JPEG decompression part of example.c
/*
* 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.doc.
*
* 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 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.doc 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.doc 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.doc 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.doc.
*/