Add a new libjpeg API function (jpeg_skip_scanlines()) to allow for partially decoding a JPEG image.
git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1582 632fc199-4ca6-4c93-a231-07263d6284db
diff --git a/jdapistd.c b/jdapistd.c
index 3be527c..51b7dc6 100644
--- a/jdapistd.c
+++ b/jdapistd.c
@@ -4,7 +4,8 @@
* This file was part of the Independent JPEG Group's software:
* Copyright (C) 1994-1996, Thomas G. Lane.
* libjpeg-turbo Modifications:
- * Copyright (C) 2010, D. R. Commander.
+ * Copyright (C) 2010, 2015, D. R. Commander.
+ * Copyright (C) 2015, Google, Inc.
* For conditions of distribution and use, see the accompanying README file.
*
* This file contains application interface code for the decompression half
@@ -16,11 +17,10 @@
* whole decompression library into a transcoder.
*/
-#define JPEG_INTERNALS
-#include "jinclude.h"
-#include "jpeglib.h"
-#include "jpegcomp.h"
-
+#include "jdmainct.h"
+#include "jdcoefct.h"
+#include "jdsample.h"
+#include "jmemsys.h"
/* Forward declarations */
LOCAL(boolean) output_pass_setup (j_decompress_ptr cinfo);
@@ -179,6 +179,242 @@
}
+/* Prepare temporary row buffer */
+
+LOCAL(void)
+dummy_buffer_setup (j_decompress_ptr cinfo)
+{
+ int nc;
+
+ if (!cinfo->master || cinfo->master->dummy_row_buffer)
+ return;
+
+ nc = (cinfo->out_color_space == JCS_RGB565) ?
+ 2 : cinfo->out_color_components;
+ cinfo->master->dummy_row_buffer =
+ jpeg_get_small((j_common_ptr) cinfo,
+ cinfo->output_width * nc * sizeof(JSAMPLE));
+}
+
+
+/*
+ * Called by jpeg_skip_scanlines(). This partially skips a decompress block by
+ * incrementing the rowgroup counter.
+ */
+
+LOCAL(void)
+increment_simple_rowgroup_ctr (j_decompress_ptr cinfo, JDIMENSION rows)
+{
+ int i;
+ JDIMENSION rows_left;
+ my_main_ptr main_ptr = (my_main_ptr) cinfo->main;
+
+ /* Increment the counter to the next row group after the skipped rows. */
+ main_ptr->rowgroup_ctr += rows / cinfo->max_v_samp_factor;
+
+ /* Partially skipping a row group would involve modifying the internal state
+ * of the upsampler, so read the remaining rows into a dummy buffer instead.
+ */
+ rows_left = rows % cinfo->max_v_samp_factor;
+ cinfo->output_scanline += rows - rows_left;
+
+ dummy_buffer_setup(cinfo);
+ for (i = 0; i < rows_left; i++)
+ jpeg_read_scanlines(cinfo, &(cinfo->master->dummy_row_buffer), 1);
+}
+
+
+/*
+ * Called by jpeg_skip_scanlines(). When we skip iMCU rows, we must update the
+ * iMCU row counter.
+ */
+
+LOCAL(void)
+increment_iMCU_ctr (j_decompress_ptr cinfo, JDIMENSION iMCU_rows)
+{
+ my_main_ptr main_ptr = (my_main_ptr) cinfo->main;
+ if (main_ptr->iMCU_row_ctr == 0 && iMCU_rows > 0)
+ set_wraparound_pointers(cinfo);
+ main_ptr->iMCU_row_ctr += iMCU_rows;
+}
+
+
+/*
+ * Skips some scanlines of data from the JPEG decompressor.
+ *
+ * The return value will be the number of lines actually skipped. If skipping
+ * num_lines would move beyond the end of the image, then the actual number of
+ * lines remaining in the image is returned. Otherwise, the return value will
+ * be equal to num_lines.
+ *
+ * Refer to libjpeg.txt for more information.
+ */
+
+GLOBAL(JDIMENSION)
+jpeg_skip_scanlines (j_decompress_ptr cinfo, JDIMENSION num_lines)
+{
+ my_main_ptr main_ptr = (my_main_ptr) cinfo->main;
+ my_coef_ptr coef = (my_coef_ptr) cinfo->coef;
+ my_upsample_ptr upsample = (my_upsample_ptr) cinfo->upsample;
+ int i, y, x;
+ JDIMENSION lines_per_iMCU_row, lines_left_in_iMCU_row, lines_after_iMCU_row;
+ JDIMENSION lines_to_skip, lines_to_read;
+
+ if (cinfo->global_state != DSTATE_SCANNING)
+ ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);
+
+ /* Do not skip past the bottom of the image. */
+ if (cinfo->output_scanline + num_lines >= cinfo->output_height) {
+ cinfo->output_scanline = cinfo->output_height;
+ return cinfo->output_height - cinfo->output_scanline;
+ }
+
+ if (num_lines == 0)
+ return 0;
+
+ lines_per_iMCU_row = cinfo->_min_DCT_scaled_size * cinfo->max_v_samp_factor;
+ lines_left_in_iMCU_row =
+ (lines_per_iMCU_row - (cinfo->output_scanline % lines_per_iMCU_row)) %
+ lines_per_iMCU_row;
+ lines_after_iMCU_row = num_lines - lines_left_in_iMCU_row;
+
+ /* Skip the lines remaining in the current iMCU row. When upsampling
+ * requires context rows, we need the previous and next rows in order to read
+ * the current row. This adds some complexity.
+ */
+ if (cinfo->upsample->need_context_rows) {
+ /* If the skipped lines would not move us past the current iMCU row, we
+ * read the lines and ignore them. There might be a faster way of doing
+ * this, but we are facing increasing complexity for diminishing returns.
+ * The increasing complexity would be a by-product of meddling with the
+ * state machine used to skip context rows. Near the end of an iMCU row,
+ * the next iMCU row may have already been entropy-decoded. In this unique
+ * case, we will read the next iMCU row if we cannot skip past it as well.
+ */
+ if ((num_lines < lines_left_in_iMCU_row + 1) ||
+ (lines_left_in_iMCU_row <= 1 && main_ptr->buffer_full &&
+ lines_after_iMCU_row < lines_per_iMCU_row + 1)) {
+ dummy_buffer_setup(cinfo);
+ for (i = 0; i < num_lines; i++)
+ jpeg_read_scanlines(cinfo, &(cinfo->master->dummy_row_buffer), 1);
+ return num_lines;
+ }
+
+ /* If the next iMCU row has already been entropy-decoded, make sure that
+ * we do not skip too far.
+ */
+ if (lines_left_in_iMCU_row <= 1 && main_ptr->buffer_full) {
+ cinfo->output_scanline += lines_left_in_iMCU_row + lines_per_iMCU_row;
+ lines_after_iMCU_row -= lines_per_iMCU_row;
+ } else {
+ cinfo->output_scanline += lines_left_in_iMCU_row;
+ }
+ main_ptr->buffer_full = FALSE;
+ main_ptr->rowgroup_ctr = 0;
+ main_ptr->context_state = CTX_PREPARE_FOR_IMCU;
+ upsample->next_row_out = cinfo->max_v_samp_factor;
+ upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;
+ }
+
+ /* Skipping is much simpler when context rows are not required. */
+ else {
+ if (num_lines < lines_left_in_iMCU_row) {
+ increment_simple_rowgroup_ctr(cinfo, num_lines);
+ return num_lines;
+ } else {
+ cinfo->output_scanline += lines_left_in_iMCU_row;
+ main_ptr->buffer_full = FALSE;
+ main_ptr->rowgroup_ctr = 0;
+ upsample->next_row_out = cinfo->max_v_samp_factor;
+ upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;
+ }
+ }
+
+ /* Calculate how many full iMCU rows we can skip. */
+ if (cinfo->upsample->need_context_rows)
+ lines_to_skip = ((lines_after_iMCU_row - 1) / lines_per_iMCU_row) *
+ lines_per_iMCU_row;
+ else
+ lines_to_skip = (lines_after_iMCU_row / lines_per_iMCU_row) *
+ lines_per_iMCU_row;
+ /* Calculate the number of lines that remain to be skipped after skipping all
+ * of the full iMCU rows that we can. We will not read these lines unless we
+ * have to.
+ */
+ lines_to_read = lines_after_iMCU_row - lines_to_skip;
+
+ /* For images requiring multiple scans (progressive, non-interleaved, etc.),
+ * all of the entropy decoding occurs in jpeg_start_decompress(), assuming
+ * that the input data source is non-suspending. This makes skipping easy.
+ */
+ if (cinfo->inputctl->has_multiple_scans) {
+ if (cinfo->upsample->need_context_rows) {
+ cinfo->output_scanline += lines_to_skip;
+ cinfo->output_iMCU_row += lines_to_skip / lines_per_iMCU_row;
+ increment_iMCU_ctr(cinfo, lines_after_iMCU_row / lines_per_iMCU_row);
+ /* It is complex to properly move to the middle of a context block, so
+ * read the remaining lines instead of skipping them.
+ */
+ dummy_buffer_setup(cinfo);
+ for (i = 0; i < lines_to_read; i++)
+ jpeg_read_scanlines(cinfo, &(cinfo->master->dummy_row_buffer), 1);
+ } else {
+ cinfo->output_scanline += lines_to_skip;
+ cinfo->output_iMCU_row += lines_to_skip / lines_per_iMCU_row;
+ increment_simple_rowgroup_ctr(cinfo, lines_to_read);
+ }
+ upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;
+ return num_lines;
+ }
+
+ /* Skip the iMCU rows that we can safely skip. */
+ for (i = 0; i < lines_to_skip; i += lines_per_iMCU_row) {
+ for (y = 0; y < coef->MCU_rows_per_iMCU_row; y++) {
+ for (x = 0; x < cinfo->MCUs_per_row; x++) {
+ /* Calling decode_mcu() with a NULL pointer causes it to discard the
+ * decoded coefficients. This is ~5% faster for large subsets, but
+ * it's tough to tell a difference for smaller images. Another
+ * advantage of discarding coefficients is that it allows us to avoid
+ * accessing the private field cinfo->coef->MCU_buffer (which would
+ * normally be a parameter to decode_mcu().)
+ */
+ (*cinfo->entropy->decode_mcu) (cinfo, NULL);
+ }
+ }
+ cinfo->input_iMCU_row++;
+ cinfo->output_iMCU_row++;
+ if (cinfo->input_iMCU_row < cinfo->total_iMCU_rows)
+ start_iMCU_row(cinfo);
+ else
+ (*cinfo->inputctl->finish_input_pass) (cinfo);
+ }
+ cinfo->output_scanline += lines_to_skip;
+
+ if (cinfo->upsample->need_context_rows) {
+ /* Context-based upsampling keeps track of iMCU rows. */
+ increment_iMCU_ctr(cinfo, lines_to_skip / lines_per_iMCU_row);
+
+ /* It is complex to properly move to the middle of a context block, so
+ * read the remaining lines instead of skipping them.
+ */
+ dummy_buffer_setup(cinfo);
+ for (i = 0; i < lines_to_read; i++)
+ jpeg_read_scanlines(cinfo, &(cinfo->master->dummy_row_buffer), 1);
+ } else {
+ increment_simple_rowgroup_ctr(cinfo, lines_to_read);
+ }
+
+ /* Since skipping lines involves skipping the upsampling step, the value of
+ * "rows_to_go" will become invalid unless we set it here. NOTE: This is a
+ * bit odd, since "rows_to_go" seems to be redundantly keeping track of
+ * output_scanline.
+ */
+ upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;
+
+ /* Always skip the requested number of lines. */
+ return num_lines;
+}
+
/*
* Alternate entry point to read raw data.
* Processes exactly one iMCU row per call, unless suspended.
@@ -271,6 +507,13 @@
if ((*cinfo->inputctl->consume_input) (cinfo) == JPEG_SUSPENDED)
return FALSE; /* Suspend, come back later */
}
+ /* Clean up row buffer */
+ if (cinfo->master->dummy_row_buffer) {
+ int nc = (cinfo->out_color_space == JCS_RGB565) ?
+ 2 : cinfo->out_color_components;
+ jpeg_free_small((j_common_ptr) cinfo, cinfo->master->dummy_row_buffer,
+ cinfo->output_width * nc * sizeof(JSAMPLE));
+ }
cinfo->global_state = DSTATE_BUFIMAGE;
return TRUE;
}