp() call to
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* establish the return point. We want the replacement error_exit to do a
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* longjmp(). But we need to make the setjmp buffer accessible to the
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* error_exit routine. To do this, we make a private extension of the
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* standard JPEG error handler object. (If we were using C++, we'd say we
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* were making a subclass of the regular error handler.)
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*
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* Here's the extended error handler struct:
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*/
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struct my_error_mgr {
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struct jpeg_error_mgr pub; /* "public" fields */
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jmp_buf setjmp_buffer; /* for return to caller */
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};
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typedef struct my_error_mgr * my_error_ptr;
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/*
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* Here's the routine that will replace the standard error_exit method:
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*/
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METHODDEF(void)
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my_error_exit (j_common_ptr cinfo)
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{
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/* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
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my_error_ptr myerr = (my_error_ptr) cinfo->err;
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/* Always display the message. */
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/* We could postpone this until after returning, if we chose. */
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(*cinfo->err->output_message) (cinfo);
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/* Return control to the setjmp point */
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longjmp(myerr->setjmp_buffer, 1);
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}
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/*
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* Sample routine for JPEG decompression. We assume that the source file name
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* is passed in. We want to return 1 on success, 0 on error.
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*/
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GLOBAL(int)
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read_JPEG_file (char * filename)
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{
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/* This struct contains the JPEG decompression parameters and pointers to
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* working space (which is allocated as needed by the JPEG library).
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*/
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struct jpeg_decompress_struct cinfo;
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/* We use our private extension JPEG error handler.
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* Note that this struct must live as long as the main JPEG parameter
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* struct, to avoid dangling-pointer problems.
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*/
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struct my_error_mgr jerr;
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/* More stuff */
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FILE * infile; /* source file */
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JSAMPARRAY buffer; /* Output row buffer */
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int row_stride; /* physical row width in output buffer */
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/* In this example we want to open the input file before doing anything else,
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* so that the setjmp() error recovery below can assume the file is open.
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* VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
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* requires it in order to read binary files.
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*/
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if ((infile = fopen(filename, "rb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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return 0;
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}
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/* Step 1: allocate and initialize JPEG decompression object */
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/* We set up the normal JPEG error routines, then override error_exit. */
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cinfo.err = jpeg_std_error(&jerr.pub);
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jerr.pub.error_exit = my_error_exit;
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/* Establish the setjmp return context for my_error_exit to use. */
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if (setjmp(jerr.setjmp_buffer)) {
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/* If we get here, the JPEG code has signaled an error.
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* We need to clean up the JPEG object, close the input file, and return.
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*/
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jpeg_destroy_decompress(&cinfo);
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fclose(infile);
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return 0;
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}
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/* Now we can initialize the JPEG decompression object. */
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jpeg_create_decompress(&cinfo);
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/* Step 2: specify data source (eg, a file) */
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jpeg_stdio_src(&cinfo, infile);
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/* Step 3: read file parameters with jpeg_read_header() */
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(void) jpeg_read_header(&cinfo, TRUE);
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/* We can ignore the return value from jpeg_read_header since
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* (a) suspension is not possible with the stdio data source, and
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* (b) we passed TRUE to reject a tables-only JPEG file as an error.
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* See libjpeg.txt for more info.
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*/
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/* Step 4: set parameters for decompression */
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/* In this example, we don't need to change any of the defaults set by
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* jpeg_read_header(), so we do nothing here.
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*/
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/* Step 5: Start decompressor */
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(void) jpeg_start_decompress(&cinfo);
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/* We can ignore the return value since suspension is not possible
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* with the stdio data source.
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*/
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/* We may need to do some setup of our own at this point before reading
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* the data. After jpeg_start_decompress() we have the correct scaled
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* output image dimensions available, as well as the output colormap
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* if we asked for color quantization.
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* In this example, we need to make an output work buffer of the right size.
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*/
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/* JSAMPLEs per row in output buffer */
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row_stride = cinfo.output_width * cinfo.output_components;
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/* Make a one-row-high sample array that will go away when done with image */
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buffer = (*cinfo.mem->alloc_sarray)
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((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
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/* Step 6: while (scan lines remain to be read) */
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/* jpeg_read_scanlines(...); */
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/* Here we use the library's state variable cinfo.output_scanline as the
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* loop counter, so that we don't have to keep track ourselves.
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*/
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while (cinfo.output_scanline < cinfo.output_height) {
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/* jpeg_read_scanlines expects an array of pointers to scanlines.
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* Here the array is only one element long, but you could ask for
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* more than one scanline at a time if that's more convenient.
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*/
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(void) jpeg_read_scanlines(&cinfo, buffer, 1);
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/* Assume put_scanline_someplace wants a pointer and sample count. */
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put_scanline_someplace(buffer[0], row_stride);
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}
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/* Step 7: Finish decompression */
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(void) jpeg_finish_decompress(&cinfo);
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/* We can ignore the return value since suspension is not possible
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* with the stdio data source.
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*/
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/* Step 8: Release JPEG decompression object */
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/* This is an important step since it will release a good deal of memory. */
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jpeg_destroy_decompress(&cinfo);
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/* After finish_decompress, we can close the input file.
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* Here we postpone it until after no more JPEG errors are possible,
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* so as to simplify the setjmp error logic above. (Actually, I don't
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* think that jpeg_destroy can do an error exit, but why assume anything...)
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*/
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fclose(infile);
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/* At this point you may want to check to see whether any corrupt-data
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* warnings occurred (test whether jerr.pub.num_warnings is nonzero).
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*/
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/* And we're done! */
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return 1;
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}
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/*
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* SOME FINE POINTS:
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*
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* In the above code, we ignored the return value of jpeg_read_scanlines,
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* which is the number of scanlines actually read. We could get away with
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* this because we asked for only one line at a time and we weren't using
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* a suspending data source. See libjpeg.txt for more info.
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*
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* We cheated a bit by calling alloc_sarray() after jpeg_start_decompress();
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* we should have done it beforehand to ensure that the space would be
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* counted against the JPEG max_memory setting. In some systems the above
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* code would risk an out-of-memory error. However, in general we don't
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* know the output image dimensions before jpeg_start_decompress(), unless we
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* call jpeg_calc_output_dimensions(). See libjpeg.txt for more about this.
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*
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* Scanlines are returned in the same order as they appear in the JPEG file,
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* which is standardly top-to-bottom. If you must emit data bottom-to-top,
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* you can use one of the virtual arrays provided by the JPEG memory manager
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* to invert the data. See wrbmp.c for an example.
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*
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* As with compression, some operating modes may require temporary files.
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* On some systems you may need to set up a signal handler to ensure that
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* temporary files are deleted if the program is interrupted. See libjpeg.txt.
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*/
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