Cull Graphics Quickly with Preview
You're faced with a folder full of images, and you need to sort through them, trashing some number and keeping the rest. For a quick way to do that, select them all, and open them in Preview (in Leopard, at least). You'll get a single window with each graphic as an item in the drawer. Use the arrow keys to move from image to image, and when you see one you want to trash, press Command-Delete to move it from its source folder to the Finder's Trash. (Delete by itself just removes the picture from Preview's drawer.)
Series: iMovie, You Movie
Can Apple's free digital video program bring out the cinematographer in you?
Article 1 of 4 in series
by Jeff Carlson
If you wanted to edit video in the recent past, you needed a room full of specialized equipment and a fair amount of training and experience just to get startedShow full article
If you wanted to edit video in the recent past, you needed a room full of specialized equipment and a fair amount of training and experience just to get started. Now, most what you need is probably sitting in your Applications folder. Apple's iMovie enables anyone with a recent Macintosh to import video footage and edit it into a professional-looking movie.
In "Dipping into Digital Video" in TidBITS-615, I covered some of the basics of digital video and what to look for when buying a digital camcorder. Now I want to share some tips I picked up while writing my latest book, iMovie 2 for Macintosh: Visual QuickStart Guide. If you're just starting to get your feet wet with digital video, these pointers will help you during shooting and when editing in iMovie.
A Stable Working Relationship -- Most people equate home video footage with the jittery movement of home movies and police pursuit shows. Except in rare circumstances, that look isn't a matter of style: the diminutive size and weight of most camcorders make it difficult to maintain stability while filming. Mounting the camera on a tripod is your best bet for keeping the camera stationary, but a tripod is often a pain to carry and set up. Instead, maintain stability when shooting by tucking your elbows into your sides and hold the camera with both hands. Although most cameras offer digital image stabilization, which does a good job of compensating for small variations in camera movement, don't rely on it to reduce larger tremors.
Zoom Zoom -- One of the first things potential camcorder buyers do is test the product's zoom capabilities. And why not? Unlike Hollywood movie directors, who can place a camera wherever it suits them, you may be trying to get a shot of a grizzly bear from hundreds of meters away (and, in the spirit of recommendations, I strongly encourage you to stay hundreds of meters away from grizzlies in the wild). Punching up the telephoto zoom can be the difference between fur, blur, or getting munched, for those that ignore these tips.
As you're zooming, try to keep the motion steady and measured. The zoom control on most camcorders is pressure sensitive, so pressing hard makes the lens zoom quicker than a lighter touch. Try not to zoom in or out (or heaven forbid, both in succession) as fast as possible, unless you're trying to nauseate your audience. If you have the opportunity, practice zooming in on your subject before you begin filming.
Also, turn off your camcorder's digital zoom feature. Unlike optical zoom, which describes the amount the lens mechanism can zoom (usually 10 times the normal setting), digital zoom is a technique where the camera's processor interpolates the image and enlarges the pixels to approximate a higher zoom level. In essence, the camera guesses what the higher zoomed-in image will look like, and it shows: digitally zoomed footage is highly pixelated, and it's often hard to tell what was originally being shot. Although a 200x digital zoom sounds nifty, it's more marketing gimmick than filmmaker's tool. Turn if off now while you're thinking about it, so you don't scold yourself later when reviewing unexpectedly blurry footage.
Do You Hear What You Hear? As you're recording, use headphones plugged into to your camera to ensure that the audio you're capturing is the sound you expect. Any pair of headphones will do, as long as what you hear is what the camera's microphone hears. You won't want to begin editing your footage and realize that traffic noise drowned out the rest of your footage's audio.
Cover Your Assets -- When you get to the editing stage, you'll want to assemble a tight movie, with no scenes that can make your audience lose interest. However, when you're out shooting, record plenty of extra coverage. Linger at the end of scenes, and don't stop recording when the action ends. Take a few minutes to shoot the scenery, the reactions of people around you, or objects that catch your eye but may have nothing to do with the subject of your video. You want to go into the editing stage with more than enough footage to work with, because in most cases you won't be able to go back and reshoot something. That extra coverage can be essential when you need to add a few seconds of footage to maintain your movie's timing and rhythm.
Dumpster Diving in iMovie -- With the shooting complete, it's time to import your footage and begin cutting together your movie. As you begin to chop, crop, and rearrange your clips, it can become difficult to know which sections were once whole in case you want to go back and try a different combination of clips. Fortunately, iMovie offers a few methods to retrieve footage. First, you'll find yourself using iMovie's ten levels of undo often, though remember that the counter resets when you close your iMovie project.
If you can't undo changes, you may still be able to restore an original clip. As you make edits, iMovie records only the changes that have been applied to the original clip you imported from the camcorder - it doesn't actually split the clip's media file on your hard disk. For example, suppose you imported a 10-minute original clip from your camera and split it into a number of smaller clips. Now suppose you deleted one of those smaller clips, not realizing until too late that you needed it. Unlike the Finder's Trash, you can't open iMovie's bin and pull out a discarded clip. Rather than reimport from the camera, select another one of the smaller clips and choose Restore Clip Media from the Advanced menu - iMovie reads the entire 10 minutes of data from the media file on the hard disk and turns that small clip into the full clip, which you can edit down to the necessary footage again. Be warned that if you use the Empty Trash command at any point, the clip is gone for good - iMovie edits the media file on your hard disk and removes the portions you threw away.
Transitions -- Leave enough padding in your clips to accommodate transitions. A transition such as Cross Dissolve overlaps portions of the two clips it's bridging in order to display both simultaneously. If the action begins immediately in the second clip, it will be partially obscured by the dissolving portion of the first clip. Leaving a few seconds of neutral footage gives you the transition effect you're looking for without disrupting the content of the scene. If you end up with too much padding, you can always trim it out later.
Speaking of transitions, don't go crazy adding every type of transition you can find (and there are plenty - in addition to Apple's, check out GeeThree's Slick Transitions and Effects). In most situations, you'll probably use Cross Dissolve, Fade In, Fade Out, and Overlap. Although others can be appropriate in context, using too many different flashy transitions in one project tends to distract from the movie itself. It's the same principle as using too many fonts in a word processing document: with more than a few on the page, it no longer matters what the words say.
Sizing Up Titles -- From an ease-of-use standpoint, the slider that determines a title's font size is wonderfully simple: slide left to reduce the size, slide right to increase the size. However, this approach can be maddeningly frustrating if you want precision. It doesn't make it any easier that the longer your title, the smaller the text will appear, even at the largest font size. Also, iMovie's rough title preview can be deceptive about text sizes and where longer phrases are wrapped to the next line. So, apply your titles to a few dummy clips that you can export back to tape and preview on a television to see exactly how the title will appear.
Using Music Tracks -- iMovie features what must have once seemed like an ingenious method of adding music to your movie: you can record song tracks from an audio CD directly into the program. You must start playing the song and record it as it plays, much the way you import video from your camcorder's videotape. However, with MP3 music files and iTunes, this technique has become archaic. Instead, use iTunes to extract music as MP3 files, then use iMovie's Import command to add the song to your movie. If you want the highest quality audio (which takes up significantly more disk space), use iTunes to extract the song in AIFF format; that's how iMovie's built-in audio recorder stores music, but iTunes provides a far superior interface to getting it done.
The iMovie Effect -- Once you start editing in iMovie, you'll never watch movies or television the same again. You see scenes in terms of shots, angles, lighting, audio effects, and visual narrative. My wife, after using iMovie only a few times, proved this to me when we watched the online trailer for the movie The Man Who Wasn't There. It's a great work of editing, but I didn't realize how good until Kim casually pointed out that each shot ended in a cross-dissolve transition, except when the main characters were on screen, which used jump cuts to show another shot of the actor before dissolving. If that doesn't demonstrate how iMovie's ease of use and editing power can get into your brain, I don't know what can.
Article 2 of 4 in series
by Charles Wu
Apple's winning iPhoto software makes it easy not only to collect and categorize your digital photos, but also to create slide shows that feature blended transitions between pictures and an accompanying sound trackShow full article
Apple's winning iPhoto software makes it easy not only to collect and categorize your digital photos, but also to create slide shows that feature blended transitions between pictures and an accompanying sound track. However, iPhoto is available only under Mac OS X. Although other applications under Mac OS 9, such as iView MediaPro, offer slide show capabilities, I use a handy program that came with my Mac to create nifty slide shows: Apple's iMovie 2.
Now, iMovie doesn't even pretend to have all the picture management tools available in iPhoto, but it does let you create QuickTime slide shows, DVDs (if you have iDVD or DVD Studio Pro), and even videotapes of your photo collection. You can employ professional-looking transitions such as wipes and dissolves, add audio and text narration, and lay down a soundtrack that works in conjunction with the photos. Because iMovie can make QuickTime movies and export to video, you can send narrated stories of your adventures to people with or without computers.
This article is a step-by-step introduction to creating a professional slide show using iMovie 2. Although designed primarily to capture and edit digital video, iMovie can also import still images and turn them into video clips, which can then be edited using all of iMovie's controls. By taking advantage of this feature, we can build a movie composed of many still images.
I'm going to assume that you've already transferred the photos from your digital camera (or scanned print photos if you don't have a digital still camera) to a folder on your hard drive. And, of course, I'm assuming that you have iMovie, which has been included free with every FireWire-equipped Mac since July 2000; the Mac OS 9 version of iMovie 2 is also available for $50 from the Apple Store.
Start the Show -- A random collection of photos is fine when they're spread out across the dining room table, but a great slide show tells a story. Start by coming up with a narrative for an event such as a birthday party or a vacation. Let's assume that you've spent the day taking pictures of your child's birthday party and you want to share the experience with grandparents who live somewhere else.
When you launch iMovie, it prompts you to create a new movie project. Call it something meaningful, like "Third Birthday." At first your project is empty, so use the File menu's Import File option to navigate to the folder containing your photos. You can import each file individually, but it's easier to select all the files by pressing Command-A, which highlights all the files. After you click the Import button and wait a few minutes (depending on how many pictures you're importing), the images appear as thumbnail clips in the right section of iMovie's interface, also called the Shelf. At this point, the photos haven't yet been added to a movie; the Shelf acts like a holding pen.
Assemble the Cast -- As you look at thumbnails in the Shelf, start thinking about which photos to use and the order in which your slides will appear. One of the dirty secrets of digital video is that because it is now so cheap, people tend to shoot far more footage than they can use; a typical movie may have five to six times more film than what appears in the final product. The same is true of most slide shows. Thankfully, you don't have to extract sections of scenes - just choose the pictures you want.
To start building your movie, drag and drop your chosen thumbnails to the bar at the bottom of the screen, which is called the Clip Viewer, keeping in mind that the movie will play from left to right. You can reorder the movie by dragging and dropping pictures to other locations in the Clip Viewer.
iMovie assigns a default time of five seconds to each picture, indicated by the numbers that appear at the top of the clip. If you want to modify the length of time a picture is displayed, select the clip and change the value in the Time field at the top of the Clip Viewer. (iMovie uses timecode notation for clip length, so a length of five seconds looks like this: "00:05:00". Broken down, this reads as "zero minutes, five seconds, and zero frames" - since each second of video is comprised of 30 frames, a number such as "00:12:26" would be zero minutes, twelve seconds, and twenty-six frames, or just four frames shy of becoming thirteen seconds.)
At this point, you've created a basic slide show. Using the controls in the Monitor (the main window), play your slide show from start to finish, or scroll forward and back through the movie by dragging the Playhead (the small triangular control located just below the Monitor's screen). It's a pretty boring slide show so far, though, so the following steps will make it much cooler.
Adding Titles -- Let's start by adding a main title to the slide show. Click the Titles button at the bottom of the Shelf to display the Titles panel, which shows a list of available title styles. Clicking an item in the list shows a rough example in the preview window at the top of the panel, so feel free to click each one to see the different styles. Let's use Centered Multiple, which displays several lines of text, faded in and out in series. Type the name of your slide show in the text fields below the title list; iMovie shows only two lines at a time in this title style, which is why the fields are broken out in pairs. The second set can be used to enter the date of the event, some comments, or whatever you choose. Click the plus-sign button to add another set of two lines. Since this is the title of the entire slide show, we want it to appear on its own instead of piggybacking on one of the slide images, so click the checkbox labeled Over Black. When you're satisfied with the results, use drag & drop to place the title name (Centered Multiple) at the beginning of the slide show. Now you've added a professional intro. A small black bar appears on the bottom of the clip's thumbnail, with a red bar inching across to indicate the progress of rendering the title clip. Don't worry, you can work with other pictures while this is going on.
Adding titles to individual slides follows the same process, but without enabling the Over Black option. Instead, select a thumbnail in the Clip Viewer and choose another title style; type the title; choose options for font, size, color, and duration using the controls in the Titles pane; and drag the title style at the position just to the left of the picture it will appear on. iMovie overlays titles on top of clips, so if the title's duration is longer than the clip, the title overlaps the next clip or creates a new clip if it's at the end of the movie.
Adding Transitions -- By now, our slides appear in the order we want, and many of them include titles. However, each slide image appears abruptly one after another, so let's make our show a little more interesting. The biggest "ooh-ahh" factor in old Kodak projector slide shows was elicited by two projectors blending into each other's image, so let's use the Overlap transition to create the same effect. Click the Transitions button in the Shelf to view the list of available transitions and select Overlap. Specify the effect's duration by dragging the Speed slider. When you're satisfied with the effect shown in the preview window, drag the transition to the space between the two slides where you want the effect. iMovie inserts a transition icon in the Clip Viewer and renders the transition.
Adding Narration -- At this point you have the equivalent of a silent film, so let's turn it into a talkie. Click the Audio button in the Shelf to bring up the Audio panel. Before you begin speaking, position the Playhead in the Monitor window at the point where the narration will begin. Using a built-in or external microphone connected to your Mac, click the Record Voice button to record some dialog about a particular image. Click Stop when you are done.
As iMovie recorded your voice, it switched to the Timeline Viewer, which displays more detail about when clips start and stop. As you record, a small orange bar appears on the audio track portion of the timeline. Click the segment to find out how long your voice clip was. If you want to match the image's duration to that of the narration, select the picture's clip, switch to the Clip Viewer, and edit the Time field. Repeat this for every slide you want to add narration. If you want to add sound effects, iMovie provides some fun ones that you can drag onto the Timeline or Clip Viewers.
Adding a Soundtrack -- The last step is to add a score to your slide show. You can import audio files in AIFF or MP3 formats, which appear as purple bars in iMovie's second sound track area. Position the Playhead at the point you want the music to begin, then use the Import File command under the File Menu to navigate to the folder containing your music and import a clip. If the music is too long, you can shorten it by dragging the triangle on the far right of the song clip. Unlike iPhoto, you can add more music clips where you want.
You're done! You've created a professional looking slide show. Now, it's time to release it to the world at large.
Exporting the Movie -- How should you pull your slide show out of iMovie? If you are going to send it via email or post it on the Web, export the show as an appropriately sized QuickTime movie. If your recipients don't have a computer, consider sending them a videotape or DVD.
Select Export Movie from the File menu and select QuickTime from the pop-up menu at the top of the dialog box that appears. iMovie includes some commonly used settings for exporting to different movie sizes, such as Web Movie, Email movie, or CD-ROM movie. The higher the quality, the more disk space the movie will occupy, so make sure you have a powerful enough machine and sufficient disk space.
If you are making a videotape, you have two options. Export a QuickTime movie using the To Camera or For iDVD export options and then copy it to a regular videotape recorder, or record directly from your computer. The first approach gives you the best quality, since you're recording from either a MiniDV tape or DVD. The latter option requires your Mac to be equipped with a video-out port and the appropriate RCA style cables that plug into your VCR. From the iMovie monitor, click on the full screen mode and press record on your VCR. It may take a few tries to synchronize the two actions.
Either way, you've put together a slide show that's more interesting than most of what's created in business presentation programs, and which didn't require upgrading to Mac OS X or purchasing third-party software. And in the process, hopefully, you've discovered that iMovie can be a fun tool to use, even if you don't own a digital camcorder.
[Charles Wu splits his time between Mountain View, CA and Denver, CO and is currently a member of the redundant economy, contemplating either business school or returning to the work force. His last position was as a Technical Marketing manager, and in the past has worked as a software engineer, product manager and in business development for various technology companies. However, he is still trying to figure out what he wants to be when he grows up and is entertaining any interesting ideas. In his spare time he runs a restaurant review site for Denver called Zig Zag Club.]
Article 3 of 4 in series
by Jeff Carlson
All during 2002, I expected Apple to announce a new version of iMovie. After all, the last major update to the free video editor happened way back when iMovie 2 was released in July of 2000Show full article
All during 2002, I expected Apple to announce a new version of iMovie. After all, the last major update to the free video editor happened way back when iMovie 2 was released in July of 2000. By the time Macworld Expo San Francisco 2003 rolled around, I wondered if Apple had forgotten about its original iApp. Apparently not: Steve Jobs showed off iMovie 3 during the Macworld keynote, and the program was released late last week as part of iLife. iMovie is available now as an 82 MB download via Software Update or from Apple's Web site.
A New Scene -- Apple couldn't update iMovie without changing its interface somehow, and the adjustments in iMovie 3 are mostly good news. The program finally runs in its own window, rather than monopolizing your screen. This lets you not only resize the window, but also drag in other media files such as photos or movies. Unlike earlier versions, iMovie 3 can import QuickTime movies that aren't specifically formatted as DV media.
iMovie's interface now offers only three playback controls while editing: Rewind, Play/Stop, and Play Full Screen. Also, the timeline viewer and clip viewer share one space, called the Liquid Timeline because of the way it morphs from one to the other when you switch between them.
I've run into at least one interface annoyance, however. Because the entire window uses Apple's brushed metallic texture, clicking anywhere that's not specifically a control can move the entire window. If I accidentally click a few pixels above the playhead, iMovie 3 can fly partially off the screen. Also, iMovie 3 requires a minimum resolution of 1024 by 768 pixels, meaning that owners of clamshell-style iBooks can't use the program on the 800 by 600 pixel display.
Sound Decisions -- One of my favorite new additions is more control over editing audio levels. Clicking the Edit Volume button in the timeline view displays audio lines that you can click and drag to change volume levels within a clip. This is great if, for example, you want to lower background music without completely fading it out. The Audio pane also accesses your iTunes library, which makes it much easier to add music clips.
Burn(s)ed -- One of the splashiest new features is the Ken Burns Effect, so named because of the way documentary filmmaker Burns animates still photos by zooming and panning across them. In iMovie 3, these still photos come from your iPhoto photo albums, which show up in iMovie's new Photos pane. You simply set the clip's duration and a zoom level and position for the start and for the end of the animated clip; iMovie calculates the images in-between.
Unfortunately, Apple is too enthusiastic about its new effect, resulting in more work for the user. Any imported photo, even if it didn't come from iPhoto, gets rendered as a movie with the Burns treatment applied. The workaround is to type Command-period before the clip renders; only then can you change the clip's duration by double-clicking it and entering a new time in the Clip Info window.
Outtakes -- iMovie 3 sports a few other notable features. The new sound effects from Skywalker Sound have the potential for actually being useful; there are new titles, transitions, and effects; and I'm looking forward to setting chapter markers for later import into iDVD when I get my hands on the full iLife package.
However, iMovie 3 suffers from degraded performance on some systems. On the two machines I've tested, a 400 MHz Titanium PowerBook G4 and a 600 MHz iBook, playback stutters noticeably, with audio and video frames dropped at random (other users report similar problems at Apple's iMovie discussion forum). The problem could be a matter of system tuning: some people have reported improvements by manually performing some of Mac OS X's background maintenance tasks or fixing permissions using Disk Utility (though I experienced no improvement); see Dan Slagle's "Unofficial" iMovie FAQ for more details. Given that I can smoothly play back video in a more complex application such as Final Cut Express, I doubt the problem is related to older hardware. If you're seeing the same problems, I encourage you to take advantage of Apple's iMovie feedback Web page or by choosing Provide iMovie Feedback from the iMovie menu.
Overall, iMovie 3 is a promising update, offering new features that give amateur video editors more options. With more work and some dedicated attention to fixing some glitches and performance issues on Apple's part, iMovie 3 could become the update I've been waiting for.
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Article 4 of 4 in series
by Jeff Carlson
When I wrote about iMovie 3 last year, I was less than enamored of Apple's digital video editor (see "iMovie 3 Tips and Gotchas" in TidBITS-697)Show full article
When I wrote about iMovie 3 last year, I was less than enamored of Apple's digital video editor (see "iMovie 3 Tips and Gotchas" in TidBITS-697). It was slow, buggy, and frustrating. Apple released iMovie 4 shortly after this year's Macworld Expo in San Francisco, and I'm happy to report that the new version is much improved - not to the extent that I'd prefer, but iMovie no longer feels like it was rushed out the door.
Peachpit Press has just released the third edition of my book on iMovie, which not only includes almost 100 new pages of material (thanks in part to the addition of iDVD coverage) but also has the longest title of anything I've published: iMovie 4 & iDVD 4 for Mac OS X: Visual QuickStart Guide (I get tired just typing it). In this short article, I want to share some worthwhile tips and iMovie improvements. I'll look at iDVD 4 in a future article.
Direct Trimming -- iMovie gains an important structural overhaul in version 4. Apple calls it Direct Trimming, but video editors know it better as non-destructive editing. In earlier versions of iMovie, you trimmed a video clip by lopping off portions that you didn't need - the last 10 seconds, for example. If you later realized you need to use that footage, the only ways to retrieve it were to use Undo (but only if the edit was made within the last 10 actions), or by restoring the entire clip to its original state.
With Direct Trimming, that deleted footage is always available. If you decide you want your clip to be two seconds longer, you simply grab the right edge of the clip in the Timeline Viewer and drag it to the right to expose the footage you want. When trimming clips down, just drag the right edge of the clip and move it to the left, hiding the frames you wish to remove from view.
The only exception to the advantages of Direct Trimming is if you empty iMovie's trash: iMovie rewrites the media files stored on disk to match the edits you've applied, deleting the footage you've hidden. (I recommend not emptying iMovie's trash at any point while editing; if you need more disk space, you're better off buying an inexpensive external FireWire hard disk and moving your project there.)
Keystrokes Make Editing Easier -- Using keystroke shortcuts doesn't sound like a sexy new feature, but I rely on them to speed up my work and reduce the amount of mousing I do. iMovie 4 added a few keystroke shortcuts that I use constantly.
Press Command-E to switch between the Clip Viewer and the Timeline Viewer (the two timeline views that occupy the same space at the bottom of iMovie's screen).
In the Timeline Viewer, press Command-Option-P to scroll to where the playhead is located, or if a clip is selected but currently off-screen, press Command-Option-S to jump to that clip.
Also in the Timeline Viewer, press Command-Option-Z with a clip selected to zoom in on just that clip so that it takes up most of the timeline. For a quick way to zoom out to see your entire movie without leaving the keyboard, press Command-A to select all clips in the timeline, then Command-Option-Z to zoom to the selection.
Press Command-B to create a bookmark, a marker used only for your own navigational use. Press Command-[ or Command-] (the bracket keys) to jump to the previous or next bookmark.
iMovie 4 features timeline snapping: as you drag the playhead through the movie, it snaps to the nearest edit point (helpful when matching audio and video clips). You can turn this option on or off in iMovie's preferences, but there's a better way: keep it turned off in the preferences, and hold down Shift as you drag the playhead to temporarily enable snapping.
Catch a Wave(form) -- iMovie 4 finally adds visible waveforms to audio clips, so you can see the peaks and valleys in the sound. Waveforms are especially helpful when you're trying to line up a song or sound clip to match action in the video. To turn on waveforms, go to iMovie's preferences and enable the Show Audio Track Waveforms checkbox. It helps to zoom in on a clip to see more waveform detail. You can also press the up and down arrow keys in a selected audio clip to raise or lower waveforms temporarily to see them better (this changes only the waveform display, not the audio levels themselves).
Unfortunately, you can only view waveforms on audio clips, not video clips, even though in iMovie the video clips also include an audio track. To view the waveforms for a video clip, you must extract the audio (press Command-J with a clip selected, or choose Extract Audio from the Advanced menu).
iMovie 4 also added the capability to scrub audio, which plays audio as you drag the playhead, not just when you play back the movie in real time. Hold down Option, then drag the playhead to scrub. However, this feature is such a performance drain that I find it unusable: the playhead lags behind where I've dragged.
Sharing Is Caring -- One last nifty feature before I roll credits is the capability to export selected clips from your movie, rather than the entire movie itself. In earlier versions, if I wanted to export a specific scene to a QuickTime movie, for example, I needed to move all the other clips off the timeline before exporting, a real hassle. In iMovie 4, simply select the clips you want to export, choose Share from the File menu (or press Command-Shift-E), and click the checkbox labeled Share Selected Clips Only.
A Word about Performance -- I mentioned earlier that iMovie 4 is improved, but not as much as I'd like. One thing about iMovie (and, it seems, all of the iLife applications except iTunes) is that they greatly benefit from faster hardware, but even then performance can vary between similar Macs. Some people export movies with no problems, while others tear their hair out due to audio problems or stuttery playback. In my experience, iMovie 4 has been quite stable on my 1.25 GHz PowerBook G4, though it's a dog when running on my older 400 MHz PowerBook G4 Titanium.
I can't offer a sure-fire solution, but I can point you to a great resource: Dan Slagel's "Unofficial" iMovie FAQ. For other information, I also maintain a companion Web site to my book, where you'll find iMovie-related news, tips, and links to other resources. And lastly, you might want to check out a recent TidBITS Talk thread, which was spurred by the latest iMovie 4.0.1 update.