Add jpeg_skip_scanlines() API to libjpeg-turbo

jpeg_skip_scanlines() API, a subset decoding optimization
aimed at Android, was submitted upstream r1582. Pull that
change, and sundry fixes, into the Chromium repo.

This new API is targetted at Android devices, not Chrome,
and should have no affect on JPEG decode behavior or perf
of Chrome.

Chrome uses suspending data source JPEG decoding, and the
new API does not support such sources. Adding support for
suspending data sources is a future TODO, should the need
arise (refer to skbug.com/4036).

BUG=515694
BUG=468914

patch from issue 256280043 at patchset 50001 (http://crrev.com/256280043#ps50001)

R=agable@chromium.org

Review URL: https://codereview.chromium.org/1271803002 .

1 files changed, 10 insertions, 7 deletions

diff --git a/jdhuff.c b/jdhuff.c
index 5d023ae..7229f02 100644
--- a/jdhuff.c
+++ b/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.
@@ -561,7 +561,7 @@ 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;
@@ -581,11 +581,13 @@ 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) */
-      (*block)[0] = (JCOEF) s;
+      if (block) {
+        /* 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 */
@@ -674,10 +676,11 @@ 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, slow_decode_mcu);