mp4: ctts v0 sample_offset seems to be signed in practice

sample file from the wild has v0 as negatice offsets ffmpeg mov.c ignored versions and always read as signed
wader · Dec 8, 2023 · aadf26f · aadf26f
1 parent 11fc0f2
commit aadf26f
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Showing 3 changed files with 25 additions and 3 deletions.
diff --git a/format/mp4/boxes.go b/format/mp4/boxes.go
@@ -878,10 +878,10 @@ func decodeBox(ctx *decodeContext, d *decode.D, typ string) {
 		var i uint64
 		d.FieldStructArrayLoop("entries", "entry", func() bool { return i < entryCount }, func(d *decode.D) {
 			d.FieldS32("sample_count")
+			// ISO/IEC14496-12 says version 0 is unsigned and version 1 is signed
+			// in preactice it seems muxers write it as signed for both version
 			switch version {
-			case 0:
-				d.FieldU32("sample_offset")
-			case 1:
+			case 0, 1:
 				d.FieldS32("sample_offset")
 			}
 			i++

diff --git a/format/mp4/testdata/ctts_v0_signed b/format/mp4/testdata/ctts_v0_signed
diff --git a/format/mp4/testdata/ctts_v0_signed.fqtest b/format/mp4/testdata/ctts_v0_signed.fqtest
@@ -0,0 +1,22 @@
+$ fq -d mp4 -o force=true dv ctts_v0_signed
+    |00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f|0123456789abcdef|.{}: ctts_v0_signed (mp4) 0x0-0x30 (48)
+    |                                               |                |  boxes[0:1]: 0x0-0x30 (48)
+    |                                               |                |    [0]{}: box 0x0-0x30 (48)
+0x00|00 00 00 30                                    |...0            |      size: 48 0x0-0x4 (4)
+0x00|            63 74 74 73                        |    ctts        |      type: "ctts" (Composition time to sample) 0x4-0x8 (4)
+0x00|                        00                     |        .       |      version: 0 0x8-0x9 (1)
+0x00|                           00 00 00            |         ...    |      flags: 0 0x9-0xc (3)
+0x00|                                    00 00 00 04|            ....|      entry_count: 4 0xc-0x10 (4)
+    |                                               |                |      entries[0:4]: 0x10-0x30 (32)
+    |                                               |                |        [0]{}: entry 0x10-0x18 (8)
+0x10|00 00 00 01                                    |....            |          sample_count: 1 0x10-0x14 (4)
+0x10|            00 00 00 0a                        |    ....        |          sample_offset: 10 0x14-0x18 (4)
+    |                                               |                |        [1]{}: entry 0x18-0x20 (8)
+0x10|                        00 00 00 01            |        ....    |          sample_count: 1 0x18-0x1c (4)
+0x10|                                    ff ff ff f6|            ....|          sample_offset: -10 0x1c-0x20 (4)
+    |                                               |                |        [2]{}: entry 0x20-0x28 (8)
+0x20|00 00 00 01                                    |....            |          sample_count: 1 0x20-0x24 (4)
+0x20|            00 00 00 0a                        |    ....        |          sample_offset: 10 0x24-0x28 (4)
+    |                                               |                |        [3]{}: entry 0x28-0x30 (8)
+0x20|                        00 00 00 01            |        ....    |          sample_count: 1 0x28-0x2c (4)
+0x20|                                    ff ff ff f6|            ....|          sample_offset: -10 0x2c-0x30 (4)