Added theme metrics for controlling random bg behavior.

Docs/Changelog_sm5.txt

@@ -4,6 +4,18 @@ The StepMania 5 Changelog covers all post-sm-ssc changes. For a list of changes
 from StepMania 4 alpha 5 to sm-ssc v1.2.5, see Changelog_sm-ssc.txt.
 ________________________________________________________________________________
 
+2015/07/02
+----------
+* [Gameplay] Random background video behavior is now controlled by these
+  three metrics in the Background section:
+  RandomBGStartBeat sets the beat of the music the first random bg occurs on.
+  RandomBGChangeMeasures sets the number of measures between changes.
+  RandomBGChangesWhenBPMChangesAtMeasureStart toggles the behavior in its name.
+
+================================================================================
+StepMania 5.0.9 | 20150520
+--------------------------------------------------------------------------------
+
 2015/06/10
 ----------
 * [global] get_music_file_length lua function added. [kyzentun]

Themes/_fallback/metrics.ini

@@ -239,6 +239,9 @@ LeftEdge=SCREEN_LEFT
 RightEdge=SCREEN_RIGHT
 TopEdge=SCREEN_TOP
 BottomEdge=SCREEN_BOTTOM
+RandomBGStartBeat=-1000
+RandomBGChangeMeasures=1
+RandomBGChangesWhenBPMChangesAtMeasureStart=true
 
 [Banner]
 # Scroll stuff when you roll over it, DDR Extreme style.

src/Background.cpp

@@ -31,6 +31,9 @@ static ThemeMetric<float> BOTTOM_EDGE				("Background","BottomEdge");
 static ThemeMetric<float> CLAMP_OUTPUT_PERCENT			("Background","ClampOutputPercent");
 static ThemeMetric<bool> SHOW_DANCING_CHARACTERS		("Background","ShowDancingCharacters");
 static ThemeMetric<bool> USE_STATIC_BG		("Background","UseStaticBackground");
+static ThemeMetric<float> RAND_BG_START_BEAT("Background", "RandomBGStartBeat");
+static ThemeMetric<float> RAND_BG_CHANGE_MEASURES("Background", "RandomBGChangeMeasures");
+static ThemeMetric<bool> RAND_BG_CHANGES_WHEN_BPM_CHANGES("Background", "RandomBGChangesWhenBPMChangesAtMeasureStart");
 
 
 static Preference<bool>	g_bShowDanger( "ShowDanger", true );
@@ -440,9 +443,10 @@ void BackgroundImpl::LoadFromRandom( float fFirstBeat, float fEndBeat, const Bac
 		int iSegmentEndRow = (i + 1 == tSigs.size()) ? iEndRow : tSigs[i+1]->GetRow();
 
 
-		for(int j=max(ts->GetRow(),iStartRow);
+		int time_signature_start= max(ts->GetRow(),iStartRow);
+		for(int j= time_signature_start;
 			j<min(iEndRow,iSegmentEndRow);
-			j+=4*ts->GetNoteRowsPerMeasure())
+			j+= RAND_BG_CHANGE_MEASURES * ts->GetNoteRowsPerMeasure())
 		{
 			// Don't fade. It causes frame rate dip, especially on slower machines.
 			BackgroundDef bd = m_Layer[0].CreateRandomBGA(m_pSong,
@@ -452,12 +456,21 @@ void BackgroundImpl::LoadFromRandom( float fFirstBeat, float fEndBeat, const Bac
 			{
 				BackgroundChange c = change;
 				c.m_def = bd;
+				if(j == time_signature_start && i == 0)
+				{
+					c.m_fStartBeat = RAND_BG_START_BEAT;
+				}
+				else
+				{
 					c.m_fStartBeat = NoteRowToBeat(j);
+				}
 				m_Layer[0].m_aBGChanges.push_back( c );
 			}
 		}
 	}
 
+	if(RAND_BG_CHANGES_WHEN_BPM_CHANGES)
+	{
 		// change BG every BPM change that is at the beginning of a measure
 		const vector<TimingSegment *> &bpms = timing.GetTimingSegments(SEGMENT_BPM);
 		for( unsigned i=0; i<bpms.size(); i++ )
@@ -492,6 +505,7 @@ void BackgroundImpl::LoadFromRandom( float fFirstBeat, float fEndBeat, const Bac
 			}
 		}
 	}
+}
 
 void BackgroundImpl::LoadFromSong( const Song* pSong )
 {