typefind: strengthen check for valid H.263 picture layer

Avoids some false positives leading to miss identification:

* Prevent picture start code emulation for the first 2 bytes read
* Add check for valid "picture coding type" and "PB-frames mode" combination

Additionally, change name on confusingly named TR var to what
it is, the layer's PTYPE.

https://bugzilla.gnome.org/show_bug.cgi?id=693263
parent 5d78aab8
......@@ -2587,12 +2587,13 @@ static void
h263_video_type_find (GstTypeFind * tf, gpointer unused)
{
DataScanCtx c = { 0, NULL, 0 };
guint64 data = 0;
guint64 data = 0xffff; /* prevents false positive for first 2 bytes */
guint64 psc = 0;
guint8 tr = 0;
guint8 ptype = 0;
guint format;
guint good = 0;
guint bad = 0;
guint pc_type, pb_mode;
while (c.offset < H263_MAX_PROBE_LENGTH) {
if (G_UNLIKELY (!data_scan_ctx_ensure_data (tf, &c, 4)))
......@@ -2603,16 +2604,21 @@ h263_video_type_find (GstTypeFind * tf, gpointer unused)
psc = data & G_GUINT64_CONSTANT (0xfffffc0000);
if (psc == 0x800000) {
/* Found PSC */
/* TR */
tr = (data & 0x3fc) >> 2;
/* PTYPE */
ptype = (data & 0x3fc) >> 2;
/* Source Format */
format = tr & 0x07;
format = ptype & 0x07;
/* Now that we have a Valid PSC, check if we also have a valid PTYPE and
the Source Format, which should range between 1 and 5 */
if (((tr >> 6) == 0x2) && (format > 0 && format < 6))
good++;
if (((ptype >> 6) == 0x2) && (format > 0 && format < 6)) {
pc_type = data & 0x02;
pb_mode = c.data[1] & 0x20 >> 4;
if (!pc_type && pb_mode)
bad++;
else
good++;
} else
bad++;
/* FIXME: maybe bail out early if we get mostly bad syncs ? */
......
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