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The Compress tab is where you specify the details of the compression for your video. (It should really be titled "Video Compress," but that doesn't fit...)
The Compress Tab may display some or all of the following depending on your output format
and your other parameters:
There are many codec choices with QuickTime, and fewer with RealMedia and VDO.
NOTE: There are several new codecs that should be available in mid-1997. Some of these may effectively replace Cinepak as the standard for CD-ROM compression. Many are aimed at low data rate movies. For more details on codecs, please use the "Codec Central" option in the Internet menu.
If you are unsure which codec to use, we suggest that you go through an interview with the Media Cleaner Wizard to see what is best for your movie. However, the Wizard only "knows" about codecs that were available when Media Cleaner was shipped new codecs will undoubtedly show up in the near future, and until a new revision of Media Cleaner is available, the Wizard won't recommend the latest codecs.
The "Bit depth" pop-up allows you to select the number of colors in the final movie. The options available depend on the codec you select. If you create a 256color movie, it is highly recommended that you create a new palette for the movie before you compress it you can set the palette options in the Video tab. See Chapter 17 for more information on palettes.
Generally, setting this to "Millions"
or "Thousands" is best.
Media Cleaner will only show you sliders that
are appropriate to the selected codec, unlike the standard QuickTime dialog.
The spatial and temporal sliders are somewhat confusing, and are often misused. In the standard QuickTime compression dialog, the "Quality" slider is really the "Spatial" slider. The "Temporal" slider is available if you press "Option" and hold the mouse over the "Quality" slider (a little known, and less frequently used item).
The higher the setting of the "Spatial" slider, the higher the quality of the key frames, but the delta frames may become lower quality to compensate for the fact you have only a fixed bandwidth. The "Temporal" slider controls how much new difference data is generated in each keyframe. Higher slider settings give you more unique data per frame, but potentially more "show through" artifacts as well.
Many people set both sliders to 100%, but this is usually not the best setting. For most Cinepak movies, setting both to 50% produces the best results. The only time you should alter these sliders is if your keyframes look significantly better or worse than your delta frames. Experimentation is often the best way to determine the best settings.
For a more complete explanation of what these sliders do with various codecs, please choose
the "Codec Central" option in the Internet menu,
and choose the codec in which you are interested.
Frame rate is the number of frames per second of your final, compressed movie. QuickTime movies for double speed CD-ROMs often have a frame rate of 12 or 15 frames per second. Normally, Macintoshes can handle higher frame rates than Windows machines. Generally, higher frame rates give you smoother movies with lower image quality.
When you select your final frame rate, you will get smoother motion if it is an even divisor of the source frame rate. You may use decimal frame rates.
For NTSC video (used in the USA) the ideal frame rates are :
30 fps (all frames)
15 fps (every other frame)
10 fps (every third frame)
7.5 fps (every fourth frame)
6 fps (every fifth frame)
25 fps (all frames)
12.5 fps (every other frame)
8.3 fps (every third frame)
6.25 fps (every fourth frame)
5 fps (every fifth frame)
NOTE: Technically, NTSC is 29.97 fps, not 30 fps.
However, for movies being used in WWW or CD-ROM projects, you normally won't notice
a difference between 30 fps or 29.97 fps.
Some programs can create variable length frames within a movie. For example, Adobe Premiere's "Optimize Stills" option increases the duration of a non-changing still image instead of repeating the same image multiple times. Media Cleaner also alters frame lengths with the "First/Last frame hold" option.
If your source movie has variable length frames (which are almost always longer than the rest of the frames), you can preserve their lengths in your final movie by selecting the "Maintain long frames" option.
If your source movie has variable length frames and you don't select this option, Media Cleaner will make multiple identical frames out of longer frames to maintain the correct timing of your movie.
NOTE: If you are planning to convert
your QuickTime to Video for Windows, do not use
the "Maintain long frames" option, since Video
for Windows does not support variable length frames.
Some codecs allow you to specify different options for keyframes. The possible settings are:
None (Natural Only)
This option turns off all keyframes, other than the first frame and naturally
occurring keyframes. Depending on your codec, you
may use the Natural keyframe slider to determine how frequently natural keyframes occur.
Every
This option places keyframes at regular
intervals you specify. Natural keyframes may
occur depending on your codec and how you have set the Natural keyframe slider (if present).
All
This option makes every frame in your movie a keyframe. This feature is highly
specialized and is not used for normal movies.
Some codecs, such as JPEG, only make keyframes, so this option may be the only one available.
In some unusual cases, you may want to minimize or turn off the keyframes. This tends to
work acceptably with static material which users
will play straight through and not randomly access.
Higher keyframe frequency is important for movies that viewers can randomly access,
because each time a viewer moves to a point in the
movie, the current delta frame must be calculated
from the nearest keyframe. If the nearest keyframe
is many frames away, it can take quite a while to generate the current frame. You should test
your results to make sure your movie will play
acceptably if you turn off the keyframes.
If your keyframe mode and codec allow for specifying when natural keyframes are generated, a slider will appear that allows you to set how much image change is needed to trigger the creation of a natural keyframe.
NOTE: Currently, only the Video codec
supports this feature. New codecs should support
this feature later in 1997.
Some data rate control modes and codecs support Quality balance sliders. These sliders allow you to control the balance between keyframes and delta frames, as well as the balance between the temporal and spatial quality of the movie. Like many aspects of digital video, these are trade-offs.
By moving the top slider toward "better keyframes" you will increase the size and quality of the keyframes at the delta frames' expense. Similarly, moving the slider toward "better difference frames" will result in better delta frames with lesser keyframe quality. This slider may be useful if you are experiencing periodic quality problems, such as movies that "pulse" between sharp and fuzzy.
By moving the bottom slider toward "less blockiness", you may gain finer image detail
with more "showthrough". By moving the
slider toward "less showthrough", you normally get
less image detail with less "showthrough".
"Showthrough" is a temporal artifact where
parts of previous frames are visible in the
current frame. Normally, showthrough occurs in
areas that are very similar between frames.
Depending on your movie content, showthrough may,
or may not, be objectionable.
The "Compare uncompressed frames" option determines whether the codec looks at the previous uncompressed or previous compressed frame to generate each new delta frame.
For live action video, this feature should be left unchecked. This usually generates the best compression for material with large areas that are significantly different between frames, especially movies with significant video noise.
If you are working with computer generated images, like
3-D animations, the "Compare uncompressed frames" opition should
generally be turned on to get the best compression quality.
You may also get better results using this option with blue screen movies that have still images
as the background running tests is usually the
best method to determine the optimal setting.
Data rate control allows you to control how the movie is constrained to the data rate you specify. Depending on the selected codec, there are up to four options to control your data rate:
NOTE: In Media Cleaner Pro 2.0, only the Video, Animation, and JPEG codecs support all of the modes listed below. All other codecs (including Cinepak) support only the "Basic" mode, which is the standard behavior of most other QuickTime applications. We are working with Apple and the other codec vendors to address this issue in a future release.
Basic
This mode uses the built-in data rate controls
of the codec, if any. This is the "standard"
behavior you get with most other QuickTime programs.
For internally data rate limited codecs such as Cinepak, this is usually acceptable, and is
the fastest mode.
Suppress spikes
This mode allows the codec to determine the
data rate, but Media Cleaner will recompress
frames that are too big. This eliminates "data spikes"
in your movie. Data spikes are short sections of the movie where the data rate increases to a very
high amount due to difficult-to-compress scenes. Spikes tend to make the movie stutter at
certain points, and play fine elsewhere.
Flat
This allows you to specify a fixed data rate.
Media Cleaner will recompress frames until it hits
your target data rate. The flat data rate is an average
of one second the key frames will be larger and
the delta frames smaller to maximize image quality.
Adaptive
Adaptive data rate control is a feature unique
to Media Cleaner Pro. It "intelligently" adjusts
the data rate within your movie to maximize your image quality. Media Cleaner does this
by analyzing your actual movie with all the
settings you have selected (including your
filtration, adjustments, codecs, etc.). It calculates
the optimal size for each frame of the movie based
on the results of the analysis, then recompresses each frame to this ideal size.
Adaptive data rate control takes into account how the buffers in QuickTime actually work, so it can allow higher data rates where needed. Adaptive data rate control takes substantially longer than any other mode of compression, since it requires an initial pass for analysis, and then a second pass for compression, with some frames being compressed multiple times to hit the ideal size.
With some hard-to-compress movies, adaptive data rate control may give significantly better
final image quality than the other modes. This improvement is most noticeable in movies
that have mixed hard-to-compress and easy-to-compress sections. Please see
Appendix A for more details on how adaptive data rate
control works and why it is useful.
Add frame anyway
This will simply leave in the frame at the larger than desired size, and may create
a data-spike. A warning dialog will come up when the compression is complete
notifying you of any oversized frames. This was
the behavior (minus the warning) of Movie Cleaner Pro 1.3 and earlier.
Stretch frame
This option changes the duration of the frame to make the average data rate
acceptable. This may result in some frames
being dropped, but the data rate will be maintained. While not ideal, stretching
frames may be the only solution for some difficult movies, especially low data rate movies
with non-optimized codecs (such as Cinepak). This was the behavior of Web-Motion
1.0 when data rate control was selected.
Abort compression
Media Cleaner will simply stop compression and notify you of its inability to make a
frame the desired size.
NOTE: If you deselect the checkbox next to "Data-Rate", your movie will be unconstrained, and may be quite large with substantial fluctuations and spikes. This is not generally recommended, except for special purposes.
The data rate you specify includes the audio channel as well. For example, if you are using a total data rate of 170 KBps, and using 22.050 kHz, 8-bit sound, your video data rate is about 150 KBps, and your audio data rate is about 20 KBps. By making this setting your total data rate, you do not have to do any calculations to figure out the total what you type in is the total data rate you will get.
Generally for CD-ROMs, a total data rate of 170
- 200 KBps is safe for cross-platform 2x speed titles.
A Macintosh-only product can usually be set to about 220 - 250 KBps. To get a better feel
for appropriate data rates for given applications,
you should go through an interview with the Media Cleaner Wizard. To do this, select "Media
Cleaner Wizard" from the Windows menu.
Media Cleaner supports the codec-specific features of many of the available codecs as of April 1997.
Because new codecs will be available in the future of which Media Cleaner will be unaware, Media Cleaner also has a "Set" button at the bottom of the "Compress" tab to allow you to use the codec's unique interface. With future releases, new codec features will be incorporated into the regular Media Cleaner interface.
NOTE: Any codec that has features which
must be set through the "Set" button may not
work properly with batch processing. This is due to
the fact that these codecs save their special settings
in a variety of methods, which are often not accessible to Media Cleaner. Generally
speaking, if you use the "Set" button, don't use batch.
Also, changes you make in the codec specific dialog may be lost when you restart
your machine, or launch other applications.