Terrific, Adrian! I'm happy you had a chance to make it more accurate!
Hopefully, I'll have a chance tomorrow to give it a try. ...Reminds me of the
"old days" when I used tape handling commands on an 8mm Exabyte tape drive to
find headers and file markers to read specific frames from tape! (Yes, I'm old.
;^) LOL )
Rich
On May 22, 2012, at 7:28 PM, Adrian Baltowski wrote:
> 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 <richb...@mac.com> 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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