Codec Central - QuickTime VR



	Introduction \| Choosing a Technology \| Index of Codecs \| Index of Architectures In-Depth Articles \| Sample Movies \| Glossary of Digital Video Terms

QuickTime VR

With QTVR, there are several factors to consider:

Type of image data
Speed of decompression
File size
System compatibility

Type of image data

There are two kinds of QTVR movies - panoramas and objects. Panoramas are made up of a series of "tiles" that create a complete panorama. Each tile is a frame in a video track. Object movies are more similar to typical video track movies - except that the frames can be referenced in an unpredictable order - so temporal compression can cause playback problems. To avoid this problem, each frame that defines a row/col should be a keyframe. Animated object movies might benefit from temporal compression depending on the options.

Hotspot tracks should be compressed with a lossless 8-bit codec (specifically, the graphics codec). Still image compressors tend to have the highest quality/size ratios for panoramas and object movies. It is best to compress 24-bit images and let the playback system dither to the screen resolution. 8-bit images compress poorly and have inferior quality on 24-bit displays.

Speed of decompression

High resolution panorama movies contain more data and take longer to decompress. If the target CPU is too slow, then larger panoramas will stutter as the tiles are decompressed. Faster CPUs, faster decoders, or small panoramas will avoid this problem.

One speed trick to consider is to use only one tile for a panorama. This prevents parts of the panorama from having to decompress multiple times as the panorama spins around. It also requires that the entire panorama fit into memory at once, so this is most appropriate for small (low resolution) panoramas.

Object movies are also sensitive to the decompression speed of codecs. If an object movie stutters when navigating, then either use a faster decompressor or reduce the size of each frame.

File size

Movies distributed on the World-Wide Web tend to be more size-sensitive than those distributed on a CD-ROM. One must trade either slower decompression or lower image quality to reduce the size of a file.

System compatibility

Cinepak was the recommended codec for cross-platform QTVR 1.0 panoramas and object movies. QTVR 1.0.3.11 for Windows now supports JPEG panoramas and QTVR 2.0 will support the entire range of codecs on both Mac and Windows.

Notes: As a rule of thumb, Cinepak works well for large QTVR movies, while JPEG works better for smaller movies.

It is important to remember that codecs that might be great for video because of temporal compression may not compress QTVR movies well. The ideal QTVR panorama codec would be a faster JPEG decompressor, while the ideal QTVR object codec would be Sorenson with smart keyframes for each row.

The Apple Video codec is useful to draft QTVR movies because of its fast compression speed during authoring and fast decompression speed during playback. Due to the size of the files it creates, however, it generally isn't used for final distribution.

QuickTime VR Codecs

Codec (Primary Use)	Pros	Cons	Availability
Cinepak Photographic images	Fast decompression, compatible with all version of QTVR	Large file sizes	Included in QuickTime
Photo-JPEG Photographic images	Best quality photographic image compression, small files	Too slow for larger images, not compatible with most QTVR 1.0 installations	Included in QuickTime
Graphics Graphics	Excellent quality, small files and fast playback	Only good for 8-bit graphics, not photographs	Included in QuickTime
Apple Video Draft authoring	Fast compression makes it handy for authoring draft movies	File sizes are too large for final distribution	Included in QuickTime

Special thanks to Joel Cannon for all his help on the QTVR section.