Track aperture mode dimension atoms

Overview

A video track in a QuickTime Movie can signal clean aperture and pixel aspect ratio information through image description extensions. The clean aperture defines the part of the encoded pixels to be displayed. The pixel aspect ratio is the aspect ratio of the encoded pixels. Conceptually the encoded pixels are decompressed, stretched (or shrunk) based on the pixel aspect ratio, and extra pixels are cropped off according to the clean aperture.

Considering this context, the dimensions recorded in the image description define the dimensions of the encoded pixels (encoded dimensions). What’s actually displayed is a result of applying the pixel aspect ratio and the clean aperture (display dimensions).

Although the result of applying the clean aperture and the pixel aspect ratio is what is intended for final display, there are cases where it is useful to display all the pixels that exist in the content for various different purposes. Readers parsing QuickTime movies require information allowing these different display modes in order to provide this flexibility:

Clean Mode

In this mode both the clean aperture and the pixel aspect ratio are applied. The dimensions of the track become equal to the clean dimensions which are equal to the display dimensions (with conformed contents).

Production Mode

This mode applies the pixel aspect ratio but not the clean aperture. The image is presented in the correct aspect ratio, but the extra pixels outside of the image that exists in the source material will be presented. The track dimensions are equal to the result of applying the pixel aspect ratio.

Classic Mode

This mode displays the image without applying either the pixel aspect ratio or the clean aperture. The image is displayed using the track header dimensions, meaning the decompressed picture is scaled into the track header dimensions if the encoded dimensions are different.

Encoded Pixels

The encoded pixels are displayed intact in this mode. Under this mode the track dimensions are equal to the encoded dimensions. No scaling or transformation takes place.

The information for each of these presentation modes are represented in the optional track aperture mode dimensions atoms.

Topics

Track aperture mode dimension atoms

• Track aperture mode dimensions atom ('tapt')
• Track clean aperture dimensions atom ('clef')
• Track production aperture dimensions atom ('prof')
• Track encoded pixels dimensions atom ('enof')

See Also

Describing tracks

• Track atom ('trak')
• Track header atom ('tkhd')
• Track exclude from autoselection atom ('txas')
• Clipping atom ('clip')
• Clipping region atom ('crgn')
• Track matte atom ('matt')
• Compressed matte atom ('kmat')
• Edit atom ('edts')
• Edit list atom ('elst')
• Playing with edit lists
• Track load settings atom ('load')
• Track reference atoms
• Track input map atoms