Compression standards
Many compression standards to choose from
Image and video compression can be done either in a lossless or lossy approach. In lossless compression, each and every pixel is kept unchanged resulting in an identical image after decompression. The downside is that the compression ratio, i.e. the data reduction, is very limited. A well-known lossless compression format is GIF (Graphics Interchange Format). Since the compression ratio is so limited, these formats are impractical for use in network video solutions where large amounts of images need to be stored and transmitted. Therefore, several lossy compression methods and standards have been developed. The fundamental idea is to reduce things that appear invisible to the human eye and by doing so, tremendously increase the compression ratio.
Compression methods also involve two different approaches to compression standards: still image compression and video compression.
Still image compression standards
All still image compression standards are focused only on one single picture at a time. The most well known and widespread standard is JPEG.
JPEG
This is short for Joint Photographic Experts Group international – a good and very popular standard for still images that is supported by many modern programs. With JPEG, decompression and viewing can be done from standard Web browsers.
JPEG compression can be done at different user-defined compression levels, which determine how much an image is to be compressed. The compression level selected is directly related to the image quality requested.
Besides the compression level, the image itself also has an impact on the resulting compression ratio. For example, a white wall may produce a relatively small image file (and a higher compression ratio), while the same compression level applied on a very complex and patterned scene will produce a larger file size, with a lower compression ratio.
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Below are examples of JPEG images with varying levels of detail. The branches of trees consist of a large amount of detail, which generate a large amount of data. |
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| Not very detailed, file size 20 KB |
Very detailed, file size 50 KB |
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Below are JPEG images using different compression ratios. |
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| Low compression, file size 45 KB |
High compression, file size 14 KB |
JPEG2000
Another still image compression standard is JPEG2000, which was developed by the same group that also developed JPEG. Its main target is for use in medical applications and for still image photography. At low compression ratios, it performs similar to JPEG but at really high compression ratios it performs slightly better than JPEG. The downside is that support for JPEG2000 in Web browsers and image displaying and processing applications is still very limited.
Video compression standards
Motion JPEG
Motion JPEG offers video as a sequence of JPEG images. Motion JPEG is the most commonly used standard in network video systems. A network camera, like a digital still picture camera, captures individual images and compresses them into JPEG format. The network camera can capture and compress, for example, 30 such individual images per second (30 fps – frames per second), and then make them available as a continuous flow of images over a network to a viewing station. At a frame rate of about 16 fps and above, the viewer perceives full motion video. We refer to this method as Motion JPEG. As each individual image is a complete JPEG compressed image, they all have the same guaranteed quality, determined by the compression level chosen for the network camera or video server.
Example of a sequence of three complete JPEG images.
H.263
The H.263 compression technique targets a fixed bit rate video transmission. The downside of having a fixed bit rate is that when an object moves, the quality of the image decreases. H.263 was originally designed for video conferencing applications and not for surveillance where details are more crucial than fixed bit rate.
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| The image of a moving person will become like a mosaic if H-series compression is used. The normally uninteresting background will, however, retain its good and clear image quality. |
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MPEG
One of the best-known audio and video streaming techniques is the standard called MPEG (initiated by the Motion Picture Experts Group in the late 1980s). This section focuses on the video part of the MPEG video standards.
MPEG's basic principle is to compare two compressed images to be transmitted over the network. The first compressed image is used as a reference frame, and only parts of the following images that differ from the reference image are sent. The network viewing station then reconstructs all images based on the reference image and the "difference data".
Despite higher complexity, applying MPEG video compression leads to lower data volumes being transmitted across the network than is the case with Motion JPEG. This is illustrated below where only information about the differences in the second and third frames is transmitted.
Naturally, MPEG is far more complex than indicated above, often involving additional techniques or tools for parameters such as prediction of motion in a scene and identifying objects. There are a number of different MPEG standards:
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MPEG-1 was released in 1993 and intended for storing digital video onto CDs. Therefore, most MPEG-1 encoders and decoders are designed for a target bit-rate of about 1.5Mbit/s at CIF resolution. For MPEG-1, the focus is on keeping the bit-rate relatively constant at the expense of a varying image quality, typically comparable to VHS video quality. The frame rate in MPEG-1 is locked at 25 (PAL)/30 (NTSC) fps.
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MPEG-2 was approved in 1994 as a standard and was designed for high quality digital video (DVD), digital high-definition TV (HDTV), interactive storage media (ISM), digital broadcast video (DBV), and cable TV (CATV). The MPEG-2 project focused on extending the MPEG-1 compression technique to cover larger pictures and higher quality at the expense of a lower compression ratio and higher bit-rate. The frame rate is locked at 25 (PAL)/30 (NTSC) fps, just as in MPEG-1.
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MPEG-4 is a major development from MPEG-2. There are many more tools in MPEG-4 to lower the bit-rate needed to achieve a certain image quality for a certain application or image scene. Furthermore, the frame rate is not locked at 25/30 fps. However, most of the tools used to lower the bit-rate are today only relevant for non real-time applications. This is because some of the new tools require so much processing power that the total time for encoding and decoding (i.e. the latency) makes them impractical for applications other than studio movie encoding, animated movie encoding, and the like. In fact, most of the tools in MPEG-4 that can be used in a real-time application are the same tools that are available in MPEG-1 and MPEG-2.
The key consideration is to select a widely used video compression standard that ensures high image quality, such as Motion JPEG or MPEG-4.
MPEG-4 (Part 10)
The two groups behind H.263 and MPEG-4 joined together to form the next generation video compression standard: AVC for Advanced Video Coding, also called H.264 or MPEG-4 Part 10. The intent is to achieve very high data compression. This standard would be capable of providing good video quality at bit rates that are substantially lower than what previous standards would need, and to do so without so much of an increase in complexity as to make the design impractical or expensive to implement.
Advantages and disadvantages of Motion JPEG, MPEG-2 and MPEG-4
Due to its simplicity, the widely used Motion JPEG--a standard in many systems--is often a good choice. There is limited delay between image capturing in a camera, encoding, transfer over the network, decoding, and finally display at the viewing station. In other words, Motion JPEG provides low latency due to its simplicity (image compression and complete individual images), and is therefore also suitable for image processing, such as in video motion detection or object tracking. Any practical image resolution, from mobile phone display size (QVGA) up to full video (4CIF) image size and above (megapixel), is available in Motion JPEG. The system guarantees image quality regardless of movement or image complexity, while offering the flexibility to select either high image quality (low compression) or lower image quality (high compression) with the benefit of lower image file sizes, thus lower bit-rate and bandwidth usage. The frame rate can easily be adjusted to limit bandwidth usage, without loss of image quality.
However, Motion JPEG generates a relatively large volume of image data to be sent across the network. In this respect, MPEG has the advantage of sending a lower volume of data per time unit across the network (bit-rate) compared with Motion JPEG, except at low frame rates as described below. If the available network bandwidth is limited, or if video is to be recorded at a high frame rate and there are storage space restraints, MPEG may be the preferred option. It provides a relatively high image quality at a lower bit-rate (bandwidth usage). Still, the lower bandwidth demands come at the cost of higher complexity in encoding and decoding, which in turn contributes to a higher latency when compared with Motion JPEG.
One other item to keep in mind: Both MPEG-2 and MPEG-4 are subject to licensing fees.
The graph at right shows how bandwidth use between Motion JPEG and MPEG-4 compares at a given image scene with motion. It is clear that at lower frame rates, MPEG-4 compression cannot make use of similarities between neighboring frames to a high degree, and due to the overhead generated by the MPEG-4 streaming format, the bandwidth consumption is similar to Motion JPEG. At higher frame rates, MPEG-4 requires much less bandwidth than Motion JPEG.
About Axis' MPEG-4 support
Most Axis network video products feature advanced real-time video encoding that can deliver simultaneous MPEG-4 and Motion JPEG streams. This gives users the flexibility to maximize image quality for recording and reduce bandwidth needs for live viewing.
Axis‘ implementation of the MPEG-4 image compression standard follows the ISO/IEC 14496-2 standard (also known as MPEG-4 Visual or MPEG-4 Part 2). Axis network video products support the Advanced Simple Profile (ASP) up to level 5 and the possibility for Simple Profile (SP). With a wide range of settings, it is possible to configure the streams to be optimized for both bandwidth and quality. The built-in Axis Media Control (AMC) software, with MPEG-4 decoder included, makes viewing of streams and integration into applications easy.
Furthermore, Axis’ multicasting support enables an unlimited number of viewers without sacrificing network system performance. Read more about multicasting.
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Does one compression standard fit all?
See also Designing a network video system |
For detailed information about digital video compression techniques, please refer to Axis’ white papers.
