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

ChangeLog.txt

@@ -8,6 +8,9 @@ to the compiler vs. x86, it was still possible to speed up compression by about
 3-4x and decompression by about 2-2.5x (relative to libjpeg v6b) through the
 use of AltiVec instructions.
 
+[2] Added a new libjpeg API function (jpeg_skip_scanlines()) that can be used
+to partially decode a JPEG image.  See libjpeg.txt for more details.
+
 
 1.4.1
 =====

djpeg.c

@@ -4,7 +4,8 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2010-2011, 2013-2014, D. R. Commander.
+ * Copyright (C) 2010-2011, 2013-2015, D. R. Commander.
+ * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains a command-line user interface for the JPEG decompressor.
@@ -88,6 +89,8 @@ static IMAGE_FORMATS requested_fmt;
 static const char * progname;   /* program name for error messages */
 static char * outfilename;      /* for -outfile switch */
 boolean memsrc;  /* for -memsrc switch */
+boolean stripe;
+JDIMENSION startY, endY;
 #define INPUT_BUF_SIZE  4096
 
 
@@ -164,6 +167,7 @@ usage (void)
   fprintf(stderr, "  -memsrc        Load input file into memory before decompressing\n");
 #endif
 
+  fprintf(stderr, "  -stripe Y0,Y1  Decode a horizontal stripe of the image [Y0, Y1)\n");
   fprintf(stderr, "  -verbose  or  -debug   Emit debug output\n");
   fprintf(stderr, "  -version       Print version information and exit\n");
   exit(EXIT_FAILURE);
@@ -189,6 +193,7 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
   requested_fmt = DEFAULT_FMT;  /* set default output file format */
   outfilename = NULL;
   memsrc = FALSE;
+  stripe = FALSE;
   cinfo->err->trace_level = 0;
 
   /* Scan command line options, adjust parameters */
@@ -361,7 +366,7 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
       /* RLE output format. */
       requested_fmt = FMT_RLE;
 
-    } else if (keymatch(arg, "scale", 1)) {
+    } else if (keymatch(arg, "scale", 2)) {
       /* Scale the output image by a fraction M/N. */
       if (++argn >= argc)       /* advance to next argument */
         usage();
@@ -369,6 +374,13 @@ parse_switches (j_decompress_ptr cinfo, int argc, char **argv,
                  &cinfo->scale_num, &cinfo->scale_denom) != 2)
         usage();
 
+    } else if (keymatch(arg, "stripe", 2)) {
+      if (++argn >= argc)
+        usage();
+      if (sscanf(argv[argn], "%d,%d", &startY, &endY) != 2 || startY > endY)
+        usage();
+      stripe = TRUE;
+
     } else if (keymatch(arg, "targa", 1)) {
       /* Targa output format. */
       requested_fmt = FMT_TARGA;
@@ -634,14 +646,48 @@ main (int argc, char **argv)
   /* Start decompressor */
   (void) jpeg_start_decompress(&cinfo);
 
-  /* Write output file header */
+  /* Stripe decode */
-  (*dest_mgr->start_output) (&cinfo, dest_mgr);
+  if (stripe) {
+    JDIMENSION tmp;
 
-  /* Process data */
+    /* Check for valid endY.  We cannot check this value until after
-  while (cinfo.output_scanline < cinfo.output_height) {
+     * jpeg_start_decompress() is called.  Note that we have already verified
-    num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+     * that startY <= endY.
-                                        dest_mgr->buffer_height);
+     */
-    (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+    if (endY > cinfo.output_height) {
+      fprintf(stderr, "%s: stripe %d-%d exceeds image height %d\n", progname,
+              startY, endY, cinfo.output_height);
+      exit(EXIT_FAILURE);
+    }
+
+    /* Write output file header.  This is a hack to ensure that the destination
+     * manager creates an image of the proper size for the partial decode.
+     */
+    tmp = cinfo.output_height;
+    cinfo.output_height = endY - startY;
+    (*dest_mgr->start_output) (&cinfo, dest_mgr);
+    cinfo.output_height = tmp;
+
+    /* Process data */
+    (void) jpeg_skip_scanlines(&cinfo, startY);
+    while (cinfo.output_scanline < endY) {
+      num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                          dest_mgr->buffer_height);
+      (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+    }
+    (void) jpeg_skip_scanlines(&cinfo, cinfo.output_height - endY);
+
+  /* Normal full image decode */
+  } else {
+    /* Write output file header */
+    (*dest_mgr->start_output) (&cinfo, dest_mgr);
+
+    /* Process data */
+    while (cinfo.output_scanline < cinfo.output_height) {
+      num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
+                                          dest_mgr->buffer_height);
+      (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
+    }
   }
 
 #ifdef PROGRESS_REPORT

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 "jdmainct.h"
-#include "jinclude.h"
+#include "jdcoefct.h"
-#include "jpeglib.h"
+#include "jdsample.h"
-#include "jpegcomp.h"
+#include "jmemsys.h"
-
 
 /* Forward declarations */
 LOCAL(boolean) output_pass_setup (j_decompress_ptr cinfo);
@@ -179,6 +179,242 @@ jpeg_read_scanlines (j_decompress_ptr cinfo, JSAMPARRAY scanlines,
 }
 
 
+/* 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 @@ jpeg_finish_output (j_decompress_ptr cinfo)
     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;
 }

jdarith.c

@@ -3,8 +3,8 @@
  *
  * This file was part of the Independent JPEG Group's software:
  * Developed 1997-2009 by Guido Vollbeding.
- * It was modified by The libjpeg-turbo Project to include only code relevant
+ * libjpeg-turbo Modifications:
- * to libjpeg-turbo.
+ * Copyright (C) 2015, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains portable arithmetic entropy decoding routines for JPEG
@@ -516,7 +516,7 @@ decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
   /* Outer loop handles each block in the MCU */
 
   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
-    block = MCU_data[blkn];
+    block = MCU_data ? MCU_data[blkn] : NULL;
     ci = cinfo->MCU_membership[blkn];
     compptr = cinfo->cur_comp_info[ci];
 
@@ -563,7 +563,8 @@ decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
       entropy->last_dc_val[ci] += v;
     }
 
-    (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
+    if (block)
+      (*block)[0] = (JCOEF) entropy->last_dc_val[ci];
 
     /* Sections F.2.4.2 & F.1.4.4.2: Decoding of AC coefficients */
 
@@ -607,7 +608,8 @@ decode_mcu (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
       while (m >>= 1)
         if (arith_decode(cinfo, st)) v |= m;
       v += 1; if (sign) v = -v;
-      (*block)[jpeg_natural_order[k]] = (JCOEF) v;
+      if (block)
+        (*block)[jpeg_natural_order[k]] = (JCOEF) v;
     }
   }
 

jdcoefct.c

@@ -4,6 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
+ * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
  * Copyright (C) 2010, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
@@ -16,53 +17,9 @@
  * Also, the input side (only) is used when reading a file for transcoding.
  */
 
-#define JPEG_INTERNALS
+#include "jdcoefct.h"
-#include "jinclude.h"
-#include "jpeglib.h"
 #include "jpegcomp.h"
 
-/* Block smoothing is only applicable for progressive JPEG, so: */
-#ifndef D_PROGRESSIVE_SUPPORTED
-#undef BLOCK_SMOOTHING_SUPPORTED
-#endif
-
-/* Private buffer controller object */
-
-typedef struct {
-  struct jpeg_d_coef_controller pub; /* public fields */
-
-  /* These variables keep track of the current location of the input side. */
-  /* cinfo->input_iMCU_row is also used for this. */
-  JDIMENSION MCU_ctr;           /* counts MCUs processed in current row */
-  int MCU_vert_offset;          /* counts MCU rows within iMCU row */
-  int MCU_rows_per_iMCU_row;    /* number of such rows needed */
-
-  /* The output side's location is represented by cinfo->output_iMCU_row. */
-
-  /* In single-pass modes, it's sufficient to buffer just one MCU.
-   * We allocate a workspace of D_MAX_BLOCKS_IN_MCU coefficient blocks,
-   * and let the entropy decoder write into that workspace each time.
-   * In multi-pass modes, this array points to the current MCU's blocks
-   * within the virtual arrays; it is used only by the input side.
-   */
-  JBLOCKROW MCU_buffer[D_MAX_BLOCKS_IN_MCU];
-
-  /* Temporary workspace for one MCU */
-  JCOEF * workspace;
-
-#ifdef D_MULTISCAN_FILES_SUPPORTED
-  /* In multi-pass modes, we need a virtual block array for each component. */
-  jvirt_barray_ptr whole_image[MAX_COMPONENTS];
-#endif
-
-#ifdef BLOCK_SMOOTHING_SUPPORTED
-  /* When doing block smoothing, we latch coefficient Al values here */
-  int * coef_bits_latch;
-#define SAVED_COEFS  6          /* we save coef_bits[0..5] */
-#endif
-} my_coef_controller;
-
-typedef my_coef_controller * my_coef_ptr;
 
 /* Forward declarations */
 METHODDEF(int) decompress_onepass
@@ -78,30 +35,6 @@ METHODDEF(int) decompress_smooth_data
 #endif
 
 
-LOCAL(void)
-start_iMCU_row (j_decompress_ptr cinfo)
-/* Reset within-iMCU-row counters for a new row (input side) */
-{
-  my_coef_ptr coef = (my_coef_ptr) cinfo->coef;
-
-  /* In an interleaved scan, an MCU row is the same as an iMCU row.
-   * In a noninterleaved scan, an iMCU row has v_samp_factor MCU rows.
-   * But at the bottom of the image, process only what's left.
-   */
-  if (cinfo->comps_in_scan > 1) {
-    coef->MCU_rows_per_iMCU_row = 1;
-  } else {
-    if (cinfo->input_iMCU_row < (cinfo->total_iMCU_rows-1))
-      coef->MCU_rows_per_iMCU_row = cinfo->cur_comp_info[0]->v_samp_factor;
-    else
-      coef->MCU_rows_per_iMCU_row = cinfo->cur_comp_info[0]->last_row_height;
-  }
-
-  coef->MCU_ctr = 0;
-  coef->MCU_vert_offset = 0;
-}
-
-
 /*
  * Initialize for an input processing pass.
  */

jdcoefct.h

@@ -0,0 +1,82 @@
+/*
+ * jdcoefct.h
+ *
+ * This file was part of the Independent JPEG Group's software:
+ * Copyright (C) 1994-1997, Thomas G. Lane.
+ * libjpeg-turbo Modifications:
+ * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
+ * For conditions of distribution and use, see the accompanying README file.
+ */
+
+#define JPEG_INTERNALS
+#include "jinclude.h"
+#include "jpeglib.h"
+
+
+/* Block smoothing is only applicable for progressive JPEG, so: */
+#ifndef D_PROGRESSIVE_SUPPORTED
+#undef BLOCK_SMOOTHING_SUPPORTED
+#endif
+
+
+/* Private buffer controller object */
+
+typedef struct {
+  struct jpeg_d_coef_controller pub; /* public fields */
+
+  /* These variables keep track of the current location of the input side. */
+  /* cinfo->input_iMCU_row is also used for this. */
+  JDIMENSION MCU_ctr;           /* counts MCUs processed in current row */
+  int MCU_vert_offset;          /* counts MCU rows within iMCU row */
+  int MCU_rows_per_iMCU_row;    /* number of such rows needed */
+
+  /* The output side's location is represented by cinfo->output_iMCU_row. */
+
+  /* In single-pass modes, it's sufficient to buffer just one MCU.
+   * We allocate a workspace of D_MAX_BLOCKS_IN_MCU coefficient blocks,
+   * and let the entropy decoder write into that workspace each time.
+   * In multi-pass modes, this array points to the current MCU's blocks
+   * within the virtual arrays; it is used only by the input side.
+   */
+  JBLOCKROW MCU_buffer[D_MAX_BLOCKS_IN_MCU];
+
+  /* Temporary workspace for one MCU */
+  JCOEF * workspace;
+
+#ifdef D_MULTISCAN_FILES_SUPPORTED
+  /* In multi-pass modes, we need a virtual block array for each component. */
+  jvirt_barray_ptr whole_image[MAX_COMPONENTS];
+#endif
+
+#ifdef BLOCK_SMOOTHING_SUPPORTED
+  /* When doing block smoothing, we latch coefficient Al values here */
+  int * coef_bits_latch;
+#define SAVED_COEFS  6          /* we save coef_bits[0..5] */
+#endif
+} my_coef_controller;
+
+typedef my_coef_controller * my_coef_ptr;
+
+
+LOCAL(void)
+start_iMCU_row (j_decompress_ptr cinfo)
+/* Reset within-iMCU-row counters for a new row (input side) */
+{
+  my_coef_ptr coef = (my_coef_ptr) cinfo->coef;
+
+  /* In an interleaved scan, an MCU row is the same as an iMCU row.
+   * In a noninterleaved scan, an iMCU row has v_samp_factor MCU rows.
+   * But at the bottom of the image, process only what's left.
+   */
+  if (cinfo->comps_in_scan > 1) {
+    coef->MCU_rows_per_iMCU_row = 1;
+  } else {
+    if (cinfo->input_iMCU_row < (cinfo->total_iMCU_rows-1))
+      coef->MCU_rows_per_iMCU_row = cinfo->cur_comp_info[0]->v_samp_factor;
+    else
+      coef->MCU_rows_per_iMCU_row = cinfo->cur_comp_info[0]->last_row_height;
+  }
+
+  coef->MCU_ctr = 0;
+  coef->MCU_vert_offset = 0;
+}

jdhuff.c

@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2009-2011, D. R. Commander.
+ * Copyright (C) 2009-2011, 2015, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains Huffman entropy decoding routines.
@@ -562,7 +562,7 @@ decode_mcu_slow (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
   ASSIGN_STATE(state, entropy->saved);
 
   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
-    JBLOCKROW block = MCU_data[blkn];
+    JBLOCKROW block = MCU_data ? MCU_data[blkn] : NULL;
     d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
     d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
     register int s, k, r;
@@ -582,11 +582,13 @@ decode_mcu_slow (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
       int ci = cinfo->MCU_membership[blkn];
       s += state.last_dc_val[ci];
       state.last_dc_val[ci] = s;
-      /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
+      if (block) {
-      (*block)[0] = (JCOEF) s;
+        /* Output the DC coefficient (assumes jpeg_natural_order[0] = 0) */
+        (*block)[0] = (JCOEF) s;
+      }
     }
 
-    if (entropy->ac_needed[blkn]) {
+    if (entropy->ac_needed[blkn] && block) {
 
       /* Section F.2.2.2: decode the AC coefficients */
       /* Since zeroes are skipped, output area must be cleared beforehand */
@@ -659,7 +661,7 @@ decode_mcu_fast (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
   ASSIGN_STATE(state, entropy->saved);
 
   for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
-    JBLOCKROW block = MCU_data[blkn];
+    JBLOCKROW block = MCU_data ? MCU_data[blkn] : NULL;
     d_derived_tbl * dctbl = entropy->dc_cur_tbls[blkn];
     d_derived_tbl * actbl = entropy->ac_cur_tbls[blkn];
     register int s, k, r, l;
@@ -675,10 +677,11 @@ decode_mcu_fast (j_decompress_ptr cinfo, JBLOCKROW *MCU_data)
       int ci = cinfo->MCU_membership[blkn];
       s += state.last_dc_val[ci];
       state.last_dc_val[ci] = s;
-      (*block)[0] = (JCOEF) s;
+      if (block)
+        (*block)[0] = (JCOEF) s;
     }
 
-    if (entropy->ac_needed[blkn]) {
+    if (entropy->ac_needed[blkn] && block) {
 
       for (k = 1; k < DCTSIZE2; k++) {
         HUFF_DECODE_FAST(s, l, actbl);

jdmainct.c

@@ -15,10 +15,7 @@
  * supplies the equivalent of the main buffer in that case.
  */
 
-#define JPEG_INTERNALS
+#include "jdmainct.h"
-#include "jinclude.h"
-#include "jpeglib.h"
-#include "jpegcomp.h"
 
 
 /*
@@ -112,36 +109,6 @@
  */
 
 
-/* Private buffer controller object */
-
-typedef struct {
-  struct jpeg_d_main_controller pub; /* public fields */
-
-  /* Pointer to allocated workspace (M or M+2 row groups). */
-  JSAMPARRAY buffer[MAX_COMPONENTS];
-
-  boolean buffer_full;          /* Have we gotten an iMCU row from decoder? */
-  JDIMENSION rowgroup_ctr;      /* counts row groups output to postprocessor */
-
-  /* Remaining fields are only used in the context case. */
-
-  /* These are the master pointers to the funny-order pointer lists. */
-  JSAMPIMAGE xbuffer[2];        /* pointers to weird pointer lists */
-
-  int whichptr;                 /* indicates which pointer set is now in use */
-  int context_state;            /* process_data state machine status */
-  JDIMENSION rowgroups_avail;   /* row groups available to postprocessor */
-  JDIMENSION iMCU_row_ctr;      /* counts iMCU rows to detect image top/bot */
-} my_main_controller;
-
-typedef my_main_controller * my_main_ptr;
-
-/* context_state values: */
-#define CTX_PREPARE_FOR_IMCU    0       /* need to prepare for MCU row */
-#define CTX_PROCESS_IMCU        1       /* feeding iMCU to postprocessor */
-#define CTX_POSTPONED_ROW       2       /* feeding postponed row group */
-
-
 /* Forward declarations */
 METHODDEF(void) process_data_simple_main
         (j_decompress_ptr cinfo, JSAMPARRAY output_buf,
@@ -237,34 +204,6 @@ make_funny_pointers (j_decompress_ptr cinfo)
 }
 
 
-LOCAL(void)
-set_wraparound_pointers (j_decompress_ptr cinfo)
-/* Set up the "wraparound" pointers at top and bottom of the pointer lists.
- * This changes the pointer list state from top-of-image to the normal state.
- */
-{
-  my_main_ptr main_ptr = (my_main_ptr) cinfo->main;
-  int ci, i, rgroup;
-  int M = cinfo->_min_DCT_scaled_size;
-  jpeg_component_info *compptr;
-  JSAMPARRAY xbuf0, xbuf1;
-
-  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
-       ci++, compptr++) {
-    rgroup = (compptr->v_samp_factor * compptr->_DCT_scaled_size) /
-      cinfo->_min_DCT_scaled_size; /* height of a row group of component */
-    xbuf0 = main_ptr->xbuffer[0][ci];
-    xbuf1 = main_ptr->xbuffer[1][ci];
-    for (i = 0; i < rgroup; i++) {
-      xbuf0[i - rgroup] = xbuf0[rgroup*(M+1) + i];
-      xbuf1[i - rgroup] = xbuf1[rgroup*(M+1) + i];
-      xbuf0[rgroup*(M+2) + i] = xbuf0[i];
-      xbuf1[rgroup*(M+2) + i] = xbuf1[i];
-    }
-  }
-}
-
-
 LOCAL(void)
 set_bottom_pointers (j_decompress_ptr cinfo)
 /* Change the pointer lists to duplicate the last sample row at the bottom

jdmainct.h

@@ -0,0 +1,71 @@
+/*
+ * jdmainct.h
+ *
+ * This file was part of the Independent JPEG Group's software:
+ * Copyright (C) 1994-1996, Thomas G. Lane.
+ * For conditions of distribution and use, see the accompanying README file.
+ */
+
+#define JPEG_INTERNALS
+#include "jinclude.h"
+#include "jpeglib.h"
+#include "jpegcomp.h"
+
+
+/* Private buffer controller object */
+
+typedef struct {
+  struct jpeg_d_main_controller pub; /* public fields */
+
+  /* Pointer to allocated workspace (M or M+2 row groups). */
+  JSAMPARRAY buffer[MAX_COMPONENTS];
+
+  boolean buffer_full;          /* Have we gotten an iMCU row from decoder? */
+  JDIMENSION rowgroup_ctr;      /* counts row groups output to postprocessor */
+
+  /* Remaining fields are only used in the context case. */
+
+  /* These are the master pointers to the funny-order pointer lists. */
+  JSAMPIMAGE xbuffer[2];        /* pointers to weird pointer lists */
+
+  int whichptr;                 /* indicates which pointer set is now in use */
+  int context_state;            /* process_data state machine status */
+  JDIMENSION rowgroups_avail;   /* row groups available to postprocessor */
+  JDIMENSION iMCU_row_ctr;      /* counts iMCU rows to detect image top/bot */
+} my_main_controller;
+
+typedef my_main_controller * my_main_ptr;
+
+
+/* context_state values: */
+#define CTX_PREPARE_FOR_IMCU    0       /* need to prepare for MCU row */
+#define CTX_PROCESS_IMCU        1       /* feeding iMCU to postprocessor */
+#define CTX_POSTPONED_ROW       2       /* feeding postponed row group */
+
+
+LOCAL(void)
+set_wraparound_pointers (j_decompress_ptr cinfo)
+/* Set up the "wraparound" pointers at top and bottom of the pointer lists.
+ * This changes the pointer list state from top-of-image to the normal state.
+ */
+{
+  my_main_ptr main_ptr = (my_main_ptr) cinfo->main;
+  int ci, i, rgroup;
+  int M = cinfo->_min_DCT_scaled_size;
+  jpeg_component_info *compptr;
+  JSAMPARRAY xbuf0, xbuf1;
+
+  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
+       ci++, compptr++) {
+    rgroup = (compptr->v_samp_factor * compptr->_DCT_scaled_size) /
+      cinfo->_min_DCT_scaled_size; /* height of a row group of component */
+    xbuf0 = main_ptr->xbuffer[0][ci];
+    xbuf1 = main_ptr->xbuffer[1][ci];
+    for (i = 0; i < rgroup; i++) {
+      xbuf0[i - rgroup] = xbuf0[rgroup*(M+1) + i];
+      xbuf1[i - rgroup] = xbuf1[rgroup*(M+1) + i];
+      xbuf0[rgroup*(M+2) + i] = xbuf0[i];
+      xbuf1[rgroup*(M+2) + i] = xbuf1[i];
+    }
+  }
+}

jdsample.c

@@ -22,51 +22,11 @@
  *   Pub. by IEEE Computer Society Press, Los Alamitos, CA. ISBN 0-8186-8944-7.
  */
 
-#define JPEG_INTERNALS
+#include "jdsample.h"
-#include "jinclude.h"
-#include "jpeglib.h"
 #include "jsimd.h"
 #include "jpegcomp.h"
 
 
-/* Pointer to routine to upsample a single component */
-typedef void (*upsample1_ptr) (j_decompress_ptr cinfo,
-                               jpeg_component_info * compptr,
-                               JSAMPARRAY input_data,
-                               JSAMPARRAY * output_data_ptr);
-
-/* Private subobject */
-
-typedef struct {
-  struct jpeg_upsampler pub;    /* public fields */
-
-  /* Color conversion buffer.  When using separate upsampling and color
-   * conversion steps, this buffer holds one upsampled row group until it
-   * has been color converted and output.
-   * Note: we do not allocate any storage for component(s) which are full-size,
-   * ie do not need rescaling.  The corresponding entry of color_buf[] is
-   * simply set to point to the input data array, thereby avoiding copying.
-   */
-  JSAMPARRAY color_buf[MAX_COMPONENTS];
-
-  /* Per-component upsampling method pointers */
-  upsample1_ptr methods[MAX_COMPONENTS];
-
-  int next_row_out;             /* counts rows emitted from color_buf */
-  JDIMENSION rows_to_go;        /* counts rows remaining in image */
-
-  /* Height of an input row group for each component. */
-  int rowgroup_height[MAX_COMPONENTS];
-
-  /* These arrays save pixel expansion factors so that int_expand need not
-   * recompute them each time.  They are unused for other upsampling methods.
-   */
-  UINT8 h_expand[MAX_COMPONENTS];
-  UINT8 v_expand[MAX_COMPONENTS];
-} my_upsampler;
-
-typedef my_upsampler * my_upsample_ptr;
-
 
 /*
  * Initialize for an upsampling pass.

jdsample.h

@@ -0,0 +1,50 @@
+/*
+ * jdsample.h
+ *
+ * This file was part of the Independent JPEG Group's software:
+ * Copyright (C) 1991-1996, Thomas G. Lane.
+ * For conditions of distribution and use, see the accompanying README file.
+ */
+
+#define JPEG_INTERNALS
+#include "jinclude.h"
+#include "jpeglib.h"
+
+
+/* Pointer to routine to upsample a single component */
+typedef void (*upsample1_ptr) (j_decompress_ptr cinfo,
+                               jpeg_component_info * compptr,
+                               JSAMPARRAY input_data,
+                               JSAMPARRAY * output_data_ptr);
+
+/* Private subobject */
+
+typedef struct {
+  struct jpeg_upsampler pub;    /* public fields */
+
+  /* Color conversion buffer.  When using separate upsampling and color
+   * conversion steps, this buffer holds one upsampled row group until it
+   * has been color converted and output.
+   * Note: we do not allocate any storage for component(s) which are full-size,
+   * ie do not need rescaling.  The corresponding entry of color_buf[] is
+   * simply set to point to the input data array, thereby avoiding copying.
+   */
+  JSAMPARRAY color_buf[MAX_COMPONENTS];
+
+  /* Per-component upsampling method pointers */
+  upsample1_ptr methods[MAX_COMPONENTS];
+
+  int next_row_out;             /* counts rows emitted from color_buf */
+  JDIMENSION rows_to_go;        /* counts rows remaining in image */
+
+  /* Height of an input row group for each component. */
+  int rowgroup_height[MAX_COMPONENTS];
+
+  /* These arrays save pixel expansion factors so that int_expand need not
+   * recompute them each time.  They are unused for other upsampling methods.
+   */
+  UINT8 h_expand[MAX_COMPONENTS];
+  UINT8 v_expand[MAX_COMPONENTS];
+} my_upsampler;
+
+typedef my_upsampler * my_upsample_ptr;

jpegint.h

@@ -4,8 +4,8 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
  * Modified 1997-2009 by Guido Vollbeding.
- * It was modified by The libjpeg-turbo Project to include only code relevant
+ * libjpeg-turbo Modifications:
- * to libjpeg-turbo.
+ * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file provides common declarations for the various JPEG modules.
@@ -137,6 +137,12 @@ struct jpeg_decomp_master {
 
   /* State variables made visible to other modules */
   boolean is_dummy_pass;        /* True during 1st pass for 2-pass quant */
+
+  /* Buffer large enough to store an output row.  This is used when
+   * jpeg_skip_scanlines() chooses to "skip" a row by reading it into this
+   * dummy buffer.
+   */
+  JSAMPROW dummy_row_buffer;
 };
 
 /* Input control module */

jpeglib.h

@@ -6,6 +6,7 @@
  * Modified 2002-2009 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
  * Copyright (C) 2009-2011, 2013-2014, D. R. Commander.
+ * Copyright (C) 2015, Google, Inc.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file defines the application interface for the JPEG library.
@@ -990,6 +991,8 @@ EXTERN(boolean) jpeg_start_decompress (j_decompress_ptr cinfo);
 EXTERN(JDIMENSION) jpeg_read_scanlines (j_decompress_ptr cinfo,
                                         JSAMPARRAY scanlines,
                                         JDIMENSION max_lines);
+EXTERN(JDIMENSION) jpeg_skip_scanlines (j_decompress_ptr cinfo,
+                                        JDIMENSION num_lines);
 EXTERN(boolean) jpeg_finish_decompress (j_decompress_ptr cinfo);
 
 /* Replaces jpeg_read_scanlines when reading raw downsampled data. */

libjpeg.txt

@@ -3,7 +3,8 @@ USING THE IJG JPEG LIBRARY
 This file was part of the Independent JPEG Group's software:
 Copyright (C) 1994-2011, Thomas G. Lane, Guido Vollbeding.
 libjpeg-turbo Modifications:
-Copyright (C) 2010, 2014, D. R. Commander.
+Copyright (C) 2010, 2014, 2015, D. R. Commander.
+Copyright (C) 2015, Google, Inc.
 For conditions of distribution and use, see the accompanying README file.
 
 
@@ -729,6 +730,49 @@ jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
 The previous discussion of aborting compression cycles applies here too.
 
 
+Skipping rows when decompressing
+--------------------------------
+
+jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines);
+
+This function provides application programmers with the ability to skip over
+multiple rows in the JPEG image, thus decoding only a subset of the image data.
+This is convenient for performance-critical applications that wish to view only
+a portion of a large JPEG image without decompressing the whole thing.  It it
+also useful in memory-constrained environments (such as on mobile devices.)
+
+Suspending data sources are not supported by this function.  Calling
+jpeg_skip_scanlines() with a suspending data source will result in undefined
+behavior.
+
+jpeg_skip_scanlines() will not allow skipping past the bottom of the image.  If
+the value of num_lines is large enough to skip past the bottom of the image,
+then the function will skip to the end of the image instead.
+
+If the value of num_lines is valid, then jpeg_skip_scanlines() will always
+skip all of the input rows requested.  There is no need to inspect the return
+value of the function in that case.
+
+Best results will be achieved by calling jpeg_skip_scanlines() for large chunks
+of rows.  The function should be viewed as a way to quickly jump to a
+particular vertical offset in the JPEG image in order to decode a subset of the
+image.  Used in this manner, it will provide significant performance
+improvements.
+
+Calling jpeg_skip_scanlines() for small values of num_lines has several
+potential drawbacks:
+    1) JPEG decompression occurs in blocks, so if jpeg_skip_scanlines() is
+       called from the middle of a decompression block, then it is likely that
+       much of the decompression work has already been done for the first
+       couple of rows that need to be skipped.
+    2) When this function returns, it must leave the decompressor in a state
+       such that it is ready to read the next line.  This may involve
+       decompressing a block that must be partially skipped.
+These issues are especially tricky for cases in which upsampling requires
+context rows.  In the worst case, jpeg_skip_scanlines() will perform similarly
+to jpeg_read_scanlines() (since it will actually call jpeg_read_scanlines().)
+
+
 Mechanics of usage: include files, linking, etc
 -----------------------------------------------