turn volume slider into mix slider

it's far more useful

src/engine/filePlayer.cpp

@@ -199,6 +199,10 @@ void DivFilePlayer::mix(float** buf, int chans, unsigned int size) {
     return;
   }
 
+  float actualVolume=volume+1.0f;
+  if (actualVolume<0.0f) actualVolume=0.0f;
+  if (actualVolume>1.0f) actualVolume=1.0f;
+
   if (wantBlock!=DIV_NO_BLOCK) {
     cacheCV.notify_one();
   }
@@ -234,7 +238,7 @@ void DivFilePlayer::mix(float** buf, int chans, unsigned int size) {
           x[5]*t1[1]+
           x[6]*t1[2]+
           x[7]*t1[3]
-        )*volume;
+        )*actualVolume;
 
         for (int j=0; j<chans; j++) {
           buf[j][i]=s;
@@ -257,7 +261,7 @@ void DivFilePlayer::mix(float** buf, int chans, unsigned int size) {
           x[5]*t1[1]+
           x[6]*t1[2]+
           x[7]*t1[3]
-        )*volume;
+        )*actualVolume;
       }
 
       // advance
@@ -463,6 +467,14 @@ void DivFilePlayer::setVolume(float vol) {
   volume=vol;
 }
 
+bool DivFilePlayer::getActive() {
+  return isActive;
+}
+
+void DivFilePlayer::setActive(bool active) {
+  isActive=active;
+}
+
 DivFilePlayer::DivFilePlayer():
   discardBuf(NULL),
   blocks(NULL),
@@ -474,11 +486,12 @@ DivFilePlayer::DivFilePlayer():
   wantBlock(DIV_NO_BLOCK),
   outRate(44100),
   rateAccum(0),
-  volume(1.0f),
+  volume(0.0f),
   playing(false),
   fileError(false),
   quitThread(false),
   threadHasQuit(false),
+  isActive(false),
   cacheThread(NULL) {
   memset(&si,0,sizeof(SF_INFO));
   sincTable=DivFilterTables::getSincTable8();

src/engine/filePlayer.h

@@ -55,6 +55,7 @@ class DivFilePlayer {
   bool fileError;
   bool quitThread;
   bool threadHasQuit;
+  bool isActive;
 
   std::thread* cacheThread;
   std::mutex cacheMutex;
@@ -89,6 +90,8 @@ class DivFilePlayer {
     void setOutputRate(int rate);
     float getVolume();
     void setVolume(float vol);
+    bool getActive();
+    void setActive(bool active);
 
     DivFilePlayer();
     ~DivFilePlayer();

src/engine/playback.cpp

@@ -3287,6 +3287,18 @@ void DivEngine::nextBuf(float** in, float** out, int inChans, int outChans, unsi
     }
   }
 
+  // calculate volume of reference file player (so we can attenuate the rest according to the mix slider)
+  // -1 to 0: player volume goes from 0% to 100%
+  // 0 to +1: tracker volume goes from 100% to 0%
+  float refPlayerVol=1.0f;
+  if (curFilePlayer!=NULL) {
+    // only if the player window is open
+    if (curFilePlayer->getActive()) {
+      refPlayerVol=1.0f-curFilePlayer->getVolume();
+      if (refPlayerVol>1.0f) refPlayerVol=1.0f;
+    }
+  }
+
   // now mix everything (resolve patchbay)
   for (unsigned int i: song.patchbay) {
     // there are 4096 portsets. each portset may have up to 16 outputs (subports).
@@ -3308,7 +3320,7 @@ void DivEngine::nextBuf(float** in, float** out, int inChans, int outChans, unsi
       // chip outputs
       if (srcPortSet<song.systemLen && playing && !halted) {
         if (srcSubPort<disCont[srcPortSet].dispatch->getOutputCount()) {
-          float vol=song.systemVol[srcPortSet]*disCont[srcPortSet].dispatch->getPostAmp()*song.masterVol;
+          float vol=song.systemVol[srcPortSet]*disCont[srcPortSet].dispatch->getPostAmp()*song.masterVol*refPlayerVol;
 
           // apply volume and panning
           switch (destSubPort&3) {