Unpacked (as a *.bmp file, for example), an image with a megapixel resolution would take up about 4 MB of the storage medium, making it much too big for transmitting and saving the image efficiently. Even in a 100 Mbit network, the image would still take about half a second to transmit. For this reason the image data needs to be compressed in the camera. There are various procedures for doing this. The most common mode of compression for a single image is JPEG compression. Depending on the quality setting, compression reduces the file to between 1/20 and 1/50 of its original size.

Image Quality

The quality of the image is directly dependent on the degree of compression and can be set as a percentage in MOBOTIX cameras. Good images can be achieved for most applications with a setting of 50 percent. In the case of settings under 20 percent, the frame creation commences within the image; configured at over 70 percent, there are barely any apparent difference from the original image. The file size is additionally dependent on the content of the image; i.e. an image which is rich in detail with fine structures is larger at the same level of quality and takes up more storage space than a landscape image which is rather monotone. For this reason a sharper focus also causes an increase in the size of the image file.

Single Frames Turned into a Motion-JPEG

If single frames are displayed in quick sequence, they create a fluid video sequence from a frame rate of 12 fps. This process is called a motion-JPEG because the individual shots are JPEG-encoded single frames. Motion images like this can be displayed via a standard browser without the need for additional plugins. Motion-JPEG is a suitable image format for making the camera images accessible to a large group of users whose PCs have been disabled for loading JAVA applets or ActiveX components for security reasons.

Normally Negligible Change between Two Images

The changes from one image to the next are normally negligible; in other words, certain areas are transferred in every single frame even though (virtually) nothing has changed. Consequently, video-encoding processes such as MPEG have been developed in order to avoid the multiple transmission of identical image areas.

Disadvantages of MPEG

However, video encoding processes like MPEG also pose some serious drawbacks for live cameras, due to the history of how they developed. MPEG was developed for the compression of cinema movies (on DVD, for example), i.e. for a task where it didn't matter how long the encoding takes. All that matters is quality and compression. In the case of a live camera however, it is very important that the transmission delay between the scene and the image display on the monitor is minimized.

The second problem with standard processes like MPEG is that moving objects are encoded in them at low resolution and low quality because the human eye can't identify the details of a moving vehicle in a cinema film anyhow, for example. In a security application however, objects which have just moved are very relevant and, therefore, have to be transferred to the screen in good quality.

MxPEG from MOBOTIX Optimizes Live Cameras

In order to get around the disadvantages of standard video encoding processes (long delay in display, low quality of moving image elements), MOBOTIX has developed the MxPEG process which based on the JPEG single frame process. Unlike motion-JPEG, MxPEG only transmits the parts of an image which are different to the previous image, i.e. in which motion has occurred. Consequently, MxPEG requires 3 to 5 times less storage capacity than a motion-JPEG display. Video transmission with MxPEG at a rate of 25 fps in CIF format typically requires a bandwidth of about one Mbit.

Advantage: Video and JPEG Single Frame Simultaneously

Since MOBOTIX cameras have multi-channel capability, they, for example, can output JPEG single frames to PDAs, motion-JPEG images to plugin-free standard browsers and video streams with an audio channel via ActiveX browser components simultaneously. The frame rates which the individual users set may also differ, if necessary. This is a decisive advantage in long-time storage, in particular, because despite a high-speed live display rate of 25 bps, the storage frame rate can be reduced to 2 fps, saving on storage space. Nevertheless, the audio channel is saved lip-synchronously.

Tip: Choose JPEG for Internet Applications
It is advisable to set the camera to JPEG single frame mode for high-grade Internet applications in which the appearance of every single image has to be optimized. In this mode the response time for the exposure control is quickest and facilitates images with optimum correction.