Selecting a Codec for Your QTVR Object Movie

Major Considerations

There are four general considerations you should make in deciding what codec to use for your QTVR Object Movie.

• Quality In general, for a given codec, the lower the quality setting, the smaller the file size. Also, for a given quality setting, different codecs will give different results. For instance, there is considerable difference in appearance between Cinepak and Photo-JPEG at the Low Quality (25%) setting, though their file sizes are fairly close.
• File Size If you are putting your QTVR movies on the web, playing the file-size/quality trade-off is crucial. Some codecs (particularly Photo-JPEG and Sorenson) give much smaller file sizes and superior quality at the lower quality settings. If you are planning to put your QTVR movies on a CD-ROM or play them directly from a hard disk (e.g. in a Kiosk), file size in a minor consideration. Quality and performance are much more important.
• Performance Codec stands for compression-decompression. When a digital picture or movie is compressed, it is put through a program that reduces the file size of the picture by taking advantage of redundancy in the picture itself and the forgiving aspects of the human visual system. Performance relates to the decompression of the compressed picture, when compressed picture is redisplayed on the computer screen. Some codecs require more processing power than others, and perform sluggishly on slower computers. Photo-JPEG and Sorenson are two of the more processor-intensive decompression routines.
• Availability Last spring, Apple shipped it's latest major revision of QuickTime (3.0). With this revision came the full support on both Macintosh and Windows computers for the 2.0 version of QTVR. Due to the relatively newness of this version, there are a great number of computers in the real world running older versions of QuickTime (if they are running it at all). QuickTime 3.0 made available for the first time the Sorenson codec on Macs and Windows computers, and Photo-JPEG on Windows. Assuming that you save your QTVR movies in the QTVR 1.0 version, you should be aware that many computers running older versions of QuickTime may not be able to view these codecs. This situation will change over time, as more and more people upgrade to QuickTime 3.0

Playing the File Size Game

If your are putting QTVR media up on the World Wide Web, the file size issue is important. Because they are made of up numerous still pictures, QTVR Object movies can reach a prohibitive file size quickly. There are five considerations that you should make when playing the file size game.

• Codec Choose a codec that gives you a smaller file size for a given subjectively-determined quality. I say "subjectively-determined" because the same quality setting can look very different using different codecs. Go by what your eye sees.
• Quality You may wish to compromise the subjectively-determined quality of the movie to get the file size smaller.
• Number of Pictures Comprising the Movie Apple likes to recommended 36 frames for the horizontal pan (columns) of object movies. This gives smooth scrolling, but isn't a requirement. 18 frame columns will halve the file size and still show 360 degrees of your object, though the scrolling may be a little jerky. If you are doing vertical tilts on your movie, you might consider doing fewer rows.
• Size (dimensions in pixels) If you halve the two dimensions of your frames (e.g. make a 160 X 120 pixel movie instead of a 320 X 240 pixel movie) you theoretically reduce the size of the movie by 1/4. (I say "theoretically" because codecs compress different features of you pictures differently, so that the savings from dimensional reduction is sometimes less than expected. It depends on a combination of the codec and the nature of your picture.)
• Number of Colors Surprisingly, this is a very minor consideration when playing the file size game. Codecs are already manipulating color to get more compression. Going from "millions of colors" to 256 colors often results in little or no file size change.

Examples

Here is a chart of examples of codec choices for the same QTVR Object movie. I've limited the codec choices to the most commonly used (Cinepak, Photo-JPEG, Sorenson, and Video). I don't list other codecs because they present either a poor file-size/quality trade-off, or have availability problems (not shipped with QuickTime). These are 36 column, single row QTVR movies, 320 X 240 pixels. For Cinepak and Photo-JPEG, I include a movies with a smaller dimension (160 X 120 pixel) so you can see what that manipulation accomplishes. Click on the file size of a given movie to view it.

Example Object Movies

Here are my personal subjective ranking judgements of the codec examples. Some of these qualities are so close that I found them very hard to determine. Where you see *, it means a very close comparison, almost impossible to tell. The file size rankings were no problem, just compare the numbers.

Subjective Rank Scores on Four Codecs for Quality, File Size, and Performance

Based on 320 X 240, 36 Column, Single Row QTVR Object Movie on a Performa 5200, a 75 mhz Power Mac

At High Quality (75%)

At Low Quality (25%)

Here are my judgements on the availability of computers capable of playing the different codecs. These are based on what I know about the past implementation history of QuickTime and discussions on Apple's QTVR discussion list. These rankings assume you saved the movie in version 1.0 of QTVR. Some Object movie features in QTVR 2.0, like hot spots, are not available in QTVR 1.0.

Availability Rankings

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