MAKING BETTER QUICKTIME MOVIES

KIP OLSON

QuickTime 1.5 makes it easier than ever to make CD-playable movies. These tips on capturing, compressing, and playing back movies will help you use the new Apple Compact Video compressor to its best advantage, creating movies that will play well off a standard CD-ROM drive on a Macintosh LC computer.

QuickTime introduced the world of digital video to the Macintosh and enabled a whole new category of multimedia content: movies. With QuickTime, it's simple to play back movies in any application and to exchange movies between applications using the standard cut/copy/paste mechanism. But no one ever said it was going to be easy to create them!

What makes movie creation tricky is the tradeoffs required to get QuickTime movies to play off a CD-ROM drive, the most effective distribution medium for digital video. Uncompressed, full-screen, full-motion video requires a data transfer rate of about 27,000 kilobytes/second, yet a typical CD- ROM drive has a data transfer rate of only 100 kilobytes/second. QuickTime solves this problem by using video compression, which requires you to make tradeoffs between frame size, frame rate, image quality, and sound quality when making a movie.

The tips in this article will help you make the right tradeoffs to produce high-quality movies that will play off CD-ROM. You'll also find tips on capturing digital video, using the MovieShop utility on this issue's CD to produce compressed movies, and playing back what you've created. I assume you're already familiar with the basics of movie making with QuickTime.

TIPS ON TRADEOFFS

PLATFORM
Before you get started, you need to decide which Macintosh platform your movie will play on. Obviously, a Macintosh Quadra 950 with a double-speed CD-ROM drive can play much larger, higher-quality movies than a Macintosh LC with a standard CD drive. Table 1 shows some common platforms and their capabilities. (See the following sections for more on frame size and rate.)

Table 1

Common QuickTime Platforms

Macintosh

• Bits/Pixel
• Built in CD Drive
• Maximum Movie Frame Size, Rate
• Market

• 1, 4, 8, 16, 24
• Optional
• 320 x 240, 24 fps
• Power user, professional

• 1, 4, 8, 16
• Optional
• 320 x 240, 15 fps
• Consumer, home

• 1, 4, 8, 16
• No
• 240 x 180, 12 fps
• Education

• Card-dependent
• No
• 160 x 120, 15 fps
• Loyal customers

For the purposes of this article, our target platform is the Macintosh LC II with an AppleCD SC CD-ROM drive (transfer rate of 100 kilobytes/second). Movies created for this platform should play back well on virtually every color Macintosh, covering as much of the installed base as possible. However, keep in mind that machines that use NuBus video, such as the Macintosh II, won't have the playback performance of the LC II. You should always test your movies on the platforms they'll play on.

FRAME SIZE
The frame size determines how large the movie will be on the screen. The larger the frame size, the greater the number of pixels that have to be updated every frame. This can be a problem for less powerful machines, so you often need to limit the frame size to get smooth playback.

Frame sizes are typically specified by horizontal and vertical pixel measurements. Some common frame sizes for digital video are shown in Table 2.

Table 2 Common Frame Sizes for Digital Video

Frame Size	Description	Pixels/Frame	Capability Required
640 x 480	Full-screen	307,200	Hardware acceleration
320 x 240	Quarter-screen	76,800	Fast CPU like a Macintosh Quadra
240 x 180	Eighth-screen	43,200	Apple Compact Video compressor
160 x 120	Sixteenth-screen	19,200	Apple Video compressor

Note that full-screen movies are practical only with hardware acceleration, and for quarter-screen movies you need a fast CPU like a Macintosh Quadra. With our LC II platform and the Apple Compact Video compressor made available by QuickTime 1.5, we can create eighth-screen movies, which have more than twice the screen area of the "postage-stamp movies" possible with QuickTime 1.0's Apple Video compressor.

For the Apple Compact Video compressor to function optimally, the frame size should be a multiple of 4 in each dimension. This is because the compression algorithm uses a 4-pixel by 4-pixel cell.

FRAME RATE
The frame rateis the number of frames displayed in each second of the movie, typically described in frames per second (fps). The frame rate to use for a movie depends on the frame rate of the source material, whether film or videotape. For the smoothest results, you should use a frame rate of which the source material frame rate is a multiple, but this may only be possible if you have hardware acceleration or a fast CPU. Still, an acceptable compromise is available if your platform is limited.

The frame rate of source material in the NTSC video format is approximately 30 fps. Much source material is shot using film at 24 fps and then transferred onto videotape. Frame rates to use for movies based on these types of source material are shown in Table 3. Other video standards such as PAL and SECAM have different frame rates; if your movie is based on one of these types of source material, you'll have to compensate accordingly. For our target platform, we can use 12 fps with good results.

Table 3
Common Frame Rates for Digital Video

If Your Source Material Is NTSC Video:

Frame Rate	Description	Capability Required
30 fps	Full-motion	Hardware acceleration
15 fps	Half-motion	Fast CPU like a Macintosh Quadra
12 fps	Half-film rate	Apple Compact Video compressor
10 fps	Third-motion	Apple Video compressor

If Your Source Material Is Film:

Frame Rate	Description	Capability Required
24 fps	Full-motion	Hardware acceleration
12 fps	Half-motion	Apple Compact Video compressor
10 fps	Third-video	rate Apple Video compressor

There are a couple of minor quirks having to do with frame rate that you should be aware of when you make a movie. First, you'll note that I said the NTSC frame rate isapproximately 30 fps. For reasons lost in the dawn of television, the NTSC frame rate is actually 29.97 fps. If you assume the frame rate is 30 fps, long movies can lose synchronization between sound and video over time, since there are fewer video frames than expected. For example, if you digitized 100 seconds of video, you would expect to get 3000 frames, but you would really only get 2997 frames in that period of time. The GrabGuy utility and the HyperCard® Movie Making Stack (found on theQuickTime 1.5 Developer CDand on this issue's CD) automatically take care of this problem, but if you find sound sync drifting over time on long movies, you may need to duplicate a video frame every 1000 frames to get things back in sync.

The second item to note involves transferring 24-fps film to 30-fps video. On videotape, each frame is composed of two fields, one containing the odd scan lines and the other containing the even scan lines. These fields are interlaced to produce the frame. When film is transferred to video, six extra frames are "made up" every second. Typically, once every four frames, two adjacent film frames are put into the two fields of a single video frame to form a fifth frame. Figure 1 shows how this works. These made-up frames have a blurred look when digitized.

Figure 1 Making Up Extra Frames When Film Is Transferred to Video

You can use a couple of different methods to digitize only the original film frames and skip the blurry made-up frames. If you have a capture system that can grab individual video fields, you can set it to capture at 12 fps, and it will skip the duplicate fields and give the original 12 film frames each second. Or simply capture at 30 fps and throw away every fifth frame, yielding the original 24 film frames.

FRAME DIFFERENCING
Frame differencingis the technique used by QuickTime of storing and updating only the pixels that differ from the previous frame, so that much less data has to be stored and displayed. For example, in Figure 2 the frame on the right contains only the information needed to update the areas of the screen that differ from the frame on the left. As a consequence, less data has to be stored on disc for the second frame and it takes less time to draw. This in turn allows larger frame sizes and frame rates, giving better-quality movies.

Figure 2 Example of Frame Differencing

When you're using the Apple Compact Video compressor, it's a good idea to create movies in which not much changes from one frame to the next, since frame differencing is one way the compressor achieves lower data transfer rates. Here are some things to keep in mind to get the most benefit from frame differencing:

• When possible, use source material with constant backgrounds and solid colors -- especially all-black and all-white areas -- to reduce the difference between frames.
• When possible, use videos of "talking heads." These are great candidates for frame differencing, since typically only the lips and head move.
• Avoid videos with lots of panning and zooming or with complex backgrounds. These effects increase the difference between frames and thus decrease the possibility of compression gains.
• Avoid source material with a lot of video noise, as this increases the difference between frames.

Frames can't be differenced indefinitely, however. At regular intervals akey frame-- a frame that refreshes the entire movie area, not just the pixels that differ from the previous frame -- is inserted. You can adjust the interval to achieve the tradeoff between data rate and movie quality that you desire. Normally, you should have one key frame per second. The more you put in, the higher the data rate and the better your movie looks but the less compression gain you get from frame differencing.

SOUND SAMPLING RATE
The standard Macintosh sound rate is 22.254 kHz, so you'll get the highest-quality audio by sampling to this rate. The Sound Manager is also more efficient at this native rate, so the movie will play back better. However, because your data transfer rate off CD-ROM is limited, using higher audio rates will decrease the quality of the video. The same constraint applies to stereo and 16-bit sound, both of which are supported by QuickTime: these formats eat up more bandwidth, so the video quality may suffer.

If your source is mostly people talking and not music, you can record audio at the alternate sampling rate of 11.127 kHz. In many cases this will make your video look sharper and give acceptable sound quality.

With most audio sampling hardware, it's best to set the hardware at the sampling rate you desire, since the hardware will often do filtering to avoid aliasing artifacts. One exception is the MacRecorder digitizer (from MacroMedia), which doesn't filter at 11.127 kHz. If possible, you should record at 22.254 kHz on a MacRecorder and downsample the audio to 11.127 kHz when you compress the movie. One other thing to keep in mind when using a MacRecorder to digitize audio at 22.254 kHz is that its sample rate tends to drift away from the standard Macintosh rate, so you should always resample it to 22.254 kHz or 11.127 kHz when you compress the movie.

TIPS ON CAPTURING

To get the highest possible quality when you're capturing the source material, you should do three things:

• Start with a clean source.
• Adjust black, white, brightness, and contrast levels.
• Grab at a larger size than you need and scale down when you compress.

I'll discuss these tips one at a time.

START WITH A CLEAN SOURCE
The less video noise, the better compression and the more benefit from frame differencing you'll get, so you should digitize from the cleanest, highest-quality video source possible. The most common video formats, in decreasing order of quality, are BetaCam, µMatic/S-VHS/Hi8, laser disc, and VHS.

Beware of tapes that have been duplicated many times or played a lot -- they can be very noisy. If your digitizing card supports S-Video inputs, use them if you can, as S-Video delivers better quality than composite video.

ADJUST LEVELS
Many digitizing cards support one or more settings with regard to black level, white level, brightness, and contrast. To enhance compression gains from frame differencing, you should adjust the black level of your card so that black areas in your source digitize as truly black pixels. A frame with truly black pixels differs much less from the original than the same frame with noisy black pixels. Thus, refreshing the screen with the noisy frame takes more data than refreshing the screen with the clean frame. Often what looks like black is quite noisy, so you should experiment with your video card. The same rule applies to white levels.

GRAB AT A LARGER SIZE THAN YOU NEED
Because many video cards do a poor job of scaling down frames when they grab, it's best to capture at a large size and let QuickDraw do a filtered scale when you compress. If you're using GrabGuy you don't need to worry, because it will grab 320 x 240 fields and use QuickDraw to scale them down. If you're grabbing from laser disc, grab frames at 640 x 480 and scale them down at compression time.

To save disc space, you should grab using JPEG compression set to the highest quality instead of grabbing raw frames. Most of the quality you might lose this way isn't used by the Apple Compact Video compressor anyway, so this won't reduce the quality of your final movie very much. A number of video cards now support hardware JPEG compression, which makes this even easier.

TIPS ON COMPRESSING

EDIT BEFORE COMPRESSING
To achieve the best possible playback performance with the smallest amount of memory, you should completely edit your movie in raw form before compressing with MovieShop. By the same token, it's not a good idea to compress the movie in pieces and then cut and paste the pieces together to form the final movie, as this will require extra input/output buffers and may cause QuickTime to run out of memory. If this happens, movie playback will slow down considerably.

Here's why: Movies are composed of tracks that typically contain video and sound data. When you capture a movie, the track data is often stored sequentially in the file, resulting in a file layout like the first one shown in Figure 3. To play this movie, QuickTime must allocate four large buffers -- one for each track -- and seek between tracks during playback. This can cause miserable playback performance off CD-ROM drives, which typically have very slow seek times (we're talking hundreds of milliseconds).

Figure 3 Ways of Storing Video and Sound Track Data in a Movie

To solve this problem, QuickTime enables you to interleave the video and sound tracks, resulting in a file layout like the second one shown in Figure 3. Because the sound and video track data are now close to each other in the file, seeking is minimized and only two buffers are needed for data transfer. This is often the file layout you get when you paste two movies together.

The most efficient layout is to append the second video/sound sequence onto the first, as illustrated by the third file layout in Figure 3, so that only one buffer is needed and playback is optimized. When you compress with MovieShop, it automatically merges all the video and sound tracks of a movie into a single video and sound track, thus giving you the most efficient layout.

SET THE DATA RATE
After importing a movie to compress, the first thing you do in MovieShop is to set the data rate. (See Figure 4.) To play off CD, a movie must be compressed to deliver a consistent data rate of 100 kilobytes/second or less, the effective data transfer rate of the first generation of CD-ROM drives. Some of the newer CD-ROM drives can now deliver twice this data rate, but you'll probably want to make your movie at 100 kilobytes/second anyway, for backward compatibility. So a data rate of 100 isusually best for CD playback, while a range from 90 to 105 kilobytes/second will usually produce good results.

Figure 4 Setting the Data Rate in MovieShop

SET THE VIDEO SETTINGS
Next you indicate your preferences relating to compression method, colors, and key frame spacing. To do so, choose Video from the Preferences menu. The dialog box shown in Figure 5 will be displayed. This is where you choose Apple Compact Video as your compression method. The Apple Compact Video compressor has been optimized for CD playback and has a built-in data rate constraining algorithm to give consistent playback from CD. Computationally, it's a highly asymmetric algorithm, taking about an hour to compress a minute of video. The results are worth it, however.

Figure 5

MovieShop tries to limit the data rate of a movie by adjusting the amount of frame differencing (motion quality) and lowering the compression quality (spatial quality). However, since the Apple Compact Video compressor determines motion and spatial quality internally, you should turn these settings off by entering the numbers shown in Figure 5. (For details about what these magic numbers mean, see the MovieShop documentation.) That way, Apple Compact Video will always make the right choices for the data rate you've chosen.

If your source material is in color, choose "Millions of colors"; if it's in black and white, choose "256 grays." "Use previous compressed video" should be checked for most video sources; however, if you're compressing raw animations or composite movies with a constant background, uncheck this setting to get more benefit from frame differencing. MovieShop will then use the last uncompressed frame instead of the last compressed frame as the basis for frame differencing.

The key frame setting should be related to the frame rate of your video. As I mentioned earlier, you should normally have one key frame per second, although in some cases you might want to have fewer than one per second to lower the data rate (which will, however, decrease quality as well).

SET THE SOUND SETTINGS
To change sound settings, choose Sound from the Preferences menu. Selecting 22 kHz will ensure that your movie uses the standard Macintosh rate of 22.254 kHz. As discussed earlier, if you're not concerned about audio quality you should probably resample to 11 kHz as a tradeoff for sharper video. The Video to Sound setting lets you set how far ahead in seconds the audio is interleaved on the file from the video on the disc. This setting should normally be at 1.90, but if you find that CD playback is choppy or the audio portion breaks up, try lowering this number to 1.5. "Interleaved sound" should always be checked so that the audio and video are interleaved as explained earlier for smooth CD playback.

SCALE AND CROP THE MOVIE
Now you need to crop the movie, since there's often tape noise and jitter on the edges of the video frames. At the same time you can scale the movie to ensure that the frame size is a multiple of 4 pixels in each dimension. (Recall that due to its algorithm, the Apple Compact Video compressor functions optimally if this is so.) To change cropping settings and scale the movie, choose Cropping from the Preferences menu. In Figure 6, the movie is being cropped by 2 pixels on all sides to eliminate noisy edges and make the output dimension values multiples of 4.

Figure 6 MovieShop's Cropping Dialog

DISABLE EXTRA COMPRESSION METHODS
MovieShop can apply a large number of techniques when compressing a movie to get the data rate you specify. These techniques are used by the Apple Compact Video compressor, but since the compressor itself takes care of all data rate limiting, all of these methods should be turned off. To turn them off, choose Methods from the Preferences menu. Then drag item 18 to the second position in the list of methods, as shown in Figure 7.

Figure 7MovieShop's Methods Dialog

The version of MovieShop on the CD (v. 1.0c2) has a bug that causes it to use the settings for methods 2, 3, 4, and 5 even when they're below item 18. If you're working with that version, you should additionally set those methods to the following values to really disable them:
2. Forced Key frame -- 255
3. Natural Key frame -- 1
4. Natural Key frame -- 200
5. Drop duplicate frame -- 255

Again, for details about what these magic numbers mean, see the MovieShop documentation.

Once you've set up MovieShop this way, click "Make the movie," choose an output file, and sit back and watch the show. It can be a long wait, but the results will be worth it.

TIPS ON PLAYBACK

• Optimize the movie's screen position and depth.
• Avoid clipping any portion of the movie.
• Hide the movie controller.
• Don't call WaitNextEvent as often.

OPTIMIZE SCREEN POSITION AND DEPTH
The position of the movie on the screen can affect playback performance. For greatest efficiency, the left edge of the movie should be aligned to a long-word boundary in video memory. A new function in QuickTime 1.5 called AlignWindow moves a window to the optimal screen location for movie playback.

The screen depth also affects playback. If the screen is set to millions of colors, the movie will play back more slowly than at 256 colors, because there are more bytes to move to the screen every frame. The Apple Compact Video compressor is optimized for thousands of colors (16 bits/pixel), so you'll get the best performance and quality at that depth.

AVOID CLIPPING
If any portion of the movie is clipped, playback performance will be substantially decreased because QuickTime will have to do a lot more work to draw the frames. Try to avoid overlapping windows and drawing to multiple screens. Be aware of the menu bar and the rounded corners on the edges of the Macintosh screen. Set the clipping region correctly for the movie.

HIDE THE MOVIE CONTROLLER
Displaying and updating the movie controller can cause the movie to play back more slowly, especially on low-end machines and for shorter movies, so you may want to hide it to achieve better playback. Still, it's desirable to have an option to show/hide the controller in your human interface. You might consider using the badge option for the movie controller to achieve this.

DON'T CALL WAITNEXTEVENT AS OFTEN
Since QuickTime performs all its drawing operations at main event loop time, the more often you call MoviesTask the better movie playback you'll get. However, most applications call WaitNextEvent once every event loop, which can go away for a fairly long time under System 7, effectively reducing the number of times MoviesTask gets called each second. To improve this, simply call WaitNextEvent only once a second or so while movies are playing. This will allow background tasks time to run but won't interfere with foreground event processing.

SEE YOU AT THE MOVIES!

In a nutshell, if you want your movie to play well on a Macintosh LC II with an AppleCD SC and thousands of colors, you should use the following setup:

• Frame size: 240 x 180
• Frame rate: 12 fps
• Sound rate: 11.127 kHz
• Data rate: 100 kilobytes/second
• Compressor: Apple Compact Video
• Key frame: every 12 frames

With this knowledge, you can really put this technology to work and produce a QuickTime movie with the best of them. While you may not become the next Steven Spielberg, you can at least see your name in lights (check out theQuickTime 1.5 Developer CDAbout Box movies to see the QuickTime team in action). See you at the movies!

KIP OLSON lives by the motto "I never met an avalanche I didn't like" and spends his winters skiing the extreme in places like Chamonix, Jackson Hole, and Hoboken. When not cornice surfing, he's been known to cast a #12 Adams to a rising brookie and score a perfect 18 at putt-putt golf. He works for Apple in his spare time. *

For more information on QuickTime 1.5, your best source is the QuickTime Developer's Kit v. 1.5, available from APDA. This kit includes the complete printed documentation for QuickTime 1.5 as well as the QuickTime 1.5 Developer CD , which is full of example code, movies, and utilities. The new Inside Macintosh: QuickTime and Inside Macintosh: QuickTime Components together provide a complete description of QuickTime 1.5. *

For more on capturing, see the article "Video Digitizing Under QuickTime" in this issue. *

For more information on AlignWindow and other alignment routines, see the Image Compression Manager chapter of the QuickTime 1.5 Developer's Kit documentation, and Inside Macintosh: QuickTime .*

THANKS TO OUR TECHNICAL REVIEWERS Dean Blackketter, Bill Guschwan, Peter Hoddie, Eric Hoffert *