[FFmpeg-devel] [PATCH v10 05/13] lavc/jpegtables: Handle multiple mappings to the same value

[velocityra at gmail.com]

From: Nick Renieris <velocityra at gmail.com>

Some JPEGs [1] have incorrect DHT entries that map 2 codes to
the same value.

The second (last) mapping does not ever actually appear in the
code stream, therefore ignoring any mappings after the first one
fixes this.

Without this, an "mjpeg_decode_dc: bad vlc: 0:0" error is thrown.

---

[1]: Embedded JPEGs in "X7 RAW" and "X7 CinemaDNG" samples here:
     https://www.dji.com/gr/zenmuse-x7/info#downloads

Signed-off-by: Nick Renieris <velocityra at gmail.com>
---
 libavcodec/jpegtables.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/libavcodec/jpegtables.c b/libavcodec/jpegtables.c
index cbe5523cb4..6f596cfc92 100644
--- a/libavcodec/jpegtables.c
+++ b/libavcodec/jpegtables.c
@@ -130,14 +130,27 @@ void ff_mjpeg_build_huffman_codes(uint8_t *huff_size, uint16_t *huff_code,
 {
     int i, j, k,nb, code, sym;
 
-    code = 0;
+    /* Zero-initialize huff_size (needed for multiple mappings check below) */
+    k = 0;
+    for(i=1;i<=16;i++) {
+        nb = bits_table[i];
+        for(j=0;j<nb;j++) {
+            sym = val_table[k++];
+            huff_size[sym] = 0;
+        }
+    }
+
     k = 0;
+    code = 0;
     for(i=1;i<=16;i++) {
         nb = bits_table[i];
         for(j=0;j<nb;j++) {
             sym = val_table[k++];
-            huff_size[sym] = i;
-            huff_code[sym] = code;
+            /* If there are multiple mappings to the same sym (bad files), keep the first code */
+            if (huff_size[sym] == 0) {
+                huff_size[sym] = i;
+                huff_code[sym] = code;
+            }
             code++;
         }
         code <<= 1;
-- 
2.21.0.windows.1