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example.c
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00001 /*
00002  * example.c
00003  *
00004  * This file illustrates how to use the IJG code as a subroutine library
00005  * to read or write JPEG image files.  You should look at this code in
00006  * conjunction with the documentation file libjpeg.doc.
00007  *
00008  * This code will not do anything useful as-is, but it may be helpful as a
00009  * skeleton for constructing routines that call the JPEG library.  
00010  *
00011  * We present these routines in the same coding style used in the JPEG code
00012  * (ANSI function definitions, etc); but you are of course free to code your
00013  * routines in a different style if you prefer.
00014  */
00015 
00016 #include <stdio.h>
00017 
00018 /*
00019  * Include file for users of JPEG library.
00020  * You will need to have included system headers that define at least
00021  * the typedefs FILE and size_t before you can include jpeglib.h.
00022  * (stdio.h is sufficient on ANSI-conforming systems.)
00023  * You may also wish to include "jerror.h".
00024  */
00025 
00026 #include "jpeglib.h"
00027 
00028 /*
00029  * <setjmp.h> is used for the optional error recovery mechanism shown in
00030  * the second part of the example.
00031  */
00032 
00033 #include <setjmp.h>
00034 
00035 
00036 
00037 /******************** JPEG COMPRESSION SAMPLE INTERFACE *******************/
00038 
00039 /* This half of the example shows how to feed data into the JPEG compressor.
00040  * We present a minimal version that does not worry about refinements such
00041  * as error recovery (the JPEG code will just exit() if it gets an error).
00042  */
00043 
00044 
00045 /*
00046  * IMAGE DATA FORMATS:
00047  *
00048  * The standard input image format is a rectangular array of pixels, with
00049  * each pixel having the same number of "component" values (color channels).
00050  * Each pixel row is an array of JSAMPLEs (which typically are unsigned chars).
00051  * If you are working with color data, then the color values for each pixel
00052  * must be adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit
00053  * RGB color.
00054  *
00055  * For this example, we'll assume that this data structure matches the way
00056  * our application has stored the image in memory, so we can just pass a
00057  * pointer to our image buffer.  In particular, let's say that the image is
00058  * RGB color and is described by:
00059  */
00060 
00061 extern JSAMPLE * image_buffer;     /* Points to large array of R,G,B-order data */
00062 extern int image_height;    /* Number of rows in image */
00063 extern int image_width;            /* Number of columns in image */
00064 
00065 
00066 /*
00067  * Sample routine for JPEG compression.  We assume that the target file name
00068  * and a compression quality factor are passed in.
00069  */
00070 
00071 GLOBAL(void)
00072 write_JPEG_file (char * filename, int quality)
00073 {
00074   /* This struct contains the JPEG compression parameters and pointers to
00075    * working space (which is allocated as needed by the JPEG library).
00076    * It is possible to have several such structures, representing multiple
00077    * compression/decompression processes, in existence at once.  We refer
00078    * to any one struct (and its associated working data) as a "JPEG object".
00079    */
00080   struct jpeg_compress_struct cinfo;
00081   /* This struct represents a JPEG error handler.  It is declared separately
00082    * because applications often want to supply a specialized error handler
00083    * (see the second half of this file for an example).  But here we just
00084    * take the easy way out and use the standard error handler, which will
00085    * print a message on stderr and call exit() if compression fails.
00086    * Note that this struct must live as long as the main JPEG parameter
00087    * struct, to avoid dangling-pointer problems.
00088    */
00089   struct jpeg_error_mgr jerr;
00090   /* More stuff */
00091   FILE * outfile;           /* target file */
00092   JSAMPROW row_pointer[1];  /* pointer to JSAMPLE row[s] */
00093   int row_stride;           /* physical row width in image buffer */
00094 
00095   /* Step 1: allocate and initialize JPEG compression object */
00096 
00097   /* We have to set up the error handler first, in case the initialization
00098    * step fails.  (Unlikely, but it could happen if you are out of memory.)
00099    * This routine fills in the contents of struct jerr, and returns jerr's
00100    * address which we place into the link field in cinfo.
00101    */
00102   cinfo.err = jpeg_std_error(&jerr);
00103   /* Now we can initialize the JPEG compression object. */
00104   jpeg_create_compress(&cinfo);
00105 
00106   /* Step 2: specify data destination (eg, a file) */
00107   /* Note: steps 2 and 3 can be done in either order. */
00108 
00109   /* Here we use the library-supplied code to send compressed data to a
00110    * stdio stream.  You can also write your own code to do something else.
00111    * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
00112    * requires it in order to write binary files.
00113    */
00114   if ((outfile = fopen(filename, "wb")) == NULL) {
00115     fprintf(stderr, "can't open %s\n", filename);
00116     exit(1);
00117   }
00118   jpeg_stdio_dest(&cinfo, outfile);
00119 
00120   /* Step 3: set parameters for compression */
00121 
00122   /* First we supply a description of the input image.
00123    * Four fields of the cinfo struct must be filled in:
00124    */
00125   cinfo.image_width = image_width;        /* image width and height, in pixels */
00126   cinfo.image_height = image_height;
00127   cinfo.input_components = 3;             /* # of color components per pixel */
00128   cinfo.in_color_space = JCS_RGB;  /* colorspace of input image */
00129   /* Now use the library's routine to set default compression parameters.
00130    * (You must set at least cinfo.in_color_space before calling this,
00131    * since the defaults depend on the source color space.)
00132    */
00133   jpeg_set_defaults(&cinfo);
00134   /* Now you can set any non-default parameters you wish to.
00135    * Here we just illustrate the use of quality (quantization table) scaling:
00136    */
00137   jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);
00138 
00139   /* Step 4: Start compressor */
00140 
00141   /* TRUE ensures that we will write a complete interchange-JPEG file.
00142    * Pass TRUE unless you are very sure of what you're doing.
00143    */
00144   jpeg_start_compress(&cinfo, TRUE);
00145 
00146   /* Step 5: while (scan lines remain to be written) */
00147   /*           jpeg_write_scanlines(...); */
00148 
00149   /* Here we use the library's state variable cinfo.next_scanline as the
00150    * loop counter, so that we don't have to keep track ourselves.
00151    * To keep things simple, we pass one scanline per call; you can pass
00152    * more if you wish, though.
00153    */
00154   row_stride = image_width * 3;    /* JSAMPLEs per row in image_buffer */
00155 
00156   while (cinfo.next_scanline < cinfo.image_height) {
00157     /* jpeg_write_scanlines expects an array of pointers to scanlines.
00158      * Here the array is only one element long, but you could pass
00159      * more than one scanline at a time if that's more convenient.
00160      */
00161     row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
00162     (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
00163   }
00164 
00165   /* Step 6: Finish compression */
00166 
00167   jpeg_finish_compress(&cinfo);
00168   /* After finish_compress, we can close the output file. */
00169   fclose(outfile);
00170 
00171   /* Step 7: release JPEG compression object */
00172 
00173   /* This is an important step since it will release a good deal of memory. */
00174   jpeg_destroy_compress(&cinfo);
00175 
00176   /* And we're done! */
00177 }
00178 
00179 
00180 /*
00181  * SOME FINE POINTS:
00182  *
00183  * In the above loop, we ignored the return value of jpeg_write_scanlines,
00184  * which is the number of scanlines actually written.  We could get away
00185  * with this because we were only relying on the value of cinfo.next_scanline,
00186  * which will be incremented correctly.  If you maintain additional loop
00187  * variables then you should be careful to increment them properly.
00188  * Actually, for output to a stdio stream you needn't worry, because
00189  * then jpeg_write_scanlines will write all the lines passed (or else exit
00190  * with a fatal error).  Partial writes can only occur if you use a data
00191  * destination module that can demand suspension of the compressor.
00192  * (If you don't know what that's for, you don't need it.)
00193  *
00194  * If the compressor requires full-image buffers (for entropy-coding
00195  * optimization or a multi-scan JPEG file), it will create temporary
00196  * files for anything that doesn't fit within the maximum-memory setting.
00197  * (Note that temp files are NOT needed if you use the default parameters.)
00198  * On some systems you may need to set up a signal handler to ensure that
00199  * temporary files are deleted if the program is interrupted.  See libjpeg.doc.
00200  *
00201  * Scanlines MUST be supplied in top-to-bottom order if you want your JPEG
00202  * files to be compatible with everyone else's.  If you cannot readily read
00203  * your data in that order, you'll need an intermediate array to hold the
00204  * image.  See rdtarga.c or rdbmp.c for examples of handling bottom-to-top
00205  * source data using the JPEG code's internal virtual-array mechanisms.
00206  */
00207 
00208 
00209 
00210 /******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************/
00211 
00212 /* This half of the example shows how to read data from the JPEG decompressor.
00213  * It's a bit more refined than the above, in that we show:
00214  *   (a) how to modify the JPEG library's standard error-reporting behavior;
00215  *   (b) how to allocate workspace using the library's memory manager.
00216  *
00217  * Just to make this example a little different from the first one, we'll
00218  * assume that we do not intend to put the whole image into an in-memory
00219  * buffer, but to send it line-by-line someplace else.  We need a one-
00220  * scanline-high JSAMPLE array as a work buffer, and we will let the JPEG
00221  * memory manager allocate it for us.  This approach is actually quite useful
00222  * because we don't need to remember to deallocate the buffer separately: it
00223  * will go away automatically when the JPEG object is cleaned up.
00224  */
00225 
00226 
00227 /*
00228  * ERROR HANDLING:
00229  *
00230  * The JPEG library's standard error handler (jerror.c) is divided into
00231  * several "methods" which you can override individually.  This lets you
00232  * adjust the behavior without duplicating a lot of code, which you might
00233  * have to update with each future release.
00234  *
00235  * Our example here shows how to override the "error_exit" method so that
00236  * control is returned to the library's caller when a fatal error occurs,
00237  * rather than calling exit() as the standard error_exit method does.
00238  *
00239  * We use C's setjmp/longjmp facility to return control.  This means that the
00240  * routine which calls the JPEG library must first execute a setjmp() call to
00241  * establish the return point.  We want the replacement error_exit to do a
00242  * longjmp().  But we need to make the setjmp buffer accessible to the
00243  * error_exit routine.  To do this, we make a private extension of the
00244  * standard JPEG error handler object.  (If we were using C++, we'd say we
00245  * were making a subclass of the regular error handler.)
00246  *
00247  * Here's the extended error handler struct:
00248  */
00249 
00250 struct my_error_mgr {
00251   struct jpeg_error_mgr pub;       /* "public" fields */
00252 
00253   jmp_buf setjmp_buffer;    /* for return to caller */
00254 };
00255 
00256 typedef struct my_error_mgr * my_error_ptr;
00257 
00258 /*
00259  * Here's the routine that will replace the standard error_exit method:
00260  */
00261 
00262 METHODDEF(void)
00263 my_error_exit (j_common_ptr cinfo)
00264 {
00265   /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
00266   my_error_ptr myerr = (my_error_ptr) cinfo->err;
00267 
00268   /* Always display the message. */
00269   /* We could postpone this until after returning, if we chose. */
00270   (*cinfo->err->output_message) (cinfo);
00271 
00272   /* Return control to the setjmp point */
00273   longjmp(myerr->setjmp_buffer, 1);
00274 }
00275 
00276 
00277 /*
00278  * Sample routine for JPEG decompression.  We assume that the source file name
00279  * is passed in.  We want to return 1 on success, 0 on error.
00280  */
00281 
00282 
00283 GLOBAL(int)
00284 read_JPEG_file (char * filename)
00285 {
00286   /* This struct contains the JPEG decompression parameters and pointers to
00287    * working space (which is allocated as needed by the JPEG library).
00288    */
00289   struct jpeg_decompress_struct cinfo;
00290   /* We use our private extension JPEG error handler.
00291    * Note that this struct must live as long as the main JPEG parameter
00292    * struct, to avoid dangling-pointer problems.
00293    */
00294   struct my_error_mgr jerr;
00295   /* More stuff */
00296   FILE * infile;            /* source file */
00297   JSAMPARRAY buffer;        /* Output row buffer */
00298   int row_stride;           /* physical row width in output buffer */
00299 
00300   /* In this example we want to open the input file before doing anything else,
00301    * so that the setjmp() error recovery below can assume the file is open.
00302    * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
00303    * requires it in order to read binary files.
00304    */
00305 
00306   if ((infile = fopen(filename, "rb")) == NULL) {
00307     fprintf(stderr, "can't open %s\n", filename);
00308     return 0;
00309   }
00310 
00311   /* Step 1: allocate and initialize JPEG decompression object */
00312 
00313   /* We set up the normal JPEG error routines, then override error_exit. */
00314   cinfo.err = jpeg_std_error(&jerr.pub);
00315   jerr.pub.error_exit = my_error_exit;
00316   /* Establish the setjmp return context for my_error_exit to use. */
00317   if (setjmp(jerr.setjmp_buffer)) {
00318     /* If we get here, the JPEG code has signaled an error.
00319      * We need to clean up the JPEG object, close the input file, and return.
00320      */
00321     jpeg_destroy_decompress(&cinfo);
00322     fclose(infile);
00323     return 0;
00324   }
00325   /* Now we can initialize the JPEG decompression object. */
00326   jpeg_create_decompress(&cinfo);
00327 
00328   /* Step 2: specify data source (eg, a file) */
00329 
00330   jpeg_stdio_src(&cinfo, infile);
00331 
00332   /* Step 3: read file parameters with jpeg_read_header() */
00333 
00334   (void) jpeg_read_header(&cinfo, TRUE);
00335   /* We can ignore the return value from jpeg_read_header since
00336    *   (a) suspension is not possible with the stdio data source, and
00337    *   (b) we passed TRUE to reject a tables-only JPEG file as an error.
00338    * See libjpeg.doc for more info.
00339    */
00340 
00341   /* Step 4: set parameters for decompression */
00342 
00343   /* In this example, we don't need to change any of the defaults set by
00344    * jpeg_read_header(), so we do nothing here.
00345    */
00346 
00347   /* Step 5: Start decompressor */
00348 
00349   (void) jpeg_start_decompress(&cinfo);
00350   /* We can ignore the return value since suspension is not possible
00351    * with the stdio data source.
00352    */
00353 
00354   /* We may need to do some setup of our own at this point before reading
00355    * the data.  After jpeg_start_decompress() we have the correct scaled
00356    * output image dimensions available, as well as the output colormap
00357    * if we asked for color quantization.
00358    * In this example, we need to make an output work buffer of the right size.
00359    */ 
00360   /* JSAMPLEs per row in output buffer */
00361   row_stride = cinfo.output_width * cinfo.output_components;
00362   /* Make a one-row-high sample array that will go away when done with image */
00363   buffer = (*cinfo.mem->alloc_sarray)
00364               ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
00365 
00366   /* Step 6: while (scan lines remain to be read) */
00367   /*           jpeg_read_scanlines(...); */
00368 
00369   /* Here we use the library's state variable cinfo.output_scanline as the
00370    * loop counter, so that we don't have to keep track ourselves.
00371    */
00372   while (cinfo.output_scanline < cinfo.output_height) {
00373     /* jpeg_read_scanlines expects an array of pointers to scanlines.
00374      * Here the array is only one element long, but you could ask for
00375      * more than one scanline at a time if that's more convenient.
00376      */
00377     (void) jpeg_read_scanlines(&cinfo, buffer, 1);
00378     /* Assume put_scanline_someplace wants a pointer and sample count. */
00379     put_scanline_someplace(buffer[0], row_stride);
00380   }
00381 
00382   /* Step 7: Finish decompression */
00383 
00384   (void) jpeg_finish_decompress(&cinfo);
00385   /* We can ignore the return value since suspension is not possible
00386    * with the stdio data source.
00387    */
00388 
00389   /* Step 8: Release JPEG decompression object */
00390 
00391   /* This is an important step since it will release a good deal of memory. */
00392   jpeg_destroy_decompress(&cinfo);
00393 
00394   /* After finish_decompress, we can close the input file.
00395    * Here we postpone it until after no more JPEG errors are possible,
00396    * so as to simplify the setjmp error logic above.  (Actually, I don't
00397    * think that jpeg_destroy can do an error exit, but why assume anything...)
00398    */
00399   fclose(infile);
00400 
00401   /* At this point you may want to check to see whether any corrupt-data
00402    * warnings occurred (test whether jerr.pub.num_warnings is nonzero).
00403    */
00404 
00405   /* And we're done! */
00406   return 1;
00407 }
00408 
00409 
00410 /*
00411  * SOME FINE POINTS:
00412  *
00413  * In the above code, we ignored the return value of jpeg_read_scanlines,
00414  * which is the number of scanlines actually read.  We could get away with
00415  * this because we asked for only one line at a time and we weren't using
00416  * a suspending data source.  See libjpeg.doc for more info.
00417  *
00418  * We cheated a bit by calling alloc_sarray() after jpeg_start_decompress();
00419  * we should have done it beforehand to ensure that the space would be
00420  * counted against the JPEG max_memory setting.  In some systems the above
00421  * code would risk an out-of-memory error.  However, in general we don't
00422  * know the output image dimensions before jpeg_start_decompress(), unless we
00423  * call jpeg_calc_output_dimensions().  See libjpeg.doc for more about this.
00424  *
00425  * Scanlines are returned in the same order as they appear in the JPEG file,
00426  * which is standardly top-to-bottom.  If you must emit data bottom-to-top,
00427  * you can use one of the virtual arrays provided by the JPEG memory manager
00428  * to invert the data.  See wrbmp.c for an example.
00429  *
00430  * As with compression, some operating modes may require temporary files.
00431  * On some systems you may need to set up a signal handler to ensure that
00432  * temporary files are deleted if the program is interrupted.  See libjpeg.doc.
00433  */