www.pudn.com > ImageVidwer.rar > jarrsrc.c

/* 
 * jdatasrc.c 
 * 
 * Copyright (C) 1994-1996, Thomas G. Lane. 
 * This file is part of the Independent JPEG Group's software. 
 * For conditions of distribution and use, see the accompanying README file. 
 * 
 * This file contains decompression data source routines for the case of 
 * reading JPEG data from a file (or any stdio stream).  While these routines 
 * are sufficient for most applications, some will want to use a different 
 * source manager. 
 * IMPORTANT: we assume that fread() will correctly transcribe an array of 
 * JOCTETs from 8-bit-wide elements on external storage.  If char is wider 
 * than 8 bits on your machine, you may need to do some tweaking. 
 */ 
 
/* this is not a core library module, so it doesn't define JPEG_INTERNALS */ 
 
#include "jinclude.h" 
#include "jpeglib.h" 
#include "jerror.h" 
 
 
/* Expanded data source object for stdio input */ 
 
typedef struct { 
  struct jpeg_source_mgr pub;	/* public fields */ 
 
  JOCTET* image_data_buffer; 
  size_t image_data_size; 
} arr_source_mgr; 
 
typedef arr_source_mgr * arr_src_ptr; 
 
/* 
 * Initialize source --- called by jpeg_read_header 
 * before any data is actually read. 
 */ 
 
METHODDEF(void) 
init_source (j_decompress_ptr cinfo) 
{ 
} 
 
 
/* 
 * Fill the input buffer --- called whenever buffer is emptied. 
 * 
 * In typical applications, this should read fresh data into the buffer 
 * (ignoring the current state of next_input_byte & bytes_in_buffer), 
 * reset the pointer & count to the start of the buffer, and return TRUE 
 * indicating that the buffer has been reloaded.  It is not necessary to 
 * fill the buffer entirely, only to obtain at least one more byte. 
 * 
 * There is no such thing as an EOF return.  If the end of the file has been 
 * reached, the routine has a choice of ERREXIT() or inserting fake data into 
 * the buffer.  In most cases, generating a warning message and inserting a 
 * fake EOI marker is the best course of action --- this will allow the 
 * decompressor to output however much of the image is there.  However, 
 * the resulting error message is misleading if the real problem is an empty 
 * input file, so we handle that case specially. 
 * 
 * In applications that need to be able to suspend compression due to input 
 * not being available yet, a FALSE return indicates that no more data can be 
 * obtained right now, but more may be forthcoming later.  In this situation, 
 * the decompressor will return to its caller (with an indication of the 
 * number of scanlines it has read, if any).  The application should resume 
 * decompression after it has loaded more data into the input buffer.  Note 
 * that there are substantial restrictions on the use of suspension --- see 
 * the documentation. 
 * 
 * When suspending, the decompressor will back up to a convenient restart point 
 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer 
 * indicate where the restart point will be if the current call returns FALSE. 
 * Data beyond this point must be rescanned after resumption, so move it to 
 * the front of the buffer rather than discarding it. 
 */ 
 
METHODDEF(boolean) 
fill_input_buffer (j_decompress_ptr cinfo) 
{ 
	arr_src_ptr src = (arr_src_ptr) cinfo->src; 
 
	src->pub.next_input_byte = src->image_data_buffer; 
	src->pub.bytes_in_buffer = src->image_data_size; 
 
	return TRUE; 
} 
 
 
/* 
 * Skip data --- used to skip over a potentially large amount of 
 * uninteresting data (such as an APPn marker). 
 * 
 * Writers of suspendable-input applications must note that skip_input_data 
 * is not granted the right to give a suspension return.  If the skip extends 
 * beyond the data currently in the buffer, the buffer can be marked empty so 
 * that the next read will cause a fill_input_buffer call that can suspend. 
 * Arranging for additional bytes to be discarded before reloading the input 
 * buffer is the application writer's problem. 
 */ 
 
METHODDEF(void) 
skip_input_data (j_decompress_ptr cinfo, long num_bytes) 
{ 
	arr_src_ptr src = (arr_src_ptr) cinfo->src; 
 
	if (num_bytes > 0) { 
		src->pub.next_input_byte += (size_t) num_bytes; 
		src->pub.bytes_in_buffer -= (size_t) num_bytes; 
	} 
} 
 
 
/* 
 * An additional method that can be provided by data source modules is the 
 * resync_to_restart method for error recovery in the presence of RST markers. 
 * For the moment, this source module just uses the default resync method 
 * provided by the JPEG library.  That method assumes that no backtracking 
 * is possible. 
 */ 
 
 
/* 
 * Terminate source --- called by jpeg_finish_decompress 
 * after all data has been read.  Often a no-op. 
 * 
 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding 
 * application must deal with any cleanup that should happen even 
 * for error exit. 
 */ 
 
METHODDEF(void) 
term_source (j_decompress_ptr cinfo) 
{ 
  /* no work necessary here */ 
} 
 
 
/* 
 * Prepare for input from a stdio stream. 
 * The caller must have already opened the stream, and is responsible 
 * for closing it after finishing decompression. 
 */ 
 
GLOBAL(void) 
jpeg_arr_src (j_decompress_ptr cinfo, JOCTET *buffer, size_t size) 
{ 
	arr_src_ptr src; 
 
	/* The source object and input buffer are made permanent so that a series 
	 * of JPEG images can be read from the same file by calling jpeg_stdio_src 
	 * only before the first one.  (If we discarded the buffer at the end of 
	 * one image, we'd likely lose the start of the next one.) 
	 * This makes it unsafe to use this manager and a different source 
	 * manager serially with the same JPEG object.  Caveat programmer. 
	 */ 
	if (cinfo->src == NULL) {	/* first time for this JPEG object? */ 
		cinfo->src = (struct jpeg_source_mgr *) 
			(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT, SIZEOF(arr_source_mgr)); 
	} 
 
	src = (arr_src_ptr) cinfo->src; 
	src->pub.init_source = init_source; 
	src->pub.fill_input_buffer = fill_input_buffer; 
	src->pub.skip_input_data = skip_input_data; 
	src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */ 
	src->pub.term_source = term_source; 
	src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */ 
	src->pub.next_input_byte = NULL; /* until buffer loaded */ 
 
	src->image_data_buffer = buffer; 
	src->image_data_size = size; 
}