QuickTime Movie Characteristics
This section discusses the characteristics that govern playing and storing movies, tracks, and media structures. This section has been divided into the following topics:
“Movie Characteristics” discusses the time, display, and sound characteristics of a QuickTime movie.
“Track Characteristics” describes the characteristics of a movie track.
“Media Characteristics” discusses the characteristics of a media.
“Spatial Properties” describes how the Movie Toolbox displays a movie, including how the data from each media is collected and transformed prior to display.
“The Transformation Matrix” describes how matrix operations transform visual elements prior to display.
“Audio Properties” describes how the Movie Toolbox works with a movie’s sound tracks.
In this section:
Movie Characteristics
Track Characteristics
Media Characteristics
Spatial Properties
The Transformation Matrix
Movie Characteristics
A QuickTime movie is represented as a private data structure. Your application never works with individual fields in that data structure. Rather, the Movie Toolbox provides functions that allow you to work with a movie’s characteristics. Figure 1-7 shows some of the characteristics of a QuickTime movie.
Every QuickTime movie has some state information, including a creation time and a modification time. These times are expressed in standard Macintosh time format, representing the number of seconds since midnight, January 1, 1904. The creation time indicates when the movie was created. The modification time indicates when the movie was last modified and saved.
Each movie has its own time coordinate system and time scale. Any time values that relate to the movie must be defined using this time scale and must be between 0 and the movie’s duration.
A movie’s preview is defined by its starting time and duration. Both of these time values are expressed in terms of the movie’s time scale. A movie’s poster is defined by its time value, which is in terms of the movie’s time scale. You assign tracks to the movie preview and the movie poster by calling the Movie Toolbox functions that are described later in this chapter.
Your current position in a movie is defined by the movie’s current time. If the movie is currently playing, this time value is changing. When you save a movie in a movie file, the Movie Toolbox updates the movie’s current time to reflect its current position. When you load a movie from a movie file, the Movie Toolbox sets the movie’s current time to the value found in the movie file.
The Movie Toolbox provides high-level editing functions that work with a movie’s current selection. The current selection defines a segment of the movie by specifying a start time, referred to as the selection time, and a duration, called the selection duration. These time values are expressed using the movie’s time scale.
For each movie currently in use, the Movie Toolbox maintains an active movie segment. The active movie segment is the part of the movie that your application is interested in playing. By default, the active movie segment is set to be the entire movie. You may wish to change this to be some segment of the movie; for example, if you wish to play a user’s selection repeatedly. By setting the active movie segment, you guarantee that the Movie Toolbox uses no samples from outside of that range while playing the movie.
A movie’s display characteristics are specified by a number of elements. The movie has a movie clipping region and a 3-by-3 transformation matrix. The Movie Toolbox uses these elements to determine the spatial characteristics of the movie. See “Spatial Properties” for a complete description of these elements and how they are used by the Movie Toolbox.
When you save a movie, you can establish preferred settings for playback rate and volume. The preferred playback rate is called the preferred rate. The preferred playback volume is called the preferred volume. These settings represent the most natural values for these movie characteristics. When the Movie Toolbox loads a movie from a movie file, it sets the movie’s volume to this preferred value. When you start playing the movie, the Movie Toolbox uses the preferred rate. You can then use Movie Toolbox functions to change the rate and volume during playback.
The Movie Toolbox allows your application to store its own data along with a movie. You define the format and content of these data objects. This application-specific data is called user data. You can use these data objects to store both text and binary data. For example, you can use text user data items to store a movie’s copyright and credit information. The Movie Toolbox provides functions that allow you to set and retrieve a movie’s user data. This data is saved with the movie when you save the movie.
Track Characteristics
A QuickTime track is represented as a private data structure. Your application never works with individual fields in that data structure. Rather, the Movie Toolbox provides functions that allow you to work with a track’s characteristics. Figure 1-8 shows the characteristics of a QuickTime track.
As with movies, each track has some state information, including a creation time and a modification time. These times are expressed in standard Macintosh time format, representing the number of seconds since midnight, January 1, 1904. The creation time indicates when the track was created. The modification time indicates when the track was last modified and saved.
Each track has its own duration value, which is expressed in the time scale of the movie that contains the track.
As has been discussed, movies can contain more than one track. In fact, a movie can contain more than one track of a given type. You might want to create a movie with several sound tracks, each in a different language, and then activate the sound track that is appropriate to the user’s native language. Your application can manage these collections of tracks by assigning each track of a given type to an alternate group. You can then choose one track from that group to be enabled at any given time. You can select a track from an alternate group based on its language or its playback quality. A track’s playback quality indicates its suitability for playback in a given environment. All tracks in an alternate group should refer to the same type of data.
A track’s display characteristics are specified by a number of elements, including track width, track height, a transformation matrix, and a clipping region. See “Spatial Properties” for a complete description of these elements and how they are used by the Movie Toolbox.
Each track has a current volume setting. This value controls how loudly the track plays relative to the movie volume.
Each track contains a media edit list. The edit list contains entries that define how the track’s media is to be used in the movie that contains the track. Each entry in the edit list indicates the starting time and duration of the media segment, along with the playback rate for that segment.
Each track has an associated media. See the next section for more information about media structures and their characteristics.
The Movie Toolbox allows your application to store its own user data along with a track. You define the format and content of these data objects. The Movie Toolbox provides functions that allow you to set and retrieve a track’s user data. This data is saved with the track when you save the movie.
Media Characteristics
As is the case with movies and tracks, a QuickTime media is represented as a private data structure. Your application never works with individual fields in that data structure. Rather, the Movie Toolbox provides functions that allow you to work with a media’s characteristics. Figure 1-9 shows the characteristics of a QuickTime media.
Each QuickTime media has some state information, including a creation time and a modification time. These times are expressed in standard Macintosh time format, representing the number of seconds since midnight, January 1, 1904. The creation time indicates when the media was created. The modification time indicates when the media was last modified and saved.
Each media has its own time coordinate system, which is defined by its time scale and duration. Any time values that relate to the media must be defined in terms of this time scale and must be between 0 and the media’s duration.
A media contains information that identifies its language and playback quality. These values are used when selecting one track to present from the tracks in an alternate group.
The media specifies a media handler, which is responsible for the details of loading, storing, and playing media data. The media handler can store state information in the media. This information is referred to as media information. The media information identifies where the media’s data is stored and how to interpret that data. Typically, this data is stored in a data reference, which identifies the file that contains the data and the type of data that is stored in the file.
The Movie Toolbox allows your application to store its own user data along with a media. You define the format and content of these data objects. The Movie Toolbox provides functions that allow you to set and retrieve a media’s user data. This data is saved with the media when you save the movie.
Spatial Properties
When you play a movie that contains visual data, the Movie Toolbox gathers the movie’s data from the appropriate tracks and media structures, transforms the data as appropriate, and displays the results in a window. The Movie Toolbox uses only those tracks that
are not empty
contain media structures that reference data at a specified time
are enabled in the current movie mode (standard playback, poster mode, or preview mode)
Consequently, the size, shape, and location of many of these regions may change during movie playback. This process is quite complicated and involves several phases of clipping and resizing.
The Movie Toolbox shields you from the intricacies of this process by providing two high-level functions, GetMovieBox and SetMovieBox, which allow you to place a movie box at a specific location in the display coordinate system. When you use these functions, the Movie Toolbox automatically adjusts the contents of the movie’s matrix to satisfy your request.
Figure 1-10 provides an overview of the entire process of gathering, transforming, and displaying visual data. Each track defines its own spatial characteristics, which are then interpreted within the context of the movie’s spatial characteristics.
This section describes the process that the Movie Toolbox uses to display a movie. The process begins with the movie data and ends with the final movie display. The phases, which are described in this section, include
the creation of a track rectangle (see Figure 1-11)
the clipping of a track’s image (see Figure 1-12)
the transformation of a track into the movie coordinate system (see Figure 1-13)
the clipping of a movie image (see Figure 1-14)
the transformation of a movie into the display coordinate system (see Figure 1-15)
the clipping of a movie for final display (see Figure 1-16)
Note: Throughout this book, the term time coordinate system denotes QuickTime’s time-based system. All other instances of the term coordinate system refer to graphic coordinates.
Each track defines a rectangle into which its media is displayed. This rectangle is referred to as the track rectangle, and it is defined by the track width and track height values assigned to the track. The upper-left corner of this rectangle defines the origin point of the track’s coordinate system.
The media handler associated with the track’s media is responsible for displaying an image into this rectangle. This process is shown in Figure 1-11.
The Movie Toolbox next mattes the image in the track rectangle by applying the track matte and the track clipping region. This does not affect the shape of the image; only the display. Both the track matte and the track clipping region are optional.
A track matte provides a mechanism for mixing images. Mattes contain several bits per pixel and are defined in the track’s coordinate system. The matte can be used to perform a deep-mask operation on the image in the track rectangle. The Movie Toolbox displays the weighted average of the track and its destination based on the corresponding pixel value in the matte.
The track clipping region is a QuickDraw region that defines a portion of the track rectangle to retain. The track clipping region is defined in the track’s coordinate system. This clipping operation creates the track boundary region, which is the intersection of the track rectangle and the track clipping region.
This process and its results are shown in Figure 1-12.
After clipping and matting the track’s image, the Movie Toolbox transforms the resulting image into the movie’s coordinate system. The Movie Toolbox uses a 3-by-3 transformation matrix to accomplish this operation (see “The Transformation Matrix” for a discussion of matrix operations in the Movie Toolbox). The image inside the track boundary region is transformed by the track’s matrix into the movie coordinate system. The resulting area is bounded by the track movie boundary region. Figure 1-13 shows the results of this transformation operation.
The Movie Toolbox performs this portion of the process for each track in the movie. Once all of the movie’s tracks have been processed, the Movie Toolbox proceeds to transform the complete movie image for display.
The union of all track movie boundary regions for a movie defines the movie’s movie boundary region. The Movie Toolbox combines a movie’s tracks into this single region where layers are applied. Therefore, tracks in back layers may be partially or completely obscured by tracks in front layers. The Movie Toolbox clips this region to obtain the clipped movie boundary region. The movie’s movie clipping region defines the portion of the movie boundary region that is to be used. Figure 1-14 shows the process by which a movie is clipped and the resulting clipped movie boundary region.
After clipping the movie’s image, the Movie Toolbox transforms the resulting image into the display coordinate system. The Movie Toolbox uses a 3-by-3 transformation matrix to accomplish this operation (see “The Transformation Matrix” for a complete discussion of matrix operations in the Movie Toolbox). The image inside the clipped movie boundary region is transformed by the movie’s matrix into the display coordinate system. The resulting area is bounded by the movie display boundary region. Figure 1-15 shows the results of this step.
The rectangle that encloses the movie display boundary region is called the movie box, as shown in Figure 1-16. You can control the location of a movie’s movie box by adjusting the movie’s transformation matrix.
Once the movie is in the display coordinate system (that is, the QuickDraw graphics world), the Movie Toolbox performs a final clipping operation to generate the image that is displayed. The movie is clipped with the movie display clipping region. When a movie is displayed, the Movie Toolbox ignores the graphics port’s clipping region; this is why there is a movie display clipping region. Figure 1-16 shows this operation.
The Transformation Matrix
The Movie Toolbox makes extensive use of transformation matrices to define graphical operations that are performed on movies when they are displayed. A transformation matrix defines how to map points from one coordinate space into another coordinate space. By modifying the contents of a transformation matrix, you can perform several standard graphical display operations, including translation, rotation, and scaling. The Movie Toolbox provides a set of functions that make it easy for you to manipulate translation matrices. Those functions are discussed in QuickTime Movie Internals Guide. The remainder of this section provides an introduction to matrix operations in a graphical environment.
The matrix used to accomplish two-dimensional transformations is described mathematically by a 3-by-3 matrix. Figure 1-17 shows a sample 3-by-3 matrix. Note that QuickTime assumes that the values of the matrix elements u and v are always 0.0, and the value of matrix element w is always 1.0.
During display operations, the contents of a 3-by-3 matrix transform a point (x,y) into a point (x',y') by means of the following equations:
x' = ax + cy + t(x)
y' = bx + dy + t(y)
For example, the matrix shown in Figure 1-18 performs no transformation. It is referred to as the identity matrix.
Using the formulas discussed earlier, you can see that this matrix would generate a new point (x',y') that is the same as the old point (x,y):
x' = 1x + 0y + 0
y' = 0x + 1y + 0
x' = y and y' = y
In order to move an image by a specified displacement, you perform a translation operation. This operation modifies the x and y coordinates of each point by a specified amount. The matrix shown in Figure 1-19 describes a translation operation.
You can stretch or shrink an image by performing a scaling operation. This operation modifies the x and y coordinates by some factor. The magnitude of the x and y factors governs whether the new image is larger or smaller than the original. In addition, by making the x factor negative, you can flip the image about the x-axis; similarly, you can flip the image horizontally, about the y-axis, by making the y factor negative. The matrix shown in Figure 1-20 describes a scaling operation.
Finally, you can rotate an image by a specified angle by performing a rotation operation. You specify the magnitude and direction of the rotation by specifying factors for both x and y. The matrix shown in Figure 1-21 rotates an image counterclockwise by an angle q.
You can combine matrices that define different transformations into a single matrix. The resulting matrix retains the attributes of both transformations. For example, you can both scale and translate an image by defining a matrix similar to that shown in Figure 1-22.
You combine two matrices by concatenating them. Mathematically, the two matrices are combined by matrix multiplication. Note that the order in which you concatenate matrices is important; matrix operations are not commutative.
Transformation matrices used by the Movie Toolbox contain the following data types:
[0] [0] Fixed [1] [0] Fixed [2] [0] Fract |
[0] [1] Fixed [1] [1] Fixed [2] [1] Fract |
[0] [2] Fixed [1] [2] Fixed [2] [2] Fract |
Each cell in this table represents the data type of the corresponding element of a 3-by-3 matrix. All of the elements in the first two columns of a matrix are represented by Fixed values. Values in the third column are represented as Fract values. The Fract data type specifies a 32-bit, fixed-point value that contains 2 integer bits and 30 fractional bits. This data type is useful for accurately representing numbers in the range from -2 to 2.
Last updated: 2007-01-08
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