avcodec/utvideodec/enc: Fix edge case of creating Huffman table

The Ut Video format stores Huffman tables in its bitstream by coding
the length of a given symbol; it does not code the actual code directly,
instead this is to be inferred by the rule that a symbol is to the left
of every shorter symbol in the Huffman tree and that for symbols of the
same length the symbol is descending from left to right. With one
exception, this is also what our de- and encoder did.

The exception only matters when there are codes of length 32, because
in this case the first symbol of this length did not get the code 0,
but 1; this is tantamount to pretending that there is a (nonexistent)
leaf of length 32. This is simply false. The reference software agrees
with this [1].

[1]: https://github.com/umezawatakeshi/utvideo/blob/2700a471a78402e5c340150b38e8a793ef3676f1/utv_core/HuffmanCode.cpp#L280

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

diff --git a/libavcodec/utvideodec.c b/libavcodec/utvideodec.c
index c07636d4351812a98dbda4fad6294d4da5b9c826..b3c4c3519bf69b0c664a485d734187c7ae8f8ba3 100644 (file)
--- a/libavcodec/utvideodec.c
+++ b/libavcodec/utvideodec.c
@@ -70,7 +70,7 @@ static int build_huff10(const uint8_t *src, VLC *vlc, int *fsym)
         return -1;
     }
 
-    code = 1;
+    code = 0;
     for (i = last; i >= 0; i--) {
         codes[i] = code >> (32 - he[i].len);
         bits[i]  = he[i].len;
@@ -113,7 +113,7 @@ static int build_huff(const uint8_t *src, VLC *vlc, int *fsym)
     if (he[last].len > 32)
         return -1;
 
-    code = 1;
+    code = 0;
     for (i = last; i >= 0; i--) {
         codes[i] = code >> (32 - he[i].len);
         bits[i]  = he[i].len;

diff --git a/libavcodec/utvideoenc.c b/libavcodec/utvideoenc.c
index f1b9d11c9605701260c04945635a48d882c4eb67..05a96140366ccb975c11ad5c568efee49843f566 100644 (file)
--- a/libavcodec/utvideoenc.c
+++ b/libavcodec/utvideoenc.c
@@ -346,7 +346,7 @@ static void calculate_codes(HuffEntry *he)
     while (he[last].len == 255 && last)
         last--;
 
-    code = 1;
+    code = 0;
     for (i = last; i >= 0; i--) {
         he[i].code  = code >> (32 - he[i].len);
         code       += 0x80000000u >> (he[i].len - 1);