Quicktime File Format (2012-08-14).pdf

Movie AtomsTrack AtomsTrack Exclude From Autoselection AtomsSome alternate tracks contain something other than a direct translation (or untranslated written form) of theprimary content. Commentary tracks are one example. These tracks should not be automatically selected. Thepresence of the Track Exclude From Autoselection atom in a track indicates that this track should not beautomatically selected.Such tracks should have user-readable names that help users to identify the purpose of the track. These namesare stored in one or more track name ('tnam') atoms, each translated into a different language, within a userdata ('udta') atom within the 'trak' atom.The type of the Track Exclude From Autoselection atom is 'txas'. This atom, if used, must be somewhereafter the 'tkhd' atom.Track exclude from autoselection atoms contain the following data elements.SizeA 32-bit integer that specifies the number of bytes in the track exclude from autoselection atom. Thismust be 8, as this atom must contain no data.TypeA 32-bit integer that identifies the atom type; this field must be set to 'txas'.Track Aperture Mode Dimension AtomsA video track in a QuickTime Movie can signal clean aperture and pixel aspect ratio information through imagedescription extensions. The clean aperture defines the part of the encoded pixels to be displayed. The pixelaspect 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 cleanaperture.Note QuickTime tracks define simple dimensions for their content in the track header dimensions.In the absence of a track aperture mode dimensions atom, the dimensions in the track header areused for all modes.Considering this context, the dimensions recorded in the image description define the dimensions of theencoded pixels (encoded dimensions). What's actually displayed is a result of applying the pixel aspect ratioand the clean aperture (display dimensions).2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.61