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Optimizing QuickTime VR Movies for Web Playback

Originally, both QuickTime movies and QuickTime VR movies had to be completely downloaded to the user’s local hard disk before they could be viewed. Starting with QuickTime 2.5, if the movie data is properly laid out in the file, standard linear QuickTime movies can be viewed almost immediately. The frames that have been downloaded so far are shown while subsequent frames continue to be downloaded.

The important change that took place to allow this to happen was for QuickTime to place global movie information at the beginning of the file. Originally, this information was at the end of the file. After that, the frame data simply needs to be in order in the file. Similarly, QuickTime VR files also need to be laid out in a certain manner in order to get some sort of quick feedback when viewing on the web. Roughly speaking this involves writing out all of the media samples in the file in a particular order. Apple now provides a movie export component that does this for you: the QTVR Flattener.

In this section:

The QTVR Flattener
Sample Atom Container for the QTVR Flattener

The QTVR Flattener

The QTVR Flattener is a movie export component that converts an existing QuickTime VR single node movie into a new movie that is optimized for the Web. Not only does the flattener reorder the media samples, but for panoramas it also creates a small preview of the panorama. When viewed on the Web, this preview appears after 5% to 10% of the movie data has been downloaded, allowing users to see a lower-resolution version of the panorama.

Using the QTVR flattener from your application is quite easy. After you have created the QuickTime VR movie, you simply open the QTVR Flattener component and call the MovieExportToFile routine as shown in Listing 5-17.

Listing 5-17  Using the flattener

ComponentDescription desc;

Component flattener;

ComponentInstance qtvrExport = nil;

desc.componentType = MovieExportType;

desc.componentSubType = MovieFileType;

desc.componentManufacturer = QTVRFlattenerType;

flattener = FindNextComponent(nil, &desc);

if (flattener) qtvrExport = OpenComponent (flattener);

if (qtvrExport)

    MovieExportToFile (qtvrExport, &myFileSpec, myQTVRMovie,  nil, 0, 0);

The code fragment shown in Listing 5-17 creates a flattened movie file specified by the myFileSpec parameter. If your QuickTime VR movie is a panorama, the flattened movie file includes a quarter size, blurred JPEG, compressed preview of the panorama image.

You can present users with the QTVR Flattener’s own dialog box to allow them to choose options such as how to compress the preview image or to select a separate preview image file. Use the following code to show the dialog box:

        err = MovieExportDoUserDialog (qtvrExport, myQTVRMovie,  nil, 0, 0,  &cancel);

If the user cancels the dialog box, then the Boolean cancel is set to true.

If you do not want to present the user with the flattener’s dialog box, you can communicate directly with the component by using the MovieExportSetSettingsFromAtomContainer routine as described in the following paragraphs.

If you want to specify a preview image other than the default, you need to create a special atom container and then call MovieExportSetSettingsFromAtomContainer before calling MovieExportToFile. You can specify how to compress the image, what resolution to use, and you can even specify your own preview image file to be used. The atom container you pass in can have various atoms that specify certain export options. These atoms must all be children of a flattener settings parent atom.

The preview resolution atom is a 16-bit value that allows you to specify the resolution of the preview image. This value, which defaults to kQTVRQuarterRes, indicates how much to reduce the preview image.

The blur preview atom is a Boolean value that indicates whether to blur the image before compressing. Blurring usually results in a much more highly compressed image. The default value is true.

The create preview atom is a Boolean value that indicates whether a preview image should be created. The default value is true.

The import preview atom is a Boolean value that is used to indicate that the preview image should be imported from an external file rather than generated from the image in the panorama file itself. This allows you to have any image you want as the preview for the panorama. You can specify which file to use by also including the import specification atom, which is an FSSpec data structure that identifies the image file. If you do not include this atom, then the flattener presents the user with a dialog box asking the user to select a file. The default for import preview is false. If an import file is used, the image is used at its natural size and the resolution setting is ignored.

Sample Atom Container for the QTVR Flattener

The sample code in Listing 5-18 creates an atom container and adds atoms to indicate an import preview file for the flattener to use.

Listing 5-18  Specifying a preview file for the flattener to use

Boolean yes = true;

QTAtomContainer exportData;

QTAtom parent;

err = QTNewAtomContainer(&exportData);

// create a parent for the other settings atoms

err = QTInsertChild (exportData, kParentAtomIsContainer,

            QTVRFlattenerParentAtomType, 1, 0, 0, nil, &parent);

// Add child atom to indicate we want to import the preview from  a file

err = QTInsertChild (exportData, parent, QTVRImportPreviewAtomType,  1, 0,

            sizeof (yes), &yes, nil);

// Add child atom to tell which file to import

err = QTInsertChild (exportData, parent, QTVRImportSpecAtomType,  1, 0,

            sizeof (previewSpec), &previewSpec, nil);

// Tell the export component

MovieExportSetSettingsFromAtomContainer (qtvrExport, exportData);

Overriding the compression settings is a bit more complicated. You need to open a standard image compression dialog component and make calls to obtain an atom container that you can then pass to the QTVR Flattener component.

Listing 5-19  Overriding the compression settings

ComponentInstance sc;

QTAtomContainer compressorData;

SCSpatialSettings ss;

sc = OpenDefaultComponent(StandardCompressionType,StandardCompressionSubType);

ss.codecType = kCinepakCodecType;

ss.codec = nil;

ss.depth = 0;

ss.spatialQuality = codecHighQuality

err = SCSetInfo(sc, scSpatialSettingsType, &ss);

err = SCGetSettingsAsAtomContainer(sc, &compressorData);

MovieExportSetSettingsFromAtomContainer (qtvrExport, compressorData);

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