From 3643f1c064ec02f33d982d7252b813a286bc12b5 Mon Sep 17 00:00:00 2001
From: Richard Harrison <rjh@zaretto.com>
Date: Fri, 31 Jul 2020 01:31:27 +0200
Subject: [PATCH] XMLSound bugfixes

- using expressions for pitch and volume didn't work
- multiple volume elements not computed properly

Discussion here: https://web.archive.org/web/20200730232940/http://fguk.me/forum/development-hangar/8360-supersonic-audio?start=40#46287

          <volume>
                <property>/a/v1</property>
                <factor>0.1</factor>
            </volume>
            <volume>
                <property>/a/v2</property>
                <offset>1</offset>
                <factor>-3</factor>
            </volume>
            <volume>
                <property>/a/v3</property>
                <offset>1</offset>
                <factor>-1</factor>
            </volume>

evaluates as follows:

update(): Have 3 volume entries
 --> 0:prop /a[0]/v1[0] => 0.110000 factor = 0.100000 v=0.011000 offset 0.000000, now 0.000000, v=0.011000 vol=0.011000 ==> 0.011000
 --> 1:prop /a[0]/v2[0] => 0.120000 factor = 3.000000 v=0.360000 -ve offset 1.000000 1.360000 = 0.014960  ==> 0.014960
 --> 2:prop /a[0]/v3[0] => 0.130000 factor = 1.000000 v=0.130000 -ve offset 1.000000 1.130000 = 0.016905  ==> 0.016905
---
 simgear/sound/xmlsound.cxx | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/simgear/sound/xmlsound.cxx b/simgear/sound/xmlsound.cxx
index 9646a9c0..89331390 100644
--- a/simgear/sound/xmlsound.cxx
+++ b/simgear/sound/xmlsound.cxx
@@ -414,9 +414,11 @@ SGXmlSound::update (double dt)
       double v = 1.0;
 
       if (_volume[i].expr) {
-         v = _volume[i].expr->getValue(nullptr);
-         expr = true;
-         continue;
+          double expression_value = _volume[i].expr->getValue(nullptr);
+          if (expression_value >= 0)
+              volume *= expression_value;
+		  expr = true;
+		  continue;
       }
 
       if (_volume[i].prop) {
@@ -437,19 +439,23 @@ SGXmlSound::update (double dt)
 
       if (_volume[i].max && (v > _volume[i].max))
          v = _volume[i].max;
-
       else if (v < _volume[i].min)
          v = _volume[i].min;
 
-      if (_volume[i].subtract)				// Hack!
-         volume = _volume[i].offset - v;
 
+      if (_volume[i].subtract){				// Hack!
+          v = v + _volume[i].offset;
+          if (v >= 0)
+            volume = volume * v;
+      }
       else {
-         volume_offset += _volume[i].offset;
-         volume *= v;
+          if (v >= 0) {
+              volume_offset += _volume[i].offset;
+              volume = volume * v;
+          }
       }
    }
-
+   
    //
    // Update the pitch
    //
@@ -462,9 +468,9 @@ SGXmlSound::update (double dt)
       double p = 1.0;
 
       if (_pitch[i].expr) {
-         p = _pitch[i].expr->getValue(nullptr);
-         expr = true;
-         continue;
+          pitch *= _pitch[i].expr->getValue(nullptr);
+          expr = true;
+          continue;
       }
 
       if (_pitch[i].prop) {