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66 lines
3 KiB
Markdown
66 lines
3 KiB
Markdown
# Controller
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The controller subsystem allows to automate element property changes. It
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works so that all parameter changes are time based and elements request
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property updates at processing time.
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## Element view
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Elements don’t need to do much. They need to: - mark object properties
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that can be changed while processing with `GST_PARAM_CONTROLLABLE` -
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call `gst_object_sync_values (self, timestamp)` in the processing
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function before accessing the parameters.
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All ordered property types can be automated (int, double, boolean,
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enum). Other property types can also be automated by using special
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control bindings. One can e.g. write a control-binding that updates a
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text property based on timestamps.
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## Application view
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Applications need to setup the property automation. For that they need
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to create a `GstControlSource` and attach it to a property using
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`GstControlBinding`. Various control-sources and control-bindings exist.
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All control sources produce control value sequences in the form of
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gdouble values. The control bindings map them to the value range and
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type of the bound property.
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One control-source can be attached to one or more properties at the same
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time. If it is attached multiple times, then each control-binding will
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scale and convert the control values to the target property type and
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range.
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One can create complex control-curves by using a
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`GstInterpolationControlSource`. This allows the classic user editable
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control-curve (often seen in audio/video editors). Another way is to use
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computed control curves. `GstLFOControlSource` can generate various
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repetitive signals. Those can be made more complex by chaining the
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control sources. One can attach another control-source to e.g. modulate
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the frequency of the first `GstLFOControlSource`.
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In most cases `GstDirectControlBinding` will be the binding to be used.
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Other control bindings are there to handle special cases, such as having
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1-4 control- sources and combine their values into a single guint to
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control a rgba-color property.
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## TODO
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* control-source value ranges - control sources should ideally emit values
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between \[0.0 and 1.0\] - right now lfo-control-sources emits values
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between \[-1.0 and 1.0\] - we can make control-sources announce that or
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fix it in a lfo2-control-source
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* ranged-control-binding - it might be a nice thing to have a
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control-binding that has scale and offset properties - when attaching a
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control-source to e.g. volume, one needs to be aware that the values go
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from \[0.0 to 4.0\] - we can also have a "mapping-mode"={AS\_IS,
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TRANSFORMED} on direct-control-binding and two extra properties that are
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used in TRANSFORMED mode
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* control-setup descriptions - it would be nice to have a way to parse a
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textual control-setup description. This could be used in gst-launch and
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in presets. It needs to be complemented with a formatter (for the preset
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storage or e.g. for debug logging). - this could be function-style:
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direct(control-source=lfo(waveform=*sine*,offset=0.5)) or gst-launch
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style (looks weird) lfo wave=sine offset=0.5 \! direct .control-source
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