www.pudn.com > imgport.rar > jdatasrc.c, change:2008-11-05,size:7816b


/* 
 * 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 */ 
 
  FILE * infile;		/* source stream */ 
  JOCTET * buffer;		/* start of buffer */ 
  boolean start_of_file;	/* have we gotten any data yet? */ 
} my_source_mgr; 
 
typedef my_source_mgr * my_src_ptr; 
 
#define INPUT_BUF_SIZE  4096	/* choose an efficiently fread'able size */ 
 
 
/* 
 * Initialize source --- called by jpeg_read_header 
 * before any data is actually read. 
 */ 
 
METHODDEF(void) 
init_source (j_decompress_ptr cinfo) 
{ 
  my_src_ptr src = (my_src_ptr) cinfo->src; 
 
  /* We reset the empty-input-file flag for each image, 
   * but we don't clear the input buffer. 
   * This is correct behavior for reading a series of images from one source. 
   */ 
  src->start_of_file = TRUE; 
} 
 
 
/* 
 * 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) 
{ 
  my_src_ptr src = (my_src_ptr) cinfo->src; 
  size_t nbytes; 
 
  nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE); 
 
  if (nbytes <= 0) { 
    if (src->start_of_file)	/* Treat empty input file as fatal error */ 
      ERREXIT(cinfo, JERR_INPUT_EMPTY); 
    WARNMS(cinfo, JWRN_JPEG_EOF); 
    /* Insert a fake EOI marker */ 
    src->buffer[0] = (JOCTET) 0xFF; 
    src->buffer[1] = (JOCTET) JPEG_EOI; 
    nbytes = 2; 
  } 
 
  src->pub.next_input_byte = src->buffer; 
  src->pub.bytes_in_buffer = nbytes; 
  src->start_of_file = FALSE; 
 
  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) 
{ 
  my_src_ptr src = (my_src_ptr) cinfo->src; 
 
  /* Just a dumb implementation for now.  Could use fseek() except 
   * it doesn't work on pipes.  Not clear that being smart is worth 
   * any trouble anyway --- large skips are infrequent. 
   */ 
  if (num_bytes > 0) { 
    while (num_bytes > (long) src->pub.bytes_in_buffer) { 
      num_bytes -= (long) src->pub.bytes_in_buffer; 
      (void) fill_input_buffer(cinfo); 
      /* note we assume that fill_input_buffer will never return FALSE, 
       * so suspension need not be handled. 
       */ 
    } 
    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_stdio_src (j_decompress_ptr cinfo, FILE * infile) 
{ 
  my_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(my_source_mgr)); 
    src = (my_src_ptr) cinfo->src; 
    src->buffer = (JOCTET *) 
      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT, 
				  INPUT_BUF_SIZE * SIZEOF(JOCTET)); 
  } 
 
  src = (my_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->infile = infile; 
  src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */ 
  src->pub.next_input_byte = NULL; /* until buffer loaded */ 
}