Use correct HRTF elevation response for negative elevations.

The computed index for negative elevations was off by one. When the
elevation is -45 deg, the selected response corresponded to the entry
for 90 deg, not 315 deg.

To simplify things, we just use a short table to map the index to the
(positive) elevation angle.

No tests for this, but see crbug.com/411191.
BUG=412442

Review URL: https://codereview.chromium.org/565303003

git-svn-id: svn://svn.chromium.org/blink/trunk@181986 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/audio/HRTFElevation.cpp

@@ -57,6 +57,15 @@ const size_t ResponseFrameSize = 256;
 const float ResponseSampleRate = 44100;
 
 #if USE(CONCATENATED_IMPULSE_RESPONSES)
+
+// This table maps the index into the elevation table with the corresponding angle. See
+// https://bugs.webkit.org/show_bug.cgi?id=98294#c9 for the elevation angles and their order in the
+// concatenated response.
+const int ElevationIndexTableSize = 10;
+const int ElevationIndexTable[ElevationIndexTableSize] = {
+    0, 15, 30, 45, 60, 75, 90, 315, 330, 345
+};
+
 // Lazily load a concatenated HRTF database for given subject and store it in a
 // local hash table to ensure quick efficient future retrievals.
 static PassRefPtr<AudioBus> getConcatenatedImpulseResponsesForSubject(const String& subjectName)
@@ -120,9 +129,20 @@ bool HRTFElevation::calculateKernelsForAzimuthElevation(int azimuth, int elevati
     if (!bus)
         return false;
 
-    int elevationIndex = positiveElevation / AzimuthSpacing;
-    if (positiveElevation > 90)
-        elevationIndex -= AzimuthSpacing;
+    // Just sequentially search the table to find the correct index.
+    int elevationIndex = -1;
+
+    for (int k = 0; k < ElevationIndexTableSize; ++k) {
+        if (ElevationIndexTable[k] == positiveElevation) {
+            elevationIndex = k;
+            break;
+        }
+    }
+
+    bool isElevationIndexGood = (elevationIndex >= 0) && (elevationIndex < ElevationIndexTableSize);
+    ASSERT(isElevationIndexGood);
+    if (!isElevationIndexGood)
+        return false;
 
     // The concatenated impulse response is a bus containing all
     // the elevations per azimuth, for all azimuths by increasing