Re: dpx file length assertion

On Wed, 1 Feb 2006, darby johnston wrote:

> Would this affect other formats, like rgba?

Yes, it does.  Formats which fit nicely into three samples like RGB, 
and YCbCr do not suffer from the issues.  Luma-only and RGBA do 
suffer.

>> Likewise, for 16-bit DPX, for best compatibility, it
>> is advisable that
>> the width be evenly divisible by 2, and for 8-bit
>> DPX, the width is
>> best evenly divisible by 4.
>
> Scary. I assume it's easier to work in scanlines and
> words though, so would it be safe to say the industry
> consensus is for 32-bit alignment?

Due to buffering and algorithmic issues, it is tremendously easier to 
work in full scanlines.  It is also beneficial if rows may be updated 
independently without re-writing the whole image.  Industry lore 
implies 32-bit alignment (e.g. mentioned in O'Reilly's GFF book) but 
this is not actually stated in the specification except for when 
compression or packing is used.  See 
http://www.simplesystems.org/users/bfriesen/dpx/dpx-issues.pdf for a 
writeup regarding current anomalies with the DPX specification.  For a 
17 page specification, there are many anomalies.  Other than 
compression/directory related issues, I think that DPX is almost as 
complex as TIFF but the TIFF specification takes about 120 pages of 
space to specify things very clearly.

> I had thought that's what the "line padding" variable
> in the dpx image header was for, but it looks like
> it's measured in bytes, which probably doesn't help
> for the 10-bit case.

Right.  It is worthless, and since it is worthless it is always 
ignored.

Bob
======================================
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/


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