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