QuickTime File Format Specification: Hot Spot Image Tracks

< Previous PageNext Page >

Hide TOC

Hot Spot Image Tracks

When a panorama contains hot spots, the movie file contains a hot spot image track, a video track that contains a parallel panorama, with the hot spots designated by colored regions. Each diced frame of the hot spot panoramic image must be compressed with a lossless compressor (such as QuickTime’s graphics compressor). The dimensions of the hot spot panoramic image are usually the same as those of the image track’s panoramic image, but this is not required. The dimensions must, however, have the same aspect ratio as the image track’s panoramic image. A hot spot image track should be 8 bits deep.

In this section:

Low-Resolution Image Tracks
Track Reference Entry Structure

Low-Resolution Image Tracks

It’s possible to store one or more low-resolution versions of a panoramic image in a movie file; those versions are called low-resolution image tracks. If there is not enough memory at runtime to use the normal image track, QuickTime VR uses a lower resolution image track if one is available. A low-resolution image track contains diced frames just like the higher resolution track, but the reconstructed panoramic image is half the height and half the width of the higher resolution image.

Important: The panoramic images in the lower resolution image tracks and the hot spot image tracks, if present, must have the same orientation (horizontal or vertical) as the panorama image track.

Track Reference Entry Structure

Since there are no fields in the pano sample data atom to indicate the presence of low-resolution image tracks, a separate sibling atom must be added to the panorama sample atom container. The track reference array atom contains an array of track reference entry structures that specify information about any low-resolution image tracks contained in a movie. Its atom type is kQTVRTrackRefArrayAtomType ('tref').

A track reference entry structure is defined by the QTVRTrackRefEntry data type:

typedef struct QTVRTrackRefEntry {

    UInt32                              trackRefType;

    UInt16                              trackResolution;

    UInt32                              trackRefIndex;

} QTVRTrackRefEntry;

trackRefType: The track reference type.
trackResolution: The track resolution.
trackRefIndex: The index of the track reference.

The number of entries in the track reference array atom is determined by dividing the size of the atom by sizeof (QTVRTrackRefEntry).

kQTVRPreviewTrackRes is a special value for the trackResolution field in the QTVRTrackRefEntry structure. This is used to indicate the presence of a special preview image track.