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/* |
* 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" |
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/* Expanded data source object for stdio input */ |
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typedef struct { |
struct jpeg_source_mgr pub; /* public fields */ |
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FILE * infile; /* source stream */ |
JOCTET * buffer; /* start of buffer */ |
boolean start_of_file; /* have we gotten any data yet? */ |
} my_source_mgr; |
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typedef my_source_mgr * my_src_ptr; |
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#define INPUT_BUF_SIZE 4096 /* choose an efficiently fread'able size */ |
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/* |
* Initialize source --- called by jpeg_read_header |
* before any data is actually read. |
*/ |
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METHODDEF(void) |
init_source (j_decompress_ptr cinfo) |
{ |
my_src_ptr src = (my_src_ptr) cinfo->src; |
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/* 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; |
} |
|
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/* |
* 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. |
*/ |
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METHODDEF(boolean) |
fill_input_buffer (j_decompress_ptr cinfo) |
{ |
my_src_ptr src = (my_src_ptr) cinfo->src; |
size_t nbytes; |
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nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE); |
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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; |
} |
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src->pub.next_input_byte = src->buffer; |
src->pub.bytes_in_buffer = nbytes; |
src->start_of_file = FALSE; |
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return TRUE; |
} |
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/* |
* 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. |
*/ |
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METHODDEF(void) |
skip_input_data (j_decompress_ptr cinfo, long num_bytes) |
{ |
my_src_ptr src = (my_src_ptr) cinfo->src; |
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/* 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; |
} |
} |
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/* |
* 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. |
*/ |
|
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/* |
* 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. |
*/ |
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METHODDEF(void) |
term_source (j_decompress_ptr cinfo) |
{ |
/* no work necessary here */ |
} |
|
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/* |
* Prepare for input from a stdio stream. |
* The caller must have already opened the stream, and is responsible |
* for closing it after finishing decompression. |
*/ |
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GLOBAL(void) |
jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile) |
{ |
my_src_ptr src; |
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/* 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)); |
} |
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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 */ |
} |