unknown zlib header (.vrscene) - zlib

Question

This is format description of Vray's .vrscene file:
http://spot3d.com/vray/help/maya/sdk22/vrscene_format.html
I interested in paragraph about "Compressed hexadecimal lists". There is said that compressed list equals to header ("ZIPB") + uncompressed size + compressed size + zlib compressed string.
For example, in my .vrscene I have such compressed list: "ZIPB2C01000015000000e7X81OT0TG4S5ENN3D8Z8IVAPODONF7EA"
It means that "e7X81OT0TG4S5ENN3D8Z8IVAPODONF7EA" -- zlib compressed string. But I dont know how to decompress it. When I do Base64 decode I receive header 0x7bb5. I dont know such a header. Maybe I shouldnt use Base64 and should do something other instead?

Answer 1

So I know this is old, but the information in here was enough to get me to a solution, the only difference being that I have a different compressed list from a different vrscene file.
As you established, you need to remove the first 20 characters from the string since they are just descriptors used by vray and not part of the compressed data.
import base64, zlib
data = "ZIPCB81900003C000000eAHt0iERAAAMxLDC9694MkYCaqCXVZMHDDDAAAMMMMAAAwwwwAADDDDAAAMMMMAAAwwwwMC7gQMbojNx"
encoded_data = data[20:] #"eAHt0iERAAAMxLDC9694MkYCaqCXVZMHDDDAAAMMMMAAAwwwwAADDDDAAAMMMMAAAwwwwMC7gQMbojNx"
compressed_data = base64.b64decode(encoded_data)
hex_data = zlib.decompress(compressed_data)
# b'\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00\x08\x00\x00\x00...'
Once you are at this point you have the raw hexidecimal list without any compression. If you want to turn that into readable python data types you can use struct to unpack them depending on what data type is stored in the list.
Further reading in the documentation that was linked explains that all such lists are stored as 4 bytes per entry (written as 8 half-bytes in the docs, but as far as python is concerned it's just 4 bytes). With that information we can determine that the number of entries in the list should be the length of the hex_data / 4 and build a struct format string that will unpack the whole list at once.
import struct
#I have a list of ints as my example, but for floats you could use "f" instead
format = "i" * int(len(hex_data)/4)
values = struct.unpack(format, hex_data)
# [8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, ...]
There are also Color lists and Vector lists, but they are just float lists that need to be unflattened.
vectors = []
for i in range(0, int(len(values)), 3):
vectors.append(tuple(values[i:i+3]))
Since this method seems to be working with all of the compressed hex lists I've been encountering up to this point it seems like there is either an issue with the file you were originally trying to parse data from, or the developers were using an encoding method other than base64 at one point but have adopted base64 encoding since then (Perhaps that's the reason for "ZIPC" appearing in my files as opposed to "ZIPB in your example).

Answer 2

The documentation says hexadecimal, but the string you provided has other characters. It is probably not base64, since you don't get a valid zlib header at the start. (By the way, I get 0x7b 0xb5 when I decode it with base64, not what you got. But either way, it's not a zlib header.) Do you have a better source for the description of the format?
The data must have been highly compressible, e.g. all zeros, since it apparently went from 4140 bytes to 21 bytes.