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diff --git a/jpeg/example.c b/jpeg/example.c new file mode 100644 index 0000000..1d6f6cc --- /dev/null +++ b/jpeg/example.c @@ -0,0 +1,433 @@ +/* + * example.c + * + * This file illustrates how to use the IJG code as a subroutine library + * to read or write JPEG image files. You should look at this code in + * conjunction with the documentation file libjpeg.txt. + * + * This code will not do anything useful as-is, but it may be helpful as a + * skeleton for constructing routines that call the JPEG library. + * + * 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 <stdio.h> + +/* + * Include file for users of JPEG library. + * You will need to have included system headers that define at least + * the typedefs FILE and size_t before you can include jpeglib.h. + * (stdio.h is sufficient on ANSI-conforming systems.) + * You may also wish to include "jerror.h". + */ + +#include "jpeglib.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). + */ + + +/* + * IMAGE DATA FORMATS: + * + * The standard input image format is a rectangular array of pixels, with + * each pixel having the same number of "component" values (color channels). + * Each pixel row is an array of JSAMPLEs (which typically are unsigned chars). + * If you are working with color data, then the color values for each pixel + * must be adjacent in the row; for example, R,G,B,R,G,B,R,G,B,... for 24-bit + * RGB color. + * + * For this example, we'll assume that this data structure matches the way + * our application has stored the image in memory, so we can just pass a + * pointer to our image buffer. In particular, let's say that the image is + * RGB color and is described by: + */ + +extern JSAMPLE * image_buffer; /* Points to large array of R,G,B-order data */ +extern int image_height; /* Number of rows in image */ +extern int image_width; /* Number of columns in image */ + + +/* + * Sample routine for JPEG compression. We assume that the target file name + * and a compression quality factor are passed in. + */ + +GLOBAL(void) +write_JPEG_file (char * filename, int quality) +{ + /* This struct contains the JPEG compression parameters and pointers to + * working space (which is allocated as needed by the JPEG library). + * It is possible to have several such structures, representing multiple + * compression/decompression processes, in existence at once. We refer + * to any one struct (and its associated working data) as a "JPEG object". + */ + struct jpeg_compress_struct cinfo; + /* This struct represents a JPEG error handler. It is declared separately + * because applications often want to supply a specialized error handler + * (see the second half of this file for an example). But here we just + * take the easy way out and use the standard error handler, which will + * print a message on stderr and call exit() if compression fails. + * Note that this struct must live as long as the main JPEG parameter + * struct, to avoid dangling-pointer problems. + */ + struct jpeg_error_mgr jerr; + /* More stuff */ + FILE * outfile; /* target file */ + JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */ + int row_stride; /* physical row width in image buffer */ + + /* Step 1: allocate and initialize JPEG compression object */ + + /* We have to set up the error handler first, in case the initialization + * step fails. (Unlikely, but it could happen if you are out of memory.) + * This routine fills in the contents of struct jerr, and returns jerr's + * address which we place into the link field in cinfo. + */ + cinfo.err = jpeg_std_error(&jerr); + /* Now we can initialize the JPEG compression object. */ + jpeg_create_compress(&cinfo); + + /* Step 2: specify data destination (eg, a file) */ + /* Note: steps 2 and 3 can be done in either order. */ + + /* Here we use the library-supplied code to send compressed data to a + * stdio stream. You can also write your own code to do something else. + * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that + * requires it in order to write binary files. + */ + if ((outfile = fopen(filename, "wb")) == NULL) { + fprintf(stderr, "can't open %s\n", filename); + exit(1); + } + jpeg_stdio_dest(&cinfo, outfile); + + /* Step 3: set parameters for compression */ + + /* First we supply a description of the input image. + * Four fields of the cinfo struct must be filled in: + */ + cinfo.image_width = image_width; /* image width and height, in pixels */ + cinfo.image_height = image_height; + cinfo.input_components = 3; /* # of color components per pixel */ + cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ + /* Now use the library's routine to set default compression parameters. + * (You must set at least cinfo.in_color_space before calling this, + * since the defaults depend on the source color space.) + */ + jpeg_set_defaults(&cinfo); + /* Now you can set any non-default parameters you wish to. + * Here we just illustrate the use of quality (quantization table) scaling: + */ + jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */); + + /* Step 4: Start compressor */ + + /* TRUE ensures that we will write a complete interchange-JPEG file. + * Pass TRUE unless you are very sure of what you're doing. + */ + jpeg_start_compress(&cinfo, TRUE); + + /* Step 5: while (scan lines remain to be written) */ + /* jpeg_write_scanlines(...); */ + + /* Here we use the library's state variable cinfo.next_scanline as the + * loop counter, so that we don't have to keep track ourselves. + * To keep things simple, we pass one scanline per call; you can pass + * more if you wish, though. + */ + row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ + + while (cinfo.next_scanline < cinfo.image_height) { + /* jpeg_write_scanlines expects an array of pointers to scanlines. + * Here the array is only one element long, but you could pass + * more than one scanline at a time if that's more convenient. + */ + row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride]; + (void) jpeg_write_scanlines(&cinfo, row_pointer, 1); + } + + /* Step 6: Finish compression */ + + jpeg_finish_compress(&cinfo); + /* After finish_compress, we can close the output file. */ + fclose(outfile); + + /* Step 7: release JPEG compression object */ + + /* This is an important step since it will release a good deal of memory. */ + jpeg_destroy_compress(&cinfo); + + /* And we're done! */ +} + + +/* + * SOME FINE POINTS: + * + * In the above loop, we ignored the return value of jpeg_write_scanlines, + * which is the number of scanlines actually written. We could get away + * with this because we were only relying on the value of cinfo.next_scanline, + * which will be incremented correctly. If you maintain additional loop + * variables then you should be careful to increment them properly. + * Actually, for output to a stdio stream you needn't worry, because + * then jpeg_write_scanlines will write all the lines passed (or else exit + * with a fatal error). Partial writes can only occur if you use a data + * destination module that can demand suspension of the compressor. + * (If you don't know what that's for, you don't need it.) + * + * If the compressor requires full-image buffers (for entropy-coding + * optimization or a multi-scan 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.) + * On some systems you may need to set up a signal handler to ensure that + * temporary files are deleted if the program is interrupted. See libjpeg.txt. + * + * Scanlines MUST be supplied 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 rdtarga.c or rdbmp.c for examples of handling bottom-to-top + * source data using the JPEG code's internal virtual-array mechanisms. + */ + + + +/******************** JPEG DECOMPRESSION SAMPLE INTERFACE *******************/ + +/* This half of the example shows how to read data from the JPEG decompressor. + * It's a bit more refined than the above, in that we show: + * (a) how to modify the JPEG library's standard error-reporting behavior; + * (b) how to allocate workspace using the library's memory manager. + * + * Just to make this example a little different from the first one, we'll + * assume that we do not intend to put the whole image into an in-memory + * buffer, but to send it line-by-line someplace else. We need a one- + * scanline-high JSAMPLE array as a work buffer, and we will let the JPEG + * memory manager allocate it for us. This approach is actually quite useful + * because we don't need to remember to deallocate the buffer separately: it + * will go away automatically when the JPEG object is cleaned up. + */ + + +/* + * ERROR HANDLING: + * + * The JPEG library's standard error handler (jerror.c) is divided into + * several "methods" which you can override individually. This lets you + * adjust the behavior without duplicating a lot of code, which you might + * have to update with each future release. + * + * Our example here shows how to override the "error_exit" method so that + * control is returned to the library's caller when a fatal error occurs, + * rather than calling exit() as the standard error_exit method does. + * + * We use C's setjmp/longjmp facility to return control. This means that the + * routine which calls the JPEG library must first execute a setjmp() call to + * establish the return point. We want the replacement error_exit to do a + * longjmp(). But we need to make the setjmp buffer accessible to the + * error_exit routine. To do this, we make a private extension of the + * standard JPEG error handler object. (If we were using C++, we'd say we + * were making a subclass of the regular error handler.) + * + * Here's the extended error handler struct: + */ + +struct my_error_mgr { + struct jpeg_error_mgr pub; /* "public" fields */ + + jmp_buf setjmp_buffer; /* for return to caller */ +}; + +typedef struct my_error_mgr * my_error_ptr; + +/* + * Here's the routine that will replace the standard error_exit method: + */ + +METHODDEF(void) +my_error_exit (j_common_ptr cinfo) +{ + /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */ + my_error_ptr myerr = (my_error_ptr) cinfo->err; + + /* Always display the message. */ + /* We could postpone this until after returning, if we chose. */ + (*cinfo->err->output_message) (cinfo); + + /* Return control to the setjmp point */ + longjmp(myerr->setjmp_buffer, 1); +} + + +/* + * Sample routine for JPEG decompression. We assume that the source file name + * is passed in. We want to return 1 on success, 0 on error. + */ + + +GLOBAL(int) +read_JPEG_file (char * filename) +{ + /* This struct contains the JPEG decompression parameters and pointers to + * working space (which is allocated as needed by the JPEG library). + */ + struct jpeg_decompress_struct cinfo; + /* We use our private extension JPEG error handler. + * Note that this struct must live as long as the main JPEG parameter + * struct, to avoid dangling-pointer problems. + */ + struct my_error_mgr jerr; + /* More stuff */ + FILE * infile; /* source file */ + JSAMPARRAY buffer; /* Output row buffer */ + int row_stride; /* physical row width in output buffer */ + + /* 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. + * VERY IMPORTANT: use "b" option to fopen() if you are on a machine that + * requires it in order to read binary files. + */ + + if ((infile = fopen(filename, "rb")) == NULL) { + fprintf(stderr, "can't open %s\n", filename); + return 0; + } + + /* Step 1: allocate and initialize JPEG decompression object */ + + /* We set up the normal JPEG error routines, then override error_exit. */ + cinfo.err = jpeg_std_error(&jerr.pub); + jerr.pub.error_exit = my_error_exit; + /* Establish the setjmp return context for my_error_exit to use. */ + if (setjmp(jerr.setjmp_buffer)) { + /* If we get here, the JPEG code has signaled an error. + * We need to clean up the JPEG object, close the input file, and return. + */ + jpeg_destroy_decompress(&cinfo); + fclose(infile); + return 0; + } + /* Now we can initialize the JPEG decompression object. */ + jpeg_create_decompress(&cinfo); + + /* Step 2: specify data source (eg, a file) */ + + jpeg_stdio_src(&cinfo, infile); + + /* Step 3: read file parameters with jpeg_read_header() */ + + (void) jpeg_read_header(&cinfo, TRUE); + /* We can ignore the return value from jpeg_read_header since + * (a) suspension is not possible with the stdio data source, and + * (b) we passed TRUE to reject a tables-only JPEG file as an error. + * See libjpeg.txt for more info. + */ + + /* Step 4: set parameters for decompression */ + + /* In this example, we don't need to change any of the defaults set by + * jpeg_read_header(), so we do nothing here. + */ + + /* Step 5: Start decompressor */ + + (void) jpeg_start_decompress(&cinfo); + /* We can ignore the return value since suspension is not possible + * with the stdio data source. + */ + + /* We may need to do some setup of our own at this point before reading + * the data. After jpeg_start_decompress() we have the correct scaled + * output image dimensions available, as well as the output colormap + * if we asked for color quantization. + * In this example, we need to make an output work buffer of the right size. + */ + /* JSAMPLEs per row in output buffer */ + row_stride = cinfo.output_width * cinfo.output_components; + /* Make a one-row-high sample array that will go away when done with image */ + buffer = (*cinfo.mem->alloc_sarray) + ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1); + + /* Step 6: while (scan lines remain to be read) */ + /* jpeg_read_scanlines(...); */ + + /* Here we use the library's state variable cinfo.output_scanline as the + * loop counter, so that we don't have to keep track ourselves. + */ + while (cinfo.output_scanline < cinfo.output_height) { + /* jpeg_read_scanlines expects an array of pointers to scanlines. + * Here the array is only one element long, but you could ask for + * more than one scanline at a time if that's more convenient. + */ + (void) jpeg_read_scanlines(&cinfo, buffer, 1); + /* Assume put_scanline_someplace wants a pointer and sample count. */ + put_scanline_someplace(buffer[0], row_stride); + } + + /* Step 7: Finish decompression */ + + (void) jpeg_finish_decompress(&cinfo); + /* We can ignore the return value since suspension is not possible + * with the stdio data source. + */ + + /* Step 8: Release JPEG decompression object */ + + /* This is an important step since it will release a good deal of memory. */ + jpeg_destroy_decompress(&cinfo); + + /* After finish_decompress, we can close the input file. + * Here we postpone it until after no more JPEG errors are possible, + * so as to simplify the setjmp error logic above. (Actually, I don't + * think that jpeg_destroy can do an error exit, but why assume anything...) + */ + fclose(infile); + + /* At this point you may want to check to see whether any corrupt-data + * warnings occurred (test whether jerr.pub.num_warnings is nonzero). + */ + + /* And we're done! */ + return 1; +} + + +/* + * SOME FINE POINTS: + * + * In the above code, we ignored the return value of jpeg_read_scanlines, + * which is the number of scanlines actually read. We could get away with + * this because we asked for only one line at a time and we weren't using + * a suspending data source. See libjpeg.txt for more info. + * + * We cheated a bit by calling alloc_sarray() after jpeg_start_decompress(); + * we should have done it beforehand to ensure that the space would be + * counted against the JPEG max_memory setting. In some systems the above + * code would risk an out-of-memory error. However, in general we don't + * know the output image dimensions before jpeg_start_decompress(), unless we + * call jpeg_calc_output_dimensions(). See libjpeg.txt for more about this. + * + * Scanlines are returned in the same order as they appear in the JPEG file, + * which is standardly top-to-bottom. If you must emit data bottom-to-top, + * you can use one of the virtual arrays provided by the JPEG memory manager + * to invert the data. See wrbmp.c for an example. + * + * As with compression, some operating modes may require temporary files. + * On some systems you may need to set up a signal handler to ensure that + * temporary files are deleted if the program is interrupted. See libjpeg.txt. + */ |