[Theora-dev] Directshow filters 0.65.7923

illiminable ogg at illiminable.com
Sat Oct 9 12:43:19 PDT 2004

----- Original Message ----- 
From: "Timothy B. Terriberry" <tterribe at vt.edu>
To: "illiminable" <ogg at illiminable.com>
Cc: <theora-dev at xiph.org>
Sent: Sunday, October 10, 2004 3:31 AM
Subject: Re: [Theora-dev] Directshow filters 0.64.7923

>> OK... just curious why files that were apparently created with the latest 
>> release, seem to work with centred offsets but not with offsets at the 
>> top or bottom ?
> Perhaps I'm not being clear: by "offset from the bottom" I mean that you 
> take the offset, and count that many pixels from the bottom, and that's 
> where the image begins. Similarly for "offset from the top". The actual 
> number of pixels you have to count is stored in the bitstream. It is 
> specified by the encoder, not the decoder. The current encoder is 
> specifying half the number of extra pixels needed, which "centers" the 
> image, but it is free to make it flush with the top, flush with the 
> bottom, off-center, etc. The offset of the image is still stored in the 
> bitstream counting from the bottom of the frame in each case.

Aah... ok... now i get you ! (my sleep deprived brain is now making the 
required neural conenctions!) So let me just make sure i do understand! :)

The y offset from the bitstream is the number of pixels from the bottom of 
the drawing surface to the bottom of the image.

Currently the encoder sets this number to half the difference between the 
image and the drawing surface, but it is not required to do so.

There is no gaurantee that the offset above the image is the same as below. 
ie don't blindly add offset to the bottom and the top. What you should do is 
add offset to the bottom and add (drawing surface size - image size - 
offset) to the top ?

For the x offset i take it this is the number of pixels from the left of the 
drawing surface to the left of the image ?




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