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Appendix A | Appendix B | Appendix C | Glossary | Index | Legal Stuff | License


Chapter 19 - The Compress Tab

Codec | Bit Depth | Frame Rate | Keyframes
Compare Uncompressed Frames | Data Rate Control

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:


Codec

The video codec you select determines how the video will be compressed. Your codec choice depends heavily on what format and data rate you are producing. Generally, codecs are optimized for Web, CD-ROM, or DVD. However, some may have application across a range of data rates.

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.



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Bit Depth

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.

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Quality Sliders

Depending on which codec you select, there may be one or more sliders available, which vary in function. For example, if you select the "Photo JPEG" codec, a slider appears that controls the spatial quality. If you select the "Cinepak" codec, both a spatial and a temporal slider will appear.

Media Cleaner will only show you sliders that are appropriate to the selected codec, unlike the standard QuickTime dialog.


Spatial and Temporal Sliders

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.

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Frame Rate

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)

For PAL video (common in Europe) the ideal frame rates are:

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.

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Maintain Long Frames

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.

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Keyframes

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.




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Recommended Keyframe Frequency

How frequently you should set keyframes in your movie depends on the content of your movie, and how the movie will be viewed. For normal material, a keyframe every second is usually a good rule. For example, if your movie was 15 fps, setting your keyframes to every 15 frames would probably be fine. For very dynamic material, every half second usually works well. For example, in a high action 12 fps clip, you might want to set your keyframes to occur every 6 frames.

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.

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Natural Keyframe Slider

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.


Quality Balance Slider

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.

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Compare Uncompressed Frames

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.

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Data Rate Control

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.

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Limit to

You may specify the data rate to which your movie will be limited. You can specify whether this total data rate is Kilobytes or Kilobits per second. Most multimedia developers are familiar with Kilobytes (kps or KBps) whereas many people working on the Internet specify files in kilobits (also called kps or kbps). KiloBits are 1/8th of a KiloByte.


If Limit Exceeded

You can specify the desired result if Media Cleaner is unable to lower the size of any given frame to the desired target. This may happen if a codec simply cannot make a frame smaller. Your options are:

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.

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Codec-Specific Features

Some codecs have unique features specific to them. For example, ClearVideo 1.2 allows you to specify how fast the video compresses versus how high its quality is.

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.

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