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opl: Add a secret DMXOPTION to reverse stereo channels.

The OPL3 support in Doom's DMX library includes stereo panning support,
but there is a bug where the stereo channels are backwards for some
reason. By default preserve this behavior so that we behave like
Vanilla, but add a secret DMXOPTION so that the channels can be
"reversed" into the correct positions by people who really want it.

Thanks to CSonicGo (@sneakernets) for the original pull request to fix
this (#586), and Alexey Khokholov for assistance in diagnosing the bug.

--- a/src/i_oplmusic.c

+++ b/src/i_oplmusic.c

@@ -352,6 +352,12 @@

 char *snd_dmxoption = "";

 int opl_io_port = 0x388;

+// If true, OPL sound channels are reversed to their correct arrangement

+// (as intended by the MIDI standard) rather than the backwards one

+// used by DMX due to a bug.

+

+static boolean opl_stereo_correct = false;

+

 // Load instrument table from GENMIDI lump:

 static boolean LoadInstrumentTable(void)

@@ -1133,6 +1139,16 @@

     unsigned int reg_pan;

     unsigned int i;

+    // The DMX library has the stereo channels backwards, maybe because

+    // Paul Radek had a Soundblaster card with the channels reversed, or

+    // perhaps it was just a bug in the OPL3 support that was never

+    // finished. By default we preserve this bug, but we also provide a

+    // secret DMXOPTION to fix it.

+    if (opl_stereo_correct)

+    {

+        pan = 144 - pan;

+    }

+

     if (opl_opl3mode)

     {

         if (pan >= 96)

@@ -1727,6 +1743,10 @@

         opl_opl3mode = 0;

         num_opl_voices = OPL_NUM_VOICES;

     }

+

+    // Secret, undocumented DMXOPTION that reverses the stereo channels

+    // into their correct orientation.

+    opl_stereo_correct = strstr(dmxoption, "-reverse") != NULL;

     // Initialize all registers.

--- a/src/setup/mainmenu.c

+++ b/src/setup/mainmenu.c

@@ -91,7 +91,7 @@

     show_endoom = 0;

     dclick_use = 0;

     novert = 1;

-    snd_dmxoption = "-opl3";

+    snd_dmxoption = "-opl3 -reverse";

     png_screenshots = 1;

 }