Hi
In fact each channel name in exr can be 31 bytes long, so even 4096 could be
too less in some extreme cases. On the other side we don't want to read too
much.
The code below fix this issue: I did small modifications to read actual size of
channels list and skip over, no matter how many channels is in the file. I
added few comments to clarify.
########################
import struct
compList = ['None', 'RLE', 'ZIP', 'ZIP 16 lines', 'PIZ', 'PXR24', 'B44', 'B44A']
n = nuke.selectedNode()
file = nuke.filename(n, nuke.REPLACE)
fd = open(file, 'rb')
# read small portion of data
header = fd.read(256)
# search for channels list tag. Well... in fact we could hardcode this position
but I prefer to write it more flexible
channels = header.find('channels')
# skip the tags...
fd.seek(channels+16)
# ...and read the actual size of channels list. We know, that it's 4-byte
integer
channelsSize = fd.read(4)
# Ok, so we know how long (in bytes) is the channels list in our file. And we
simply jump over
fd.seek(struct.unpack('i', channelsSize)[0], 1)
# Here we are
position = fd.tell()
# read next portion of data and search for compression
header = fd.read(256)
index = header.find('compression')
fd.seek(position + index+28)
comp = ord(fd.read(1))
print compList[comp]
######################################
Best
Adrian
> Nathan,
>
> Thanks for the PDF link to the OpenEXR file layout - I was looking for
> something like that!
>
> Rich
>
> On May 22, 2012, at 1:23 PM, Nathan Rusch wrote:
>
> > The compression attribute will be in a different place within the header
> > depending on the number of channels in the file (due to the layout of the
> > EXR header).
> >
> > Adrian's snippet is simply finding the 'compression' attribute name within
> > the header, since that is one of the standard EXR attributes. From there,
> > based on known information about the EXR file layout, he can determine
> > where the actual compression attribute value is stored.
> >
> > The read size of 1024 is pretty much an arbitrary value; you may need to
> > read a larger chunk in order to get far enough into the file to locate the
> > compression attribute if your file has a lot of channels. If you know the
> > names of all the channels within the file, you could figure out exactly how
> > many bytes to read, or even start your read from a predefined offset to
> > keep your data buffer as small as possible; otherwise, you'll just need to
> > make a safe estimate.
> >
> > Check out this doc for a nice simple example of the structure of an EXR
> > file: http://www.openexr.com/openexrfilelayout.pdf
> >
> > Hope this helps.
> >
> > -Nathan
> >
> >
> > -----Original Message----- From: Dan Rosen
> > Sent: Tuesday, May 22, 2012 9:59 AM
> > To: Nuke user discussion
> > Subject: Re: [Nuke-users] How to check zip compression type for EXR
> > images...?
> >
> > This is great! Can you let us know what index number the datatype is
> > in? I'm finding it hard to tell.
> >
> > thx
> > Dan
> >
> > On Mon, May 21, 2012 at 7:30 PM, Richard Bobo <[email protected]> wrote:
> >> Adrian,
> >>
> >> Brilliant - I'll be making it into a nice little pulldown menu utility
> >> function! And, looking a bit deeper at your code, of course, so I can learn
> >> some more Python... 8^)
> >>
> >> Thanks!
> >>
> >> Rich
> >>
> >> Rich Bobo
> >> Senior VFX Compositor
> >>
> >> Mobile: (248) 840-2665
> >> Web: http://richbobo.com/
> >>
> >> "Man has been endowed with reason, with the power to create, so that he can
> >> add to what he's been given."
> >> - Anton Chekhov
> >>
> >>
> >>
> >> On May 21, 2012, at 6:40 PM, Adrian Baltowski wrote:
> >>
> >> Hi
> >> With just few lines of code and totally simplified
> >>
> >> **********************************
> >> compList = ['None', 'RLE', 'ZIP', 'ZIP 16 lines', 'PIZ', 'PXR24', 'B44',
> >> 'B44A']
> >>
> >> n = nuke.selectedNode()
> >> file = nuke.filename(n, nuke.REPLACE)
> >> fd = open(file, 'rb')
> >> header = fd.read(1024)
> >> index = header.find('compression')
> >> comp =ord(header[(index+28):(index+29)])
> >> print compList[comp]
> >>
> >> ***********************************
> >>
> >> Each exr file MUST have compression info in the header and this info is
> >> placed just after channels info. It's simple to get actual size of channels
> >> list but I quickly set 1024 bytes of a headroom.
> >>
> >> Best
> >> Adrian
> >
> > _______________________________________________
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>
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