Gstreamer Pad

• The relationship between pad and caps

                         |-name_template(eg:wavparse_src, wavparse_sink)
                         |
                         |-direction(src or sink)
    GstStaticPadTemplate-|
            |            |-presence(Sometimes, Always, request)
            |            |
            |            |                  |-GstCaps(String is stored in GstStruct)
            |            |-GstStaticCaps----|
            |                               |-String(support caps)
     gst_static_pad_template_get()
            |
            |           |-GstObject
            V           |
    GstPadTemplate------|-direction
            |           |
            |           |-presence
            |           |
            |           |-GstCaps
            |
     gst_pad_new_from_static_template()
            |
            |
            V    |-GstObject
    GstPad-------|-GstPadTemplate
                 |...
  

• Add pad into element and set relation

  gst_element_add_pad(element, pad)
    {
        _set_parent(element, pad) {
            pad->parent = element
        }
    
        if(pad->direction == src)
            element->srcpads = pad
        else if(pad->direction == sink)
            element->sinkpads = pad
    
        element->pads = pad
    }
    
    +---------+         +------------+         +---------+
    |      parent------>|  element   |<------parent      |
    | Gstpad  |<-----sinkpad      srcpad------>|  GstPad |
    +---------+         +------------+         +---------+
    
  

• Set pad active from srcpad to sinkpad
  1. element state change from READY to PAUSE, set pad active

     gst_element_pads_activate(element, TRUE)
       {
           //set srcpad
           activate_pads(element->srcpads, TRUE);
           //set sinkpad
           activate_pads(element->sinkpads, TRUE);
       }
     

  2. check pad current mode and call pad->activefunc

     gst_pad_set_active(pad)
       {
           mode = GST_PAD_ACTIVATE_MODE(pad);
           if(mode == GST_ACTIVATE_PUSH || mode == GST_ACTIVATE_PULL) {
               //pad already activation
               return TRUE;
           }
           else if (mode == GST_ACTIVATE_NONE)
               GST_PAD_ACTIVATEFUNC(pad) (pad);
       }
     

  3. call pad->activepushfunc or pad->activepullfunc, default is pushfunc

     gst_pad_activate_push(pad)
       {
           GST_PAD_ACTIVATE_MODE(pad) = GST_ACTIVATE_PUSH;
       
           GST_PAD_ACTIVATEPUSHFUNC(pad) (pad);
       }
       gst_pad_activate_pull(pad)
       {
           GST_PAD_ACTIVATE_MODE(pad) = GST_ACTIVATE_PULL;
       
           GST_PAD_ACTIVATEPULLFUNC(pad) (pad);
       }