/* * Copyright (c) 1995, 2015, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file was based upon the example.c stub file included in the * release 6 of the Independent JPEG Group's free JPEG software. * It has been updated to conform to release 6b. */ /* First, if system header files define "boolean" map it to "system_boolean" */ #define boolean system_boolean #include #include #include #include #include #include "jni.h" #include "jni_util.h" /* undo "system_boolean" hack and undef FAR since we don't use it anyway */ #undef boolean #undef FAR #include #include "jerror.h" #ifdef __APPLE__ /* use setjmp/longjmp versions that do not save/restore the signal mask */ #define setjmp _setjmp #define longjmp _longjmp #endif /* The method IDs we cache. Note that the last two belongs to the * java.io.InputStream class. */ static jmethodID sendHeaderInfoID; static jmethodID sendPixelsByteID; static jmethodID sendPixelsIntID; static jmethodID InputStream_readID; static jmethodID InputStream_availableID; /* Initialize the Java VM instance variable when the library is first loaded */ JavaVM *jvm; JNIEXPORT jint JNICALL DEF_JNI_OnLoad(JavaVM *vm, void *reserved) { jvm = vm; return JNI_VERSION_1_2; } /* * 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 sun_jpeg_error_mgr { struct jpeg_error_mgr pub; /* "public" fields */ jmp_buf setjmp_buffer; /* for return to caller */ }; typedef struct sun_jpeg_error_mgr * sun_jpeg_error_ptr; /* * Here's the routine that will replace the standard error_exit method: */ METHODDEF(void) sun_jpeg_error_exit (j_common_ptr cinfo) { /* cinfo->err really points to a sun_jpeg_error_mgr struct */ sun_jpeg_error_ptr myerr = (sun_jpeg_error_ptr) cinfo->err; /* Always display the message. */ /* We could postpone this until after returning, if we chose. */ /* (*cinfo->err->output_message) (cinfo); */ /* For Java, we will format the message and put it in the error we throw. */ /* Return control to the setjmp point */ longjmp(myerr->setjmp_buffer, 1); } /* * Error Message handling * * This overrides the output_message method to send JPEG messages * */ METHODDEF(void) sun_jpeg_output_message (j_common_ptr cinfo) { char buffer[JMSG_LENGTH_MAX]; /* Create the message */ (*cinfo->err->format_message) (cinfo, buffer); /* Send it to stderr, adding a newline */ fprintf(stderr, "%s\n", buffer); } /* * INPUT HANDLING: * * The JPEG library's input management is defined by the jpeg_source_mgr * structure which contains two fields to convey the information in the * buffer and 5 methods which perform all buffer management. The library * defines a standard input manager that uses stdio for obtaining compressed * jpeg data, but here we need to use Java to get our data. * * We need to make the Java class information accessible to the source_mgr * input routines. We also need to store a pointer to the start of the * Java array being used as an input buffer so that it is not moved or * garbage collected while the JPEG library is using it. To store these * things, we make a private extension of the standard JPEG jpeg_source_mgr * object. * * Here's the extended source manager struct: */ struct sun_jpeg_source_mgr { struct jpeg_source_mgr pub; /* "public" fields */ jobject hInputStream; int suspendable; unsigned long remaining_skip; JOCTET *inbuf; jbyteArray hInputBuffer; size_t inbufoffset; /* More stuff */ union pixptr { int *ip; unsigned char *bp; } outbuf; jobject hOutputBuffer; }; typedef struct sun_jpeg_source_mgr * sun_jpeg_source_ptr; /* We use Get/ReleasePrimitiveArrayCritical functions to avoid * the need to copy buffer elements. * * MAKE SURE TO: * * - carefully insert pairs of RELEASE_ARRAYS and GET_ARRAYS around * callbacks to Java. * - call RELEASE_ARRAYS before returning to Java. * * Otherwise things will go horribly wrong. There may be memory leaks, * excessive pinning, or even VM crashes! * * Note that GetPrimitiveArrayCritical may fail! */ static void RELEASE_ARRAYS(JNIEnv *env, sun_jpeg_source_ptr src) { if (src->inbuf) { if (src->pub.next_input_byte == 0) { src->inbufoffset = -1; } else { src->inbufoffset = src->pub.next_input_byte - src->inbuf; } (*env)->ReleasePrimitiveArrayCritical(env, src->hInputBuffer, src->inbuf, 0); src->inbuf = 0; } if (src->outbuf.ip) { (*env)->ReleasePrimitiveArrayCritical(env, src->hOutputBuffer, src->outbuf.ip, 0); src->outbuf.ip = 0; } } static int GET_ARRAYS(JNIEnv *env, sun_jpeg_source_ptr src) { if (src->hInputBuffer) { assert(src->inbuf == 0); src->inbuf = (JOCTET *)(*env)->GetPrimitiveArrayCritical (env, src->hInputBuffer, 0); if (src->inbuf == 0) { return 0; } if ((int)(src->inbufoffset) >= 0) { src->pub.next_input_byte = src->inbuf + src->inbufoffset; } } if (src->hOutputBuffer) { assert(src->outbuf.ip == 0); src->outbuf.ip = (int *)(*env)->GetPrimitiveArrayCritical (env, src->hOutputBuffer, 0); if (src->outbuf.ip == 0) { RELEASE_ARRAYS(env, src); return 0; } } return 1; } /* * Initialize source. This is called by jpeg_read_header() before any * data is actually read. Unlike init_destination(), it may leave * bytes_in_buffer set to 0 (in which case a fill_input_buffer() call * will occur immediately). */ GLOBAL(void) sun_jpeg_init_source(j_decompress_ptr cinfo) { sun_jpeg_source_ptr src = (sun_jpeg_source_ptr) cinfo->src; src->pub.next_input_byte = 0; src->pub.bytes_in_buffer = 0; } /* * This is called whenever bytes_in_buffer has reached zero and more * data is wanted. In typical applications, it should read fresh data * into the buffer (ignoring the current state of next_input_byte and * 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. bytes_in_buffer MUST be set to a positive value * if TRUE is returned. A FALSE return should only be used when I/O * suspension is desired (this mode is discussed in the next section). */ /* * Note that with I/O suspension turned on, this procedure should not * do any work since the JPEG library has a very simple backtracking * mechanism which relies on the fact that the buffer will be filled * only when it has backed out to the top application level. When * suspendable is turned on, the sun_jpeg_fill_suspended_buffer will * do the actual work of filling the buffer. */ GLOBAL(boolean) sun_jpeg_fill_input_buffer(j_decompress_ptr cinfo) { sun_jpeg_source_ptr src = (sun_jpeg_source_ptr) cinfo->src; JNIEnv *env = (JNIEnv *)JNU_GetEnv(jvm, JNI_VERSION_1_2); int ret, buflen; if (src->suspendable) { return FALSE; } if (src->remaining_skip) { src->pub.skip_input_data(cinfo, 0); } RELEASE_ARRAYS(env, src); buflen = (*env)->GetArrayLength(env, src->hInputBuffer); ret = (*env)->CallIntMethod(env, src->hInputStream, InputStream_readID, src->hInputBuffer, 0, buflen); if (ret > buflen) ret = buflen; if ((*env)->ExceptionOccurred(env) || !GET_ARRAYS(env, src)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } if (ret <= 0) { /* Silently accept truncated JPEG files */ WARNMS(cinfo, JWRN_JPEG_EOF); src->inbuf[0] = (JOCTET) 0xFF; src->inbuf[1] = (JOCTET) JPEG_EOI; ret = 2; } src->pub.next_input_byte = src->inbuf; src->pub.bytes_in_buffer = ret; return TRUE; } /* * Note that with I/O suspension turned on, the JPEG library requires * that all buffer filling be done at the top application level. Due * to the way that backtracking works, this procedure should save all * of the data that was left in the buffer when suspension occurred and * only read new data at the end. */ GLOBAL(void) sun_jpeg_fill_suspended_buffer(j_decompress_ptr cinfo) { sun_jpeg_source_ptr src = (sun_jpeg_source_ptr) cinfo->src; JNIEnv *env = (JNIEnv *)JNU_GetEnv(jvm, JNI_VERSION_1_2); size_t offset, buflen; int ret; RELEASE_ARRAYS(env, src); ret = (*env)->CallIntMethod(env, src->hInputStream, InputStream_availableID); if ((*env)->ExceptionOccurred(env) || !GET_ARRAYS(env, src)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } if (ret < 0 || (unsigned int)ret <= src->remaining_skip) { return; } if (src->remaining_skip) { src->pub.skip_input_data(cinfo, 0); } /* Save the data currently in the buffer */ offset = src->pub.bytes_in_buffer; if (src->pub.next_input_byte > src->inbuf) { memmove(src->inbuf, src->pub.next_input_byte, offset); } RELEASE_ARRAYS(env, src); buflen = (*env)->GetArrayLength(env, src->hInputBuffer) - offset; if (buflen <= 0) { if (!GET_ARRAYS(env, src)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } return; } ret = (*env)->CallIntMethod(env, src->hInputStream, InputStream_readID, src->hInputBuffer, offset, buflen); if ((ret > 0) && ((unsigned int)ret > buflen)) ret = buflen; if ((*env)->ExceptionOccurred(env) || !GET_ARRAYS(env, src)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } if (ret <= 0) { /* Silently accept truncated JPEG files */ WARNMS(cinfo, JWRN_JPEG_EOF); src->inbuf[offset] = (JOCTET) 0xFF; src->inbuf[offset + 1] = (JOCTET) JPEG_EOI; ret = 2; } src->pub.next_input_byte = src->inbuf; src->pub.bytes_in_buffer = ret + offset; return; } /* * Skip num_bytes worth of data. The buffer pointer and count should * be advanced over num_bytes input bytes, refilling the buffer as * needed. This is used to skip over a potentially large amount of * uninteresting data (such as an APPn marker). In some applications * it may be possible to optimize away the reading of the skipped data, * but it's not clear that being smart is worth much trouble; large * skips are uncommon. bytes_in_buffer may be zero on return. * A zero or negative skip count should be treated as a no-op. */ /* * Note that with I/O suspension turned on, this procedure should not * do any I/O since the JPEG library has a very simple backtracking * mechanism which relies on the fact that the buffer will be filled * only when it has backed out to the top application level. */ GLOBAL(void) sun_jpeg_skip_input_data(j_decompress_ptr cinfo, long num_bytes) { sun_jpeg_source_ptr src = (sun_jpeg_source_ptr) cinfo->src; JNIEnv *env = (JNIEnv *)JNU_GetEnv(jvm, JNI_VERSION_1_2); int ret; int buflen; if (num_bytes < 0) { return; } num_bytes += src->remaining_skip; src->remaining_skip = 0; ret = (int)src->pub.bytes_in_buffer; /* this conversion is safe, because capacity of the buffer is limited by jnit */ if (ret >= num_bytes) { src->pub.next_input_byte += num_bytes; src->pub.bytes_in_buffer -= num_bytes; return; } num_bytes -= ret; if (src->suspendable) { src->remaining_skip = num_bytes; src->pub.bytes_in_buffer = 0; src->pub.next_input_byte = src->inbuf; return; } /* Note that the signature for the method indicates that it takes * and returns a long. Casting the int num_bytes to a long on * the input should work well enough, and if we assume that the * return value for this particular method should always be less * than the argument value (or -1), then the return value coerced * to an int should return us the information we need... */ RELEASE_ARRAYS(env, src); buflen = (*env)->GetArrayLength(env, src->hInputBuffer); while (num_bytes > 0) { ret = (*env)->CallIntMethod(env, src->hInputStream, InputStream_readID, src->hInputBuffer, 0, buflen); if (ret > buflen) ret = buflen; if ((*env)->ExceptionOccurred(env)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } if (ret < 0) { break; } num_bytes -= ret; } if (!GET_ARRAYS(env, src)) { cinfo->err->error_exit((struct jpeg_common_struct *) cinfo); } if (num_bytes > 0) { /* Silently accept truncated JPEG files */ WARNMS(cinfo, JWRN_JPEG_EOF); src->inbuf[0] = (JOCTET) 0xFF; src->inbuf[1] = (JOCTET) JPEG_EOI; src->pub.bytes_in_buffer = 2; src->pub.next_input_byte = src->inbuf; } else { src->pub.bytes_in_buffer = -num_bytes; src->pub.next_input_byte = src->inbuf + ret + num_bytes; } } /* * Terminate source --- called by jpeg_finish_decompress() after all * data has been read. Often a no-op. */ GLOBAL(void) sun_jpeg_term_source(j_decompress_ptr cinfo) { } JNIEXPORT void JNICALL Java_sun_awt_image_JPEGImageDecoder_initIDs(JNIEnv *env, jclass cls, jclass InputStreamClass) { CHECK_NULL(sendHeaderInfoID = (*env)->GetMethodID(env, cls, "sendHeaderInfo", "(IIZZZ)Z")); CHECK_NULL(sendPixelsByteID = (*env)->GetMethodID(env, cls, "sendPixels", "([BI)Z")); CHECK_NULL(sendPixelsIntID = (*env)->GetMethodID(env, cls, "sendPixels", "([II)Z")); CHECK_NULL(InputStream_readID = (*env)->GetMethodID(env, InputStreamClass, "read", "([BII)I")); CHECK_NULL(InputStream_availableID = (*env)->GetMethodID(env, InputStreamClass, "available", "()I")); } /* * The Windows Itanium Aug 2002 SDK generates bad code * for this routine. Disable optimization for now. */ #ifdef _M_IA64 #pragma optimize ("", off) #endif JNIEXPORT void JNICALL Java_sun_awt_image_JPEGImageDecoder_readImage(JNIEnv *env, jobject this, jobject hInputStream, jbyteArray hInputBuffer) { /* 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 sun_jpeg_error_mgr jerr; struct sun_jpeg_source_mgr jsrc; int ret; unsigned char *bp; int *ip, pixel; int grayscale; int hasalpha; int buffered_mode; int final_pass; /* Step 0: verify the inputs. */ if (hInputBuffer == 0 || hInputStream == 0) { JNU_ThrowNullPointerException(env, 0); return; } jsrc.outbuf.ip = 0; jsrc.inbuf = 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 = sun_jpeg_error_exit; /* We need to setup our own print routines */ jerr.pub.output_message = sun_jpeg_output_message; /* Establish the setjmp return context for sun_jpeg_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); RELEASE_ARRAYS(env, &jsrc); if (!(*env)->ExceptionOccurred(env)) { char buffer[JMSG_LENGTH_MAX]; (*cinfo.err->format_message) ((struct jpeg_common_struct *) &cinfo, buffer); JNU_ThrowByName(env, "sun/awt/image/ImageFormatException", buffer); } return; } /* Now we can initialize the JPEG decompression object. */ jpeg_create_decompress(&cinfo); /* Step 2: specify data source (eg, a file) */ cinfo.src = &jsrc.pub; jsrc.hInputStream = hInputStream; jsrc.hInputBuffer = hInputBuffer; jsrc.hOutputBuffer = 0; jsrc.suspendable = FALSE; jsrc.remaining_skip = 0; jsrc.inbufoffset = -1; jsrc.pub.init_source = sun_jpeg_init_source; jsrc.pub.fill_input_buffer = sun_jpeg_fill_input_buffer; jsrc.pub.skip_input_data = sun_jpeg_skip_input_data; jsrc.pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */ jsrc.pub.term_source = sun_jpeg_term_source; if (!GET_ARRAYS(env, &jsrc)) { jpeg_destroy_decompress(&cinfo); return; } /* Step 3: read file parameters with jpeg_read_header() */ (void) jpeg_read_header(&cinfo, TRUE); /* select buffered-image mode if it is a progressive JPEG only */ buffered_mode = cinfo.buffered_image = jpeg_has_multiple_scans(&cinfo); grayscale = (cinfo.out_color_space == JCS_GRAYSCALE); #ifdef YCCALPHA hasalpha = (cinfo.out_color_space == JCS_RGBA); #else hasalpha = 0; #endif /* We can ignore the return value from jpeg_read_header since * (a) suspension is not possible with the stdio data source, and * (nor with the Java input source) * (b) we passed TRUE to reject a tables-only JPEG file as an error. * See libjpeg.doc for more info. */ RELEASE_ARRAYS(env, &jsrc); ret = (*env)->CallBooleanMethod(env, this, sendHeaderInfoID, cinfo.image_width, cinfo.image_height, grayscale, hasalpha, buffered_mode); if ((*env)->ExceptionOccurred(env) || !ret) { /* No more interest in this image... */ jpeg_destroy_decompress(&cinfo); return; } /* Make a one-row-high sample array with enough room to expand to ints */ if (grayscale) { jsrc.hOutputBuffer = (*env)->NewByteArray(env, cinfo.image_width); } else { jsrc.hOutputBuffer = (*env)->NewIntArray(env, cinfo.image_width); } if (jsrc.hOutputBuffer == 0 || !GET_ARRAYS(env, &jsrc)) { jpeg_destroy_decompress(&cinfo); return; } /* 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. */ /* For the first pass for Java, we want to deal with RGB for simplicity */ /* Unfortunately, the JPEG code does not automatically convert Grayscale */ /* to RGB, so we have to deal with Grayscale explicitly. */ if (!grayscale && !hasalpha) { cinfo.out_color_space = JCS_RGB; } /* Step 5: Start decompressor */ jpeg_start_decompress(&cinfo); /* 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. */ /* 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. */ if (buffered_mode) { final_pass = FALSE; cinfo.dct_method = JDCT_IFAST; } else { final_pass = TRUE; } do { if (buffered_mode) { do { sun_jpeg_fill_suspended_buffer(&cinfo); jsrc.suspendable = TRUE; ret = jpeg_consume_input(&cinfo); jsrc.suspendable = FALSE; } while (ret != JPEG_SUSPENDED && ret != JPEG_REACHED_EOI); if (ret == JPEG_REACHED_EOI) { final_pass = TRUE; cinfo.dct_method = JDCT_ISLOW; } jpeg_start_output(&cinfo, cinfo.input_scan_number); } while (cinfo.output_scanline < cinfo.output_height) { if (! final_pass) { do { sun_jpeg_fill_suspended_buffer(&cinfo); jsrc.suspendable = TRUE; ret = jpeg_consume_input(&cinfo); jsrc.suspendable = FALSE; } while (ret != JPEG_SUSPENDED && ret != JPEG_REACHED_EOI); if (ret == JPEG_REACHED_EOI) { break; } } (void) jpeg_read_scanlines(&cinfo, (JSAMPARRAY) &(jsrc.outbuf), 1); if (grayscale) { RELEASE_ARRAYS(env, &jsrc); ret = (*env)->CallBooleanMethod(env, this, sendPixelsByteID, jsrc.hOutputBuffer, cinfo.output_scanline - 1); } else { if (hasalpha) { ip = jsrc.outbuf.ip + cinfo.image_width; bp = jsrc.outbuf.bp + cinfo.image_width * 4; while (ip > jsrc.outbuf.ip) { pixel = (*--bp) << 24; pixel |= (*--bp); pixel |= (*--bp) << 8; pixel |= (*--bp) << 16; *--ip = pixel; } } else { ip = jsrc.outbuf.ip + cinfo.image_width; bp = jsrc.outbuf.bp + cinfo.image_width * 3; while (ip > jsrc.outbuf.ip) { pixel = (*--bp); pixel |= (*--bp) << 8; pixel |= (*--bp) << 16; *--ip = pixel; } } RELEASE_ARRAYS(env, &jsrc); ret = (*env)->CallBooleanMethod(env, this, sendPixelsIntID, jsrc.hOutputBuffer, cinfo.output_scanline - 1); } if ((*env)->ExceptionOccurred(env) || !ret || !GET_ARRAYS(env, &jsrc)) { /* No more interest in this image... */ jpeg_destroy_decompress(&cinfo); return; } } if (buffered_mode) { jpeg_finish_output(&cinfo); } } while (! final_pass); /* 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. * (nor with the Java 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...) */ /* Not needed for Java - the Java code will close the file */ /* 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! */ RELEASE_ARRAYS(env, &jsrc); return; } #ifdef _M_IA64 #pragma optimize ("", on) #endif /* * 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. */