m_option, vf: add label support

Can be used to refer to filters by name. Intended to be used when the
filter chain is changed at runtime.

A label can be assigned to a filter by prefixing it with '@name:', where
'name' is an user-chosen identifier. For example, a filter added with
'-vf-add @label1:gradfun=123' can be removed with '-vf-del @label1'.
If a filter with an already existing label is added, the existing filter
is replaced with the new filter (this happens for both -vf-add and
-vf-pre). If a filter is replaced, the new filter takes the position of
the old filter, instead of being appended/prepended to the filter chain
as usual. For -vf-toggle, labels are compared if at least one of the
filters has a label; otherwise they are compared by filter name and
arguments (like before). This means two filters are never considered
equal if one has a label and the other one does not.

1 files changed, 5 insertions, 0 deletions

diff --git a/DOCS/man/en/input.rst b/DOCS/man/en/input.rst
index 671e05bf05..59d48f5a28 100644
--- a/DOCS/man/en/input.rst
+++ b/DOCS/man/en/input.rst
@@ -260,6 +260,11 @@ vf set|add|toggle|del "filter1=params,filter2,..."
         indexes start from the last filter, and ``-1`` denotes the last
         filter.
 
+    You can assign labels to filter by prefixing them with ``@name:`` (where
+    ``name`` is a user-chosen arbitrary identifiers). Labels can be used to
+    refer to filters by name in all of the filter chain modification commands.
+    For ``add``, using an already used label will replace the existing filter.
+
     *EXAMPLE for input.conf*:
 
     - ``a vf set flip`` turn video upside-down on the ``a`` key