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C/C++ Source or Header | 1992-12-17 | 26.2 KB | 632 lines

/* * example.c * * This file is not actually part of the JPEG software. Rather, it provides * a skeleton that may be useful for constructing applications that use the * JPEG software as subroutines. This code will NOT do anything useful as is. * * This file illustrates how to use the JPEG code as a subroutine library * to read or write JPEG image files. We assume here that you are not * merely interested in converting the image to yet another image file format * (if you are, you should be adding another I/O module to cjpeg/djpeg, not * constructing a new application). Instead, we show how to pass the * decompressed image data into or out of routines that you provide. For * example, a viewer program might use the JPEG decompressor together with * routines that write the decompressed image directly to a display. * * We present these routines in the same coding style used in the JPEG code * (ANSI function definitions, etc); but you are of course free to code your * routines in a different style if you prefer. */ /* * Include file for declaring JPEG data structures. * This file also includes some system headers like <stdio.h>; * if you prefer, you can include "jconfig.h" and "jpegdata.h" instead. */ #include "jinclude.h" /* * <setjmp.h> is used for the optional error recovery mechanism shown in * the second part of the example. */ #include <setjmp.h> /******************** JPEG COMPRESSION SAMPLE INTERFACE *******************/ /* This half of the example shows how to feed data into the JPEG compressor. * We present a minimal version that does not worry about refinements such * as error recovery (the JPEG code will just exit() if it gets an error). */ /* * To supply the image data for compression, you must define three routines * input_init, get_input_row, and input_term. These routines will be called * from the JPEG compressor via function pointer values that you store in the * cinfo data structure; hence they need not be globally visible and the exact * names don't matter. (In fact, the "METHODDEF" macro expands to "static" if * you use the unmodified JPEG include files.) * * The input file reading modules (jrdppm.c, jrdgif.c, jrdtarga.c, etc) may be * useful examples of what these routines should actually do, although each of * them is encrusted with a lot of specialized code for its own file format. */ METHODDEF void input_init (compress_info_ptr cinfo) /* Initialize for input; return image size and component data. */ { /* This routine must return five pieces of information about the incoming * image, and must do any setup needed for the get_input_row routine. * The image information is returned in fields of the cinfo struct. * (If you don't care about modularity, you could initialize these fields * in the main JPEG calling routine, and make this routine be a no-op.) * We show some example values here. */ cinfo->image_width = 640; /* width in pixels */ cinfo->image_height = 480; /* height in pixels */ /* JPEG views an image as being a rectangular array of pixels, with each * pixel having the same number of "component" values (color channels). * You must specify how many components there are and the colorspace * interpretation of the components. Most applications will use RGB data or * grayscale data. If you want to use something else, you'll need to study * and perhaps modify jcdeflts.c, jccolor.c, and jdcolor.c. */ cinfo->input_components = 3; /* or 1 for grayscale */ cinfo->in_color_space = CS_RGB; /* or CS_GRAYSCALE for grayscale */ cinfo->data_precision = 8; /* bits per pixel component value */ /* In the current JPEG software, data_precision must be set equal to * BITS_IN_JSAMPLE, which is 8 unless you twiddle jconfig.h. Future * versions might allow you to say either 8 or 12 if compiled with * 12-bit JSAMPLEs, or up to 16 in lossless mode. In any case, * it is up to you to scale incoming pixel values to the range * 0 .. (1<<data_precision)-1. * If your image data format is fixed at a byte per component, * then saying "8" is probably the best long-term solution. */ } /* * This function is called repeatedly and must supply the next row of pixels * on each call. The rows MUST be returned in top-to-bottom order if you want * your JPEG files to be compatible with everyone else's. (If you cannot * readily read your data in that order, you'll need an intermediate array to * hold the image. See jrdtarga.c or jrdrle.c for examples of handling * bottom-to-top source data using the JPEG code's portable mechanisms.) * The data is to be returned into a 2-D array of JSAMPLEs, indexed as * JSAMPLE pixel_row[component][column] * where component runs from 0 to cinfo->input_components-1, and column runs * from 0 to cinfo->image_width-1 (column 0 is left edge of image). Note that * this is actually an array of pointers to arrays rather than a true 2D array, * since C does not support variable-size multidimensional arrays. * JSAMPLE is typically typedef'd as "unsigned char". */ METHODDEF void get_input_row (compress_info_ptr cinfo, JSAMPARRAY pixel_row) /* Read next row of pixels into pixel_row[][] */ { /* This example shows how you might read RGB data (3 components) * from an input file in which the data is stored 3 bytes per pixel * in left-to-right, top-to-bottom order. */ register FILE * infile = cinfo->input_file; register JSAMPROW ptr0, ptr1, ptr2; register long col; ptr0 = pixel_row[0]; ptr1 = pixel_row[1]; ptr2 = pixel_row[2]; for (col = 0; col < cinfo->image_width; col++) { *ptr0++ = (JSAMPLE) getc(infile); /* red */ *ptr1++ = (JSAMPLE) getc(infile); /* green */ *ptr2++ = (JSAMPLE) getc(infile); /* blue */ } } METHODDEF void input_term (compress_info_ptr cinfo) /* Finish up at the end of the input */ { /* This termination routine will very often have no work to do, */ /* but you must provide it anyway. */ /* Note that the JPEG code will only call it during successful exit; */ /* if you want it called during error exit, you gotta do that yourself. */ } /* * That's it for the routines that deal with reading the input image data. * Now we have overall control and parameter selection routines. */ /* * This routine must determine what output JPEG file format is to be written, * and make any other compression parameter changes that are desirable. * This routine gets control after the input file header has been read * (i.e., right after input_init has been called). You could combine its * functions into input_init, or even into the main control routine, but * if you have several different input_init routines, it's a definite win * to keep this separate. You MUST supply this routine even if it's a no-op. */ METHODDEF void c_ui_method_selection (compress_info_ptr cinfo) { /* If the input is gray scale, generate a monochrome JPEG file. */ if (cinfo->in_color_space == CS_GRAYSCALE) j_monochrome_default(cinfo); /* For now, always select JFIF output format. */ jselwjfif(cinfo); } /* * OK, here is the main function that actually causes everything to happen. * We assume here that the target filename is supplied by the caller of this * routine, and that all JPEG compression parameters can be default values. */ GLOBAL void write_JPEG_file (char * filename) { /* These three structs contain JPEG parameters and working data. * They must survive for the duration of parameter setup and one * call to jpeg_compress; typically, making them local data in the * calling routine is the best strategy. */ struct Compress_info_struct cinfo; struct Compress_methods_struct c_methods; struct External_methods_struct e_methods; /* Initialize the system-dependent method pointers. */ cinfo.methods = &c_methods; /* links to method structs */ cinfo.emethods = &e_methods; /* Here we use the default JPEG error handler, which will just print * an error message on stderr and call exit(). See the second half of * this file for an example of more graceful error recovery. */ jselerror(&e_methods); /* select std error/trace message routines */ /* Here we use the standard memory manager provided with the JPEG code. * In some cases you might want to replace the memory manager, or at * least the system-dependent part of it, with your own code. */ jselmemmgr(&e_methods); /* select std memory allocation routines */ /* If the compressor requires full-image buffers (for entropy-coding * optimization or a noninterleaved JPEG file), it will create temporary * files for anything that doesn't fit within the maximum-memory setting. * (Note that temp files are NOT needed if you use the default parameters.) * You can change the default maximum-memory setting by changing * e_methods.max_memory_to_use after jselmemmgr returns. * On some systems you may also need to set up a signal handler to * ensure that temporary files are deleted if the program is interrupted. * (This is most important if you are on MS-DOS and use the jmemdos.c * memory manager back end; it will try to grab extended memory for * temp files, and that space will NOT be freed automatically.) * See jcmain.c or jdmain.c for an example signal handler. */ /* Here, set up pointers to your own routines for input data handling * and post-init parameter selection. */ c_methods.input_init = input_init; c_methods.get_input_row = get_input_row; c_methods.input_term = input_term; c_methods.c_ui_method_selection = c_ui_method_selection; /* Set up default JPEG parameters in the cinfo data structure. */ j_c_defaults(&cinfo, 75, FALSE); /* Note: 75 is the recommended default quality level; you may instead pass * a user-specified quality level. Be aware that values below 25 will cause * non-baseline JPEG files to be created (and a warning message to that * effect to be emitted on stderr). This won't bother our decoder, but some * commercial JPEG implementations may choke on non-baseline JPEG files. * If you want to force baseline compatibility, pass TRUE instead of FALSE. * (If non-baseline files are fine, but you could do without that warning * message, set e_methods.trace_level to -1.) */ /* At this point you can modify the default parameters set by j_c_defaults * as needed. For a minimal implementation, you shouldn't need to change * anything. See jcmain.c for some examples of what you might change. */ /* Select the input and output files. * Note that cinfo.input_file is only used if your input reading routines * use it; otherwise, you can just make it NULL. * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that * requires it in order to write binary files. */ cinfo.input_file = NULL; /* if no actual input file involved */ if ((cinfo.output_file = fopen(filename, "wb")) == NULL) { fprintf(stderr, "can't open %s\n", filename); exit(1); } /* Here we go! */ jpeg_compress(&cinfo); /* That's it, son. Nothin' else to do, except close files. */ /* Here we assume only the output file need be closed. */ fclose(cinfo.output_file); /* Note: if you want to compress more than one image, we recommend you * repeat this whole routine. You MUST repeat the j_c_defaults()/alter * parameters/jpeg_compress() sequence, as some data structures allocated * in j_c_defaults are freed upon exit from jpeg_compress. */ } /******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************/ /* This half of the example shows how to read data from the JPEG decompressor. * It's a little more refined than the above in that we show how to do your * own error recovery. If you don't care about that, you don't need these * next two routines. */ /* * These routines replace the default trace/error routines included with the * JPEG code. The example trace_message routine shown here is actually the * same as the standard one, but you could modify it if you don't want messages * sent to stderr. The example error_exit routine is set up to return * control to read_JPEG_file() rather than calling exit(). You can use the * same routines for both compression and decompression error recovery. */ /* These static variables are needed by the error routines. */ static jmp_buf setjmp_buffer; /* for return to caller */ static external_methods_ptr emethods; /* needed for access to message_parm */ /* This routine is used for any and all trace, debug, or error printouts * from the JPEG code. The parameter is a printf format string; up to 8 * integer data values for the format string have been stored in the * message_parm[] field of the external_methods struct. */ METHODDEF void trace_message (const char *msgtext) { fprintf(stderr, msgtext, emethods->message_parm[0], emethods->message_parm[1], emethods->message_parm[2], emethods->message_parm[3], emethods->message_parm[4], emethods->message_parm[5], emethods->message_parm[6], emethods->message_parm[7]); fprintf(stderr, "\n"); /* there is no \n in the format string! */ } /* * The error_exit() routine should not return to its caller. The default * routine calls exit(), but here we assume that we want to return to * read_JPEG_file, which has set up a setjmp context for the purpose. * You should make sure that the free_all method is called, either within * error_exit or after the return to the outer-level routine. */ METHODDEF void error_exit (const char *msgtext) { trace_message(msgtext); /* report the error message */ (*emethods->free_all) (); /* clean up memory allocation & temp files */ longjmp(setjmp_buffer, 1); /* return control to outer routine */ } /* * To accept the image data from decompression, you must define four routines * output_init, put_color_map, put_pixel_rows, and output_term. * * You must understand the distinction between full color output mode * (N independent color components) and colormapped output mode (a single * output component representing an index into a color map). You should use * colormapped mode to write to a colormapped display screen or output file. * Colormapped mode is also useful for reducing grayscale output to a small * number of gray levels: when using the 1-pass quantizer on grayscale data, * the colormap entries will be evenly spaced from 0 to MAX_JSAMPLE, so you * can regard the indexes are directly representing gray levels at reduced * precision. In any other case, you should not depend on the colormap * entries having any particular order. * To get colormapped output, set cinfo->quantize_colors to TRUE and set * cinfo->desired_number_of_colors to the maximum number of entries in the * colormap. This can be done either in your main routine or in * d_ui_method_selection. For grayscale quantization, also set * cinfo->two_pass_quantize to FALSE to ensure the 1-pass quantizer is used * (presently this is the default, but it may not be so in the future). * * The output file writing modules (jwrppm.c, jwrgif.c, jwrtarga.c, etc) may be * useful examples of what these routines should actually do, although each of * them is encrusted with a lot of specialized code for its own file format. */ METHODDEF void output_init (decompress_info_ptr cinfo) /* This routine should do any setup required */ { /* This routine can initialize for output based on the data passed in cinfo. * Useful fields include: * image_width, image_height Pretty obvious, I hope. * data_precision bits per pixel value; typically 8. * out_color_space output colorspace previously requested * color_out_comps number of color components in same * final_out_comps number of components actually output * final_out_comps is 1 if quantize_colors is true, else it is equal to * color_out_comps. * * If you have requested color quantization, the colormap is NOT yet set. * You may wish to defer output initialization until put_color_map is called. */ } /* * This routine is called if and only if you have set cinfo->quantize_colors * to TRUE. It is given the selected colormap and can complete any required * initialization. This call will occur after output_init and before any * calls to put_pixel_rows. Note that the colormap pointer is also placed * in a cinfo field, whence it can be used by put_pixel_rows or output_term. * num_colors will be less than or equal to desired_number_of_colors. * * The colormap data is supplied as a 2-D array of JSAMPLEs, indexed as * JSAMPLE colormap[component][indexvalue] * where component runs from 0 to cinfo->color_out_comps-1, and indexvalue * runs from 0 to num_colors-1. Note that this is actually an array of * pointers to arrays rather than a true 2D array, since C does not support * variable-size multidimensional arrays. * JSAMPLE is typically typedef'd as "unsigned char". If you want your code * to be as portable as the JPEG code proper, you should always access JSAMPLE * values with the GETJSAMPLE() macro, which will do the right thing if the * machine has only signed chars. */ METHODDEF void put_color_map (decompress_info_ptr cinfo, int num_colors, JSAMPARRAY colormap) /* Write the color map */ { /* You need not provide this routine if you always set cinfo->quantize_colors * FALSE; but a safer practice is to provide it and have it just print an * error message, like this: */ fprintf(stderr, "put_color_map called: there's a bug here somewhere!\n"); } /* * This function is called repeatedly, with a few more rows of pixels supplied * on each call. With the current JPEG code, some multiple of 8 rows will be * passed on each call except the last, but it is extremely bad form to depend * on this. You CAN assume num_rows > 0. * The data is supplied in top-to-bottom row order (the standard order within * a JPEG file). If you cannot readily use the data in that order, you'll * need an intermediate array to hold the image. See jwrrle.c for an example * of outputting data in bottom-to-top order. * * The data is supplied as a 3-D array of JSAMPLEs, indexed as * JSAMPLE pixel_data[component][row][column] * where component runs from 0 to cinfo->final_out_comps-1, row runs from 0 to * num_rows-1, and column runs from 0 to cinfo->image_width-1 (column 0 is * left edge of image). Note that this is actually an array of pointers to * pointers to arrays rather than a true 3D array, since C does not support * variable-size multidimensional arrays. * JSAMPLE is typically typedef'd as "unsigned char". If you want your code * to be as portable as the JPEG code proper, you should always access JSAMPLE * values with the GETJSAMPLE() macro, which will do the right thing if the * machine has only signed chars. * * If quantize_colors is true, then there is only one component, and its values * are indexes into the previously supplied colormap. Otherwise the values * are actual data in your selected output colorspace. */ METHODDEF void put_pixel_rows (decompress_info_ptr cinfo, int num_rows, JSAMPIMAGE pixel_data) /* Write some rows of output data */ { /* This example shows how you might write full-color RGB data (3 components) * to an output file in which the data is stored 3 bytes per pixel. */ register FILE * outfile = cinfo->output_file; register JSAMPROW ptr0, ptr1, ptr2; register long col; register int row; for (row = 0; row < num_rows; row++) { ptr0 = pixel_data[0][row]; ptr1 = pixel_data[1][row]; ptr2 = pixel_data[2][row]; for (col = 0; col < cinfo->image_width; col++) { putc(GETJSAMPLE(*ptr0), outfile); /* red */ ptr0++; putc(GETJSAMPLE(*ptr1), outfile); /* green */ ptr1++; putc(GETJSAMPLE(*ptr2), outfile); /* blue */ ptr2++; } } } METHODDEF void output_term (decompress_info_ptr cinfo) /* Finish up at the end of the output */ { /* This termination routine may not need to do anything. */ /* Note that the JPEG code will only call it during successful exit; */ /* if you want it called during error exit, you gotta do that yourself. */ } /* * That's it for the routines that deal with writing the output image. * Now we have overall control and parameter selection routines. */ /* * This routine gets control after the JPEG file header has been read; * at this point the image size and colorspace are known. * The routine must determine what output routines are to be used, and make * any decompression parameter changes that are desirable. For example, * if it is found that the JPEG file is grayscale, you might want to do * things differently than if it is color. You can also delay setting * quantize_colors and associated options until this point. * * j_d_defaults initializes out_color_space to CS_RGB. If you want grayscale * output you should set out_color_space to CS_GRAYSCALE. Note that you can * force grayscale output from a color JPEG file (though not vice versa). */ METHODDEF void d_ui_method_selection (decompress_info_ptr cinfo) { /* if grayscale input, force grayscale output; */ /* else leave the output colorspace as set by main routine. */ if (cinfo->jpeg_color_space == CS_GRAYSCALE) cinfo->out_color_space = CS_GRAYSCALE; /* select output routines */ cinfo->methods->output_init = output_init; cinfo->methods->put_color_map = put_color_map; cinfo->methods->put_pixel_rows = put_pixel_rows; cinfo->methods->output_term = output_term; } /* * OK, here is the main function that actually causes everything to happen. * We assume here that the JPEG filename is supplied by the caller of this * routine, and that all decompression parameters can be default values. * The routine returns 1 if successful, 0 if not. */ GLOBAL int read_JPEG_file (char * filename) { /* These three structs contain JPEG parameters and working data. * They must survive for the duration of parameter setup and one * call to jpeg_decompress; typically, making them local data in the * calling routine is the best strategy. */ struct Decompress_info_struct cinfo; struct Decompress_methods_struct dc_methods; struct External_methods_struct e_methods; /* Select the input and output files. * In this example we want to open the input file before doing anything else, * so that the setjmp() error recovery below can assume the file is open. * Note that cinfo.output_file is only used if your output handling routines * use it; otherwise, you can just make it NULL. * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that * requires it in order to read binary files. */ if ((cinfo.input_file = fopen(filename, "rb")) == NULL) { fprintf(stderr, "can't open %s\n", filename); return 0; } cinfo.output_file = NULL; /* if no actual output file involved */ /* Initialize the system-dependent method pointers. */ cinfo.methods = &dc_methods; /* links to method structs */ cinfo.emethods = &e_methods; /* Here we supply our own error handler; compare to use of standard error * handler in the previous write_JPEG_file example. */ emethods = &e_methods; /* save struct addr for possible access */ e_methods.error_exit = error_exit; /* supply error-exit routine */ e_methods.trace_message = trace_message; /* supply trace-message routine */ e_methods.trace_level = 0; /* default = no tracing */ e_methods.num_warnings = 0; /* no warnings emitted yet */ e_methods.first_warning_level = 0; /* display first corrupt-data warning */ e_methods.more_warning_level = 3; /* but suppress additional ones */ /* prepare setjmp context for possible exit from error_exit */ if (setjmp(setjmp_buffer)) { /* If we get here, the JPEG code has signaled an error. * Memory allocation has already been cleaned up (see free_all call in * error_exit), but we need to close the input file before returning. * You might also need to close an output file, etc. */ fclose(cinfo.input_file); return 0; } /* Here we use the standard memory manager provided with the JPEG code. * In some cases you might want to replace the memory manager, or at * least the system-dependent part of it, with your own code. */ jselmemmgr(&e_methods); /* select std memory allocation routines */ /* If the decompressor requires full-image buffers (for two-pass color * quantization or a noninterleaved JPEG file), it will create temporary * files for anything that doesn't fit within the maximum-memory setting. * You can change the default maximum-memory setting by changing * e_methods.max_memory_to_use after jselmemmgr returns. * On some systems you may also need to set up a signal handler to * ensure that temporary files are deleted if the program is interrupted. * (This is most important if you are on MS-DOS and use the jmemdos.c * memory manager back end; it will try to grab extended memory for * temp files, and that space will NOT be freed automatically.) * See jcmain.c or jdmain.c for an example signal handler. */ /* Here, set up the pointer to your own routine for post-header-reading * parameter selection. You could also initialize the pointers to the * output data handling routines here, if they are not dependent on the * image type. */ dc_methods.d_ui_method_selection = d_ui_method_selection; /* Set up default decompression parameters. */ j_d_defaults(&cinfo, TRUE); /* TRUE indicates that an input buffer should be allocated. * In unusual cases you may want to allocate the input buffer yourself; * see jddeflts.c for commentary. */ /* At this point you can modify the default parameters set by j_d_defaults * as needed; for example, you can request color quantization or force * grayscale output. See jdmain.c for examples of what you might change. */ /* Set up to read a JFIF or baseline-JPEG file. */ /* This is the only JPEG file format currently supported. */ jselrjfif(&cinfo); /* Here we go! */ jpeg_decompress(&cinfo); /* That's it, son. Nothin' else to do, except close files. */ /* Here we assume only the input file need be closed. */ fclose(cinfo.input_file); /* You might want to test e_methods.num_warnings to see if bad data was * detected. In this example, we just blindly forge ahead. */ return 1; /* indicate success */ /* Note: if you want to decompress more than one image, we recommend you * repeat this whole routine. You MUST repeat the j_d_defaults()/alter * parameters/jpeg_decompress() sequence, as some data structures allocated * in j_d_defaults are freed upon exit from jpeg_decompress. */ }