From 3cae10ab40a4e1688b46ebe62e1bc5e298611861 Mon Sep 17 00:00:00 2001
From: Robin Gareus <robin@gareus.org>
Date: Wed, 14 Oct 2020 01:56:36 +0200
Subject: [PATCH] Elaborate MIDI rewrite example script

---
 share/scripts/_midi_rewrite.lua | 35 +++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/share/scripts/_midi_rewrite.lua b/share/scripts/_midi_rewrite.lua
index 42f266211d..c23c61cb6d 100644
--- a/share/scripts/_midi_rewrite.lua
+++ b/share/scripts/_midi_rewrite.lua
@@ -13,18 +13,37 @@ function factory ()
 		for p in t[2]:iter () do
 			if not p:receives_input () then goto next end
 
-			if not p:name () == "MIDI/midi_in 1" then goto next end
+			-- search-filter port
+			if not p:name () == "MIDITrackName/midi_in 1" then
+				goto next -- skip
+			end
 
-			midiport = p:to_midiport ()
-			assert (not midiport:isnil ())
-			mb = midiport:get_midi_buffer (n_samples);
+			-- ensure that the port is-a https://manual.ardour.org/lua-scripting/class_reference/#ARDOUR:MidiPort
+			assert (not p:to_midiport ():isnil ())
 
-			events = mb:table() -- copy event list into lua table
-			mb:silence (n_samples, 0); -- clear existing buffer
+			-- https://manual.ardour.org/lua-scripting/class_reference/#ARDOUR:MidiBuffer
+			local mb = p:to_midiport ():get_midi_buffer (n_samples)
+			-- When an I/O port is connected (source -> sink), the
+			-- buffer is shared. The MidiBuffer is in fact the buffer
+			-- from the backend. Changing events in this buffer affects
+			-- all destinations that use this buffer as source.
 
+			local events = mb:table() -- *copy* event list to a Lua table
+			mb:silence (n_samples, 0) -- clear existing buffer
+
+			-- now iterate over events that were in the buffer
 			for _,e in pairs (events) do
-				-- e is-a http://manual.ardour.org/lua-scripting/class_reference/#Evoral:MidiEvent
-				e:set_channel (2)
+				-- e is-a http://manual.ardour.org/lua-scripting/class_reference/#Evoral:Event
+				if e:size () == 3 then
+					-- found a 3 byte event
+					local buffer = e:buffer():array()
+					local ev_type = buffer[1] >> 4 -- get MIDI event type (upper 4 bits)
+					if ev_type == 8 or ev_type == 9 then -- note on or note off event
+						buffer[1] = (buffer[1] & 0xf0) + 2 -- change the MIDI channel to "2"
+						buffer[3] = buffer[3] >> 1 -- reduce the velocity by half
+					end
+				end
+				-- finally place the event back into the Port's MIDI buffers
 				mb:push_event (e)
 			end