Today we highlight a part of Marc Drossaers's Kinect Client Server (KCS) project, where he discusses how you can deal with the shear amount of data you can get from the Kinect for Windows device and how you can apply data compression to make it manageable
A couple other times we've highlighted Marc's work
Transmitting uncompressed Kinect depth and color data requires a network bandwidth of about 460Mbit/s. Using the RleCodec or the LZ4 library we achieve tremendous compression – a compression ratio of 10 or 22 respectively, at lightning speed – over 1600Mbytes/s. We achieve this not so much by the compression algorithms, but by removing undesirable effects (jitter, by the DiscreteMedianFilter) and redundancy (already sent data, by taking the Delta).
From the start, one goal of the Kinect Client Server (KCS) project was to provide a version of the KCS viewer, called 3D-TV, from the Windows Store. Because of certification requirement 3.1 (V5.0)
“Windows Store apps must not communicate with local desktop applications or services via local mechanisms,..”
3D-TV has to connect to a KinectColorDepth server application on another PC. In practice, the network bandwidth that is required to transfer uncompressed Kinect depth and color data over Ethernet LAN using TCP is about 460Mbit/s, see e.g. the blog post on the jitter filter. This is a lot, and we would like to reduce it using data compression.
This is the final post in a series of three on the Kinect Client Server system, an Open Source project at CodePlex, where the source code of the software discussed here can be obtained.
Data Compression: Background, Existing Solutions
If you are looking for an introduction to data compression, you might want to take a look at Rui del-Negro’s excellent 3 part introduction to data compression. In short: there are lossless compression techniques and lossy compression techniques. The lossy ones achieve better compression, but at the expense of some loss of the original data. This loss can be a nuisance, or irrelevant, e.g. because it defines information that cannot be detected by our senses. Both types of compression are applied, often in combination, to images, video and sound.
The simplest compression technique is Run Length Encoding, a lossless compression technique. It simply replaces a sequence of identical tokens by one occurrence of the token and the count of occurrences. A very popular somewhat more complex family of compression techniques is the LZ (Lempel-Ziv) family (e.g. LZ, LZ77, LZ78, LZW) which is a dictionary based, lossless compression. For video, the MPEG family of codecs is a well known solution.
I decided to write my own data compression codec, and chose the Run Length Encoding algorithm as a starting point. Why?
Well, I expected a custom algorithm, tailored to the situation at hand would outperform the general purpose LZ4 library. And the assumption turned out to be correct. A prototype implementation of the RleCodec supported by both the DiscreteMedianFilter and creating a Delta before compressing data really outperformed the LZ4 reference implementation, as can be read from the performance data in the Performance section.
It only dawned on me much later that removing undesired effects (like jitter, by the DiscreteMedianFilter) and redundant information (already sent data, by taking the Delta) before compressing and transmitting data is not an improvement of just the RLE algorithm, but should be applied before any compression and transmission takes place. So, I adjusted my approach and in the performance comparison below, we compare the core RLE and LZ4 algorithms, and see that LZ4 is indeed the better algorithm.
In compressing, transmitting, and decompressing data the KinectColorDepth server application takes the following steps:
- Apply the DiscreteMedianFilter.
- Take the Delta of the current input with the previous input.
- Compress the data.
- Transmit the data over Ethernet using TCP.
- Decompress the data at the client side.
- Update the previous frame with the Delta.
Since the first frame has no predecessor, it is a Delta itself and send over the network as a whole.
The RleCodec was implemented in C++ as a template class. Like with the DiscreteMedianFilter, traits classes have been defined to inject the properties that are specific to color and depth data at compile time.
The interface consists of:
How does our custom RLE codec perform in test environment and in the practice of transmitting Kinect data over a network? How does its performance compare to that of LZ4?. Let’s find out.
Using the RleCodec or the LZ4 library we achieve tremendous compression, a compression ratio of 10 or 22 respectively , at lightning speed – over 1600Mbytes/s. We achieve this not so much by the compression algorithms, but by removing undesirable effects (jitter, by the DiscreteMedianFilter) and redundancy (already sent data, by taking the Delta).
Project Information URL: http://thebytekitchen.com/2014/03/24/data-compression-for-the-kinect/
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