The ImageDescription QuickTime structure contains informations that describe the characteristics of a compressed image or sequence.
The QTPlgImageDesc class provides you with properties and methods that let you whether access or create new instances of such a QuickTime description structure. Moreover, it implements a few properties useful when dealing with compression of sequences. Also, you can use instances of this class as a convenient container for image settings that you will use in any way you like it. Many of the QTPlugin classes make use of this class, or its , in order to allow you to get or set the appropriate image parameters.
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The QTPlgImageDescComp class constructors are called when you create a new instance of the class, while the ˜QTPlgImageDescComp class destructor, is called automatically when the class is no more in use. Also, you can invoke the class destructor by setting the instance of the class to nil.
Once a new class instance is returned, it is good practice to check the class property to be sure that the new object can be used. See the validity requirements for more details.
The class provides you with the following initializer methods:
()
This is the default constructor. Use this constructor to create a completely new data descriptor container in memory.
All of the class properties are intialized to 0
(other as QTPlgImageDescComp)
This is the copy constructor. The properties of the object to be copied will be used to initialize the
new class instance properties
specifies the maximum number of frames allowed between key frames.
The compressor determines the optimum placement for key frames based upon the amount of
redundancy between adjacent images in the sequence. Consequently, the compressor may insert key
frames more frequently than you have requested. However, the compressor never places fewer key frames
than is indicated by the setting of this property. The compressor ignores this parameter
if you have not requested temporal compression (that is, you have set the to 0 the
class
property).
Setting this property to 0 indicates that there are no key frames in the sequence
specifies the maximum number of bytes of compressed data your application wants to receive per second.
Use this parameter to modulate the rate at which the component passes compressed data to your application.
This can be useful to account for hardware limitations during sequence playback
specifies the duration of a single sample
specifies the minimum acceptable spatial quality. In order to meet your specified
, the compressor component
may have to adjust the spatial quality setting. Use this parameter to set a minimum level, which the component
may not exceed
specifies the minimum acceptable temporal quality. As with spatial quality, in order to meet your specified
,
the compressor component may have to adjust the temporal quality setting. Use this parameter to set a minimum level,
which the component may not exceed
specifies the the number of frames per second in the image sequence to be compressed.
This value will be used when compressing a sequence of images
enables frame reordering. In order to encode B frames, a compressor must reorder frames, which means that the order in which they will be
emitted and stored (the decode order) is different from the order in which they were presented to the compressor (the display order). Note that,
in order to encode using B frames, you must enable frame reordering
requires QuickTime 7.0 or later
enables multipass compression
requires QuickTime 7.0 or later
enables temporal compression. Note that, in order to encode using B frames and/or P frames, you must set this property to
true