Re: Matroska file decoding - Question on seekhead

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Thanks Dave Rice for your kind response.

On Sun, Sep 3, 2017 at 9:25 PM, Dave Rice <[email protected]> wrote:

> Hi Austin,
>
> On Sep 3, 2017, at 9:08 AM, Austin Einter <[email protected]> wrote:
>
> First of all a BIG THANKS to Matroska, for providing a open source media
> file container.
>
> I have a requirement, where I need to parse a matroska file. Initial few
> bytes of file is given as below.
>
> 0x1a 0x45 0xdf 0xa3 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x23 0x42 0x86 0x81
> 0x01
> 0x42 0xf7 0x81 0x01 0x42 0xf2 0x81 0x04 0x42 0xf3 0x81 0x08 0x42 0x82 0x88
> 0x6d
> 0x61 0x74 0x72 0x6f 0x73 0x6b 0x61 0x42 0x87 0x81 0x04 0x42 0x85 0x81 0x02
> 0x18
> 0x53 0x80 0x67 0x01 0x00 0x00 0x00 0x00 0x33 0xdb 0x10 *0x11 0x4d 0x9b
> 0x74* 0x40
> 0x42 0xbf 0x84 0x11 0xac 0x83 0x8a 0x4d 0xbb 0x8b 0x53 0xab 0x84 0x15 0x49
> 0xa9
> 0x66 0x53 0xac 0x81 0xe5 0x4d 0xbb 0x8c 0x53 0xab 0x84 0x16 0x54 0xae 0x6b
> 0x53
> 0xac 0x82 0x01 0x56 0x4d 0xbb 0x8c 0x53 0xab 0x84 0x12 0x54 0xc3 0x67 0x53
> 0xac
> 0x82 0x11 0x5c 0x4d 0xbb 0x8d 0x53 0xab 0x84 0x1c 0x53 0xbb 0x6b 0x53 0xac
> 0x83
> 0x33 0xd9 0x1c 0xec 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x94 0x00 0x00 0x00
> 0x00
> 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> 0x00
> 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> 0x00
>
> I am trying to parse this file. I have parsed first 59 bytes successfully.
> Now I am at 60th byte. From 6th byte bytes are 0x11 0x4d 0x9b 0x74, so it
> means seekhead is starting.
>
> I used mkvinfo to view parsed data. As per mkvinfo as shown below,
> seekhead starts at 59 which is fine.
>
>  <image.png>
>
>
> Looks the first seek entry starts at 71. Now from 59th to 71st position
> what is there. This part I am not able to understand.
>
> Can somebody please help me to understand this part.
>
>
> As noted the SeekHead element starts at position 59. The first Child
> Element of SeekHead is then a CRC-32 element, which is:
>
> 0xbf 0x84 0x11 0xac 0x83 0x8a
>
>
> Element ID: 0xbf
> Element Data Size: 0x84
> Element Value: 0x11 0xac 0x83 0x8a
>
> After the CRC-32 Element is the first Seek Element.
>
> More info about the CRC-32 element is in the EBML specification, see
> https://tools.ietf.org/html/draft-ietf-cellar-ebml.
> Dave Rice
>
> _______________________________________________
> Matroska-users mailing list
> [email protected]
> https://lists.matroska.org/cgi-bin/mailman/listinfo/matroska-users
> Read Matroska-Users on GMane: http://dir.gmane.org/gmane.
> comp.multimedia.matroska.user
>
>

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<div dir=3D"ltr">Thanks Dave Rice for your kind response.</div><div class=
=3D"gmail_extra"><br><div class=3D"gmail_quote">On Sun, Sep 3, 2017 at 9:25=
 PM, Dave Rice <span dir=3D"ltr">&lt;<a href=3D"mailto:[email protected]" ta=
rget=3D"_blank">[email protected]</a>&gt;</span> wrote:<br><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex"><div style=3D"word-wrap:break-word">Hi Austin,<div><br><div><=
blockquote type=3D"cite"><span class=3D""><div>On Sep 3, 2017, at 9:08 AM, =
Austin Einter &lt;<a href=3D"mailto:[email protected]" target=3D"_bla=
nk">[email protected]</a>&gt; wrote:</div><br class=3D"m_728928339938=
5397415Apple-interchange-newline"></span><div><div dir=3D"ltr"><span class=
=3D"">First of all a BIG THANKS to Matroska, for providing a open source me=
dia file container.=C2=A0<div><br></div><div>I have a requirement, where I =
need to parse a matroska file. Initial few bytes of file is given as below.=
</div><div><br></div><div><div>0x1a 0x45 0xdf 0xa3 0x01 0x00 0x00 0x00 0x00=
 0x00 0x00 0x23 0x42 0x86 0x81 0x01</div><div>0x42 0xf7 0x81 0x01 0x42 0xf2=
 0x81 0x04 0x42 0xf3 0x81 0x08 0x42 0x82 0x88 0x6d</div><div>0x61 0x74 0x72=
 0x6f 0x73 0x6b 0x61 0x42 0x87 0x81 0x04 0x42 0x85 0x81 0x02 0x18</div><div=
>0x53 0x80 0x67 0x01 0x00 0x00 0x00 0x00 0x33 0xdb 0x10 <b>0x11 0x4d 0x9b 0=
x74</b> 0x40</div><div>0x42 0xbf 0x84 0x11 0xac 0x83 0x8a 0x4d 0xbb 0x8b 0x=
53 0xab 0x84 0x15 0x49 0xa9</div><div>0x66 0x53 0xac 0x81 0xe5 0x4d 0xbb 0x=
8c 0x53 0xab 0x84 0x16 0x54 0xae 0x6b 0x53</div><div>0xac 0x82 0x01 0x56 0x=
4d 0xbb 0x8c 0x53 0xab 0x84 0x12 0x54 0xc3 0x67 0x53 0xac</div><div>0x82 0x=
11 0x5c 0x4d 0xbb 0x8d 0x53 0xab 0x84 0x1c 0x53 0xbb 0x6b 0x53 0xac 0x83</d=
iv><div>0x33 0xd9 0x1c 0xec 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x94 0x00 0x=
00 0x00 0x00</div><div>0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x=
00 0x00 0x00 0x00 0x00 0x00</div><div>0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x=
00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00</div></div><div><br></div><div>I=
 am trying to parse this file. I have parsed first 59 bytes successfully. N=
ow I am at 60th byte. From 6th byte bytes are 0x11 0x4d 0x9b 0x74, so it me=
ans=C2=A0seekhead is starting.=C2=A0</div><div><br></div><div>I used mkvinf=
o to view parsed data. As per mkvinfo as shown below, seekhead starts at 59=
 which is fine. =C2=A0</div><div><br></div></span><div>=C2=A0<span id=3D"m_=
7289283399385397415cid:ii_15e47d9c56acaf21">&lt;image.png&gt;</span></div><=
span class=3D""><div><br></div><div><br></div><div>Looks the first seek ent=
ry starts at 71. Now from 59th to 71st position what is there. This part I =
am not able to understand.</div><div><br></div><div>Can somebody please hel=
p me to understand this part.</div></span></div></div></blockquote></div><d=
iv><br></div>As noted the SeekHead element starts at position 59. The first=
 Child Element of SeekHead is then a CRC-32 element, which is:<br></div><sp=
an class=3D""><div><blockquote type=3D"cite"><div dir=3D"ltr"><div><div>0xb=
f 0x84 0x11 0xac 0x83 0x8a</div></div></div></blockquote><br></div></span><=
div>Element ID: 0xbf</div><div>Element Data Size: 0x84</div><div>Element Va=
lue: 0x11 0xac 0x83 0x8a</div><div><br></div><div>After the CRC-32 Element =
is the first Seek Element.</div><div><br></div><div>More info about the CRC=
-32 element is in the EBML specification, see=C2=A0<a href=3D"https://tools=
.ietf.org/html/draft-ietf-cellar-ebml" target=3D"_blank">https://tools.ietf=
.org/<wbr>html/draft-ietf-cellar-ebml</a>.</div><span class=3D"HOEnZb"><fon=
t color=3D"#888888"><div>Dave Rice</div></font></span></div><br>___________=
___________________<wbr>_________________<br>
Matroska-users mailing list<br>
<a href=3D"mailto:[email protected]">[email protected]=
atroska.<wbr>org</a><br>
<a href=3D"https://lists.matroska.org/cgi-bin/mailman/listinfo/matroska-use=
rs" rel=3D"noreferrer" target=3D"_blank">https://lists.matroska.org/<wbr>cg=
i-bin/mailman/listinfo/<wbr>matroska-users</a><br>
Read Matroska-Users on GMane: <a href=3D"http://dir.gmane.org/gmane.comp.mu=
ltimedia.matroska.user" rel=3D"noreferrer" target=3D"_blank">http://dir.gma=
ne.org/gmane.<wbr>comp.multimedia.matroska.user</a><br>
<br></blockquote></div><br></div>

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