The document at http://www.openexr.com/openexrfilelayout.pdf
describes the on-disk format of OpenEXR files. The document
should be enough to calculate how much space is used by the
file header and other auxiliary data.
The source code and comments in file ImfB44Compressor.cpp
are currently the only detailed description of how B44
compression works. It will take a bit of work, but you
should be able to derive a relatively formula that tells
you exactly how big the compressed pixel data are.
If you work out an exact formula for the file sizes, would
you mind publishing it?
Hai Nguyen wrote:
Thanks, Florian.
That information was actually on the web site - I should have been more
paying more attention. :)
What I'm trying to do is to compress an image in memory to B44 and save
it off in a chunk of memory.
Then derive something form IStream to read it back during play back. I
think someone was trying to do something similar
sometime ago - I saw it in the archives.
The motivation for my initial question was to avoid having to write out
to a temporary file and read it back in.
This requires me to make a guess at what the maximum size of the
compressed data would be.
Do you have any recommendations/suggestions for a process like this?
- Hai
Florian Kainz wrote:
Hai,
the B44 compression rate is fixed; it does not depend on image content.
If you store the pixels as RGB data, the compression rate for the raw
pixels is 32:14 (approximately 2.28:1). If you store the pixels as
luminance/chroma data, the compression rate is 32:7 (approximately
4.57:1).
In addition to the pixels, the file contains header data and some
padding,
so the overall compression rate will be slightly less.
Since the compression rate is fixed, the easiest way to determine the
size of a file with given dimensions, is to compress one image; other
images will have the same size.
I don't have an exact formula for the size of B44-compressed files,
but if necessary, we can probably work it out.
Florian
Hai Nguyen wrote:
Hi,
I am wondering if there was a formula of some type of find the
maximum size of a B44 compressed image
give its dimensions? I briefly looked through the header files but I
couldn't find what type the final compressed
data is stored in.
Thanks,
- Hai
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