What does passthrough & transform_ip_on_passthrough do?

Plugin and library development
1

When setting BaseTransform to passthrough mode, and with transform_ip_on_passthrough set, the documentation states:

gboolean transform_ip_on_passthrough;

If set to TRUE, transform_ip will be called in passthrough mode. The passed buffer might not be writable. When FALSE, neither transform nor transform_ip will be called in passthrough mode. Set to TRUE by default.

What would cause a buffer to be not writable in passthrough mode vs when passthrough mode is off? Does gstreamer do anything differently when calling transform_ip when in passthrough mode vs when passthrough mode is off?

I have some custom elements I have written which are not running with passthrough mode on that could be & I am wondering if there are any benefits to fixing them to have it enabled.

Thank you for any insight you might be able to provide.

2

I believe I have found the relevant section of gstbasetransform.c;

  /* now perform the needed transform */
  if (priv->passthrough) {
    /* In passthrough mode, give transform_ip a look at the
     * buffer, without making it writable, or just push the
     * data through */
    if (bclass->transform_ip_on_passthrough && bclass->transform_ip) {
      GST_DEBUG_OBJECT (trans, "doing passthrough transform_ip");
      ret = bclass->transform_ip (trans, *outbuf);
    } else {
      GST_DEBUG_OBJECT (trans, "element is in passthrough");
    }
  } else {
    want_in_place = (bclass->transform_ip != NULL) && priv->always_in_place;

    if (want_in_place) {
      GST_DEBUG_OBJECT (trans, "doing inplace transform");
      ret = bclass->transform_ip (trans, *outbuf);
    } else {
      GST_DEBUG_OBJECT (trans, "doing non-inplace transform");

      if (bclass->transform)
        ret = bclass->transform (trans, inbuf, *outbuf);
      else
        ret = GST_FLOW_NOT_SUPPORTED;
    }
  }

As far as I can tell, it does not handle the passthrough transform_ip and non-passthrough transform_ip cases differently. The only thing I don’t totally understand is the comment, as it states:

In passthrough mode, give transform_ip a look at the buffer, without making it writable

But as far as I can tell, we haven’t done anything to make the buffer “writeable” in the case where we are not operating in passthrough mode & are in the want_in_place case.

Is there any the difference in behavior here I am not seeing / understanding?