torchzlib

A utility library for zlib compression / decompression of Torch7 tensors.

Installation

After cloning the library, do a local luarocks build:

git clone git@github.com:jonathantompson/torchzlib.git
cd torchzlib
luarocks make rocks/torchzlib-1.0-1.rockspec

API / Usage

The main (and only) API entry point is a new class torch.CompressedTensor. This is a super simple class that creates a compressed ByteTensor of an input tensor (using zlib deflate) and has a single decompress() method to return the original data.

The constructor signature is:

torch.CompressedTensor(tensor, 
                       quality,  -- optional: default 1 
                       apply_sub_filter)  -- optional: default false

Where tensor is the tensor to be compressed, quality is a integer value in [0, 1, 2] and controls the amount of compression and apply_sub_filter is a boolean and indicates whether a PNG subfilter should be applied to each scanline before each compression (and inverse applied on decompression). Note: if the tensor is a float type, then the sub-filter will result in lossy compression.

As per the zlib library, all compression is lossless.

Usage:

require 'torchzlib'

data = torch.rand(5,5):double() -- Can be any other tensor
data_compressed = torch.CompressedTensor(data)  -- Compress using default compression level (1)

-- Alternatively:
data_compressed = torch.CompressedTensor(data, 0)  -- Compress using fast compression (low compression ratio)
data_compressed = torch.CompressedTensor(data, 2)  -- Compress using slow compression (high compression ratio)

-- Do whatever you want in here (including saving and loading data_compressed to file)

data_decompressed = data_compressed:decompress()

To compare libpng vs torchzlib compression ratios:

require 'torchzlib'
require 'image'

im = image.lena():mul(255):clamp(0,255):byte()
torch.save('image.bin', im)  -- Baseline
image.savePNG('image.png', im)  -- PNG compression
torch.save('image.zlib', torch.CompressedTensor(im, 2))  -- zlib compression
torch.save('image.filtered.zlib', torch.CompressedTensor(im, 2, true))
os.execute('gzip -c image.bin > image.bin.gz')  -- zlib compression from the command line (Linux)

Note: png is not just zlib. It's also uses a preprocessing step (called delta filtering) that enables extremely high compression ratios on images once the output is passed through DEFLATE. In the above example image.bin.gz should be approximately the same size as image.zlib.