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 */