Adding DVD-ROM Content

One of the reasons why the DVD format is so versatile is that it can accommodate not only video, sound, and pictures, but also any disk files that you may want to distribute.

Here's the scoop on this aspect of DVD authoring, along with a peek under the hood to see how MPEG-2 compression manages to squeeze up to two hours of video onto a 4.7GB DVD.

The DVD-ROM Zone

A DVD can hold more than video and slide shows; it can also hold "computer files"Microsoft Word documents, PDF files, JPEG images, and so on. You might take advantage of this to distribute files that are related to your DVD's content.

If you've created an in-house training DVD for new employees, you might want to include a PDF of the employee handbook. If you've created a DVD containing a couple of rough edits of a TV commercial, you might also include some PDFs that show the print versions of your ad campaign. If you've created a DVD promoting your band, you might include some audio files of your tunes.

When a DVD-Video disc also contains files intended to be used by a computer, it's said to have a DVD-ROM portion. If users play the DVD in a living-room DVD player, those files are invisible. However, if they use that same DVD with a personal computer, they can access the files.

Including Photos

As described on page 294, when creating DVDs containing slide shows, you can have iDVD copy the original images to the DVD-ROM portion. In the slide show editor, click Settings, then check the box labeled Add Image Files to DVD-ROM.

This option is ideal for photographers who want to distribute high-resolution versions of their images along with slide shows. You might also find it a useful way to back up a set of digital photos. The slide shows serve as a handy way of viewing the images, while the original, high-resolution files are archived in the DVD-ROM portion of the disc.

Note

If your slide show includes raw-format photos from iPhoto, iDVD includes both the raw-format originals and the JPEG stand-ins that iPhoto created. If you'd rather not include the raw originals (or the JPEGs), save your DVD as a disc image and edit the contents of the disc image as described on page 311.

Tip

If you always want to include a slide show's original images on your DVD, choose Preferences from the iDVD menu, click the Slideshow button, then check the box labeled Always Add Original Photos to DVD-ROM Contents.

Managing DVD-ROM Content

To add other types of files to your DVD, choose Edit DVD-ROM Contents from the Advanced menu. Use the DVD-ROM Contents window to manage and organize the contents of the DVD-ROM folder.

MPEG: Compressing Space and Time

One of the jobs iDVD performs is to compress your movies into MPEG format, the standard method of storing video on DVD-Video discs. Like image and audio compression, MPEG is a lossy format: the final product lacks some of the quality of the original. But as with image and audio compression, the amount of quality loss depends on the degree to which the original material is compressed.

Like JPEG, MPEG performs spatial compression that reduces the storage requirements of individual images. But video adds the dimension of time, and MPEG takes this into account by also performing temporal compression.

The key to temporal compression is to describe only those details that have changed since the previous video frame. In an MPEG video stream, some video frames contain the entire image; these are called I-frames. There are usually two I frames per second.

An I-frame describes an entire scene: "There's a basketball on a concrete driveway."

Sandwiched between those I-frames are much smaller frames that don't contain the entire image, but rather only those pixels that have changed since the previous frame.

"It's rolling toward the street."

To perform temporal compression, the video frame is divided into a grid of blocks, and each square is examined to see if anything has changed. Areas that haven't changedsuch as the stationary background in this example are simply repeated in the next frame.

This is why video with relatively little motion often tends to look better than video that contains a great deal of motion. When little changes from one frame to the next, the quality of each frame can be higher.