ref: bf4f158707ebfeacb15826ad4283ff247d7d81ae
dir: /sys/src/cmd/gs/jpeg/jdatasrc.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 */
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 */
}