Animations provide one of the most powerful means for communicating environmental data. They offer all of the advantages of images and many of the advantages of VRML models. Processing or playing back a sequence of individual raster images creates animations. Animations can be used to clarify three-dimensionality by rotating a model about one or more axes, however they can do much more. Animations can also include moving objects, changing plume levels, moving cut or slice planes. They can even be used to present temporal data such as measured water table variation over time. In fact all of the above techniques can be combined into a single animation.
Although animations cannot be manipulated by the user as a VRML model can, animations can incorporate complex model changes that are beyond the capabilities of VRML. Furthermore, VRML's flexibility can sometimes be a problem. The end user can become confused or can miss the point of the model since VRML viewers allow the user to manipulate the model without limitation. Animations follow a script. If the script is properly planned and executed, animations lead the viewer down a path communicating the proper message. Animations are not limited to rendered images. Title images can be used to introduce sequences and sound can be added. The addition of sound allows for background music for dramatic affect as well as narration.
The process of creating animations begins with the creation of the image sequences. In C Tech's EVS, a script is generated based on a series of key frames (representing transition points in the animation) and interframe interpolation method. When the script is played, the software renders each image and writes them as a raster image. Conversion software is then used to encode the images into an animation file format.
There are many animation file formats available. The standard format for Windows computers is Audio-Video Interleaved (AVI). AVI files can employ many different Codecs (compression / decompression methods). Virtually all Codecs utilized by AVI converters employ lossy compression. Therefore if an individual frame of an animation were viewed or extracted, it would be noticeably degraded from the original. As with image file formats the compression artifacts are most pronounced in areas of high contrast such as lines and edges. Photographs of people and landscapes do not tend to have these high contrast regions (as compared with digitally created images) and therefore the compression is usually more acceptable. MPEG (developed by the Motion Picture Engineering Group) and Apple's QuickTime are competing animation formats. These also generally employ lossy compression.
A noticeable exception in animation file formats is the proprietary HAV file format developed by Gromada (http://www.gromada.com). HAV uses a lossless compression format that often creates files as small or smaller than lossy codecs. Gromada offers shareware converter software and freeware HAV player. Their player is also capable of playing image sequences without the need to convert them to an animation format. C Tech's EVS software includes an animation converter that creates AVI, MPEG and HAV files.
The creation of animations intended for playback on televisions requires consideration. The U.S. and foreign video formats (NTSC, PAL and SECAM) all use Interleaved Video fields. The first color TV broadcast system was implemented in the United States in 1953. This was based on the NTSC (National Television System Committee) standard. Many countries on the American continent as well as many Asian countries including Japan use NTSC. NTSC runs on 525 lines/frame. The PAL (Phase Alternating Line) standard was introduced in the early 1960's and implemented in most European countries except for France. The PAL standard utilizes a wider channel bandwidth than NTSC that allows for better picture quality. PAL runs on 625 lines/frame. The SECAM (Sequential Couleur Avec Memoire or Sequential Color with Memory) standard was introduced in the early 1960's and implemented primarily in France. SECAM uses the same bandwidth as PAL but transmits the color information sequentially. SECAM runs on 625 lines/frame.
Interleaved video displays alternating odd and even scan lines every 60th of a second (50 Hz. for some PAL and all SECAM formats). These results in even and odd scan lines flickering at 30 Hz. When lines or edges are drawn only one pixel wide and are nearly horizontal, the lines will exist in only the odd or even fields. This causes the lines to flicker in a very objectionable manner. The best solution to this problem is to employ special processing techniques on the images before conversion to broadcast video. The processing reduces the contrast of lines and blurs them slightly to avoid having the entire line being represented in only the odd or even fields. Although this may seem like it would introduce additional negative consequences, the results are usually excellent.
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