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

Profile Atom GuidelinesUniversal FeaturesWriter ResponsibilitiesA writer of the Average Video Frame Rate feature should record a 16.16 fixed-point value that is equal to orgreater than the average video frame rate. A writer (such as a CE device) may choose to record a constant forthe feature based on its current recording mode, even if the actual frame rate is less.A writer of a new video track (such as a CE device recorder) may set the average frame rate feature value to avalue set during video encoder initialization, so long as this frame rate is not exceeded by the actual average,as determined by the feature value algorithm described below.If the average calculated frame rate is fractional (such as 22.3 fps), a writer may choose to round the value upto the nearest integer value (such as 23.0 fps for 22.3 fps).Feature Value AlgorithmThis feature value is calculated by dividing the the total number of frames (samples) by the duration of thetrack. It is permissible to omit the first and last frames from this calculation, as they may have significantlydifferent duration than the average.Reader ResponsibilitiesA reader of this feature code should understand that each frame is a video sample with its own independentand explicit duration. While it is possible for all frames to have the same duration, it is equally possible for theduration of any frame to be radically different from any other. Therefore, the average frame rate may not alwaysbe meaningful information.The reader should not interpret the value of this feature as the current frame rate. To determine the currentframe rate, the reader should use the video track’s sample table.CommentsA writer may choose to round up any fractional value of the fixed-point number to the nearest 16-bit integerleaving the lower 16 bits of the Fixed value set to 0. So, in the case of the 29.97 approximation of 0x001DF853,the writer could round this up to 0x001E0000 (which equals 30).Video Variable Frame Rate IndicationContaining profile atomTrack (video), moviepart-ID0x20202020 (universal feature)2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.416