Property Loading/Saving

To allow for an easy serializing/desrializing of gstreamer filter state tcambin and tcamsrc have gobject properties tcam-properties and tcam-properties-json.

You can use these properties to load/save the current tcam-properties state.

tcam-properties

The property tcam-properties allows for easy initialization at the command line.

E.g.:

gst-launch-1.0 tcambin tcam-properties=tcam,ExposureAuto=Off,ExposureTime=33333 ! ...

tcam-properties is of type GstStructure. You can read/write this structure via the gobject interface.

Setting several properties via GstStructure:

c

python

GstElement* source = ...;

// Create a new structure
GstStructure* new_property_struct = gst_structure_new_empty("tcam-properties");

// Change 2 properties so that we can see a 'difference'
gst_structure_set(new_property_struct,
                  "ExposureAuto", G_TYPE_STRING, "Off",
                  "ExposureTime", G_TYPE_DOUBLE, 35000.,
                  NULL);

GValue new_state = G_VALUE_INIT;
g_value_init(&new_state, GST_TYPE_STRUCTURE);
gst_value_set_structure(&new_state, new_property_struct);

// Set the new property settings
g_object_set_property(G_OBJECT(source), "tcam-properties", &new_state);

source = ...

new_property_struct = Gst.Structure.empty("tcam-properties")

new_property_struct.set_value("ExposureAuto", "Off")
new_property_struct.set_value("ExposureTime", 35000.0)

source.set_property("tcam-properties", new_state)

Reading the GstStructure of a opened device:

c

python

GstElement* src = ...;

// Initialize the GValue
GValue current_properties = G_VALUE_INIT;
g_value_init(&current_properties, GST_TYPE_STRUCTURE);

// Get the GObject property
g_object_get_property(G_OBJECT(source), "tcam-properties", &current_properties);

// get a string to print the current property state
char* string = gst_structure_to_string(gst_value_get_structure(&current_properties));
printf("Current properties:\n%s\n", string);
g_free(string); // free the string

g_value_unset(&current_properties); // free the GstStructure in the GValue

source = ...

state = source.get_property("tcam-properties")
print(f"New state of device is:\n{state.to_string()}")

Note

• If a property is locked when loading it, writing to the property is retried after all other properties are written. (This circumvents the problem of property order for e.g. “ExposureTime” and “ExposureAuto”)
• Failed writing/reading of properties gets logged to the gstreamer log.
• Writing to the property in GST_STATE_NULL sets an internal cache which gets applied in the state transition to GST_STATE_READY

tcam-properties-json

The property tcam-properties-json provides the current device properties as a json string.

A simple state dump would look like this:

{
    "ExposureTime": 35000.0,
    "ExposureAuto": "Off"
}

A Property consists out of the fields ‘name’ and ‘value’.

• name is a string containing the property identifier.
• value contains the actual value. The interpretation of this field is done automatically.

In the case of an error a message will the written to the GStreamer log. The property will be ignored.

Properties of the type ‘button’ are not added to a state description.