Don't clip FFT output values to minDecibels.

If the (linear) FFT value is zero, the dB value was set to analyser.minDecibels. The spec doesn't require this and it makes more sense to return the expected -Infinity instead. BUG=588853 TEST=realtimeanalyser-zero.html Review URL: https://codereview.chromium.org/1729783002 Cr-Commit-Position: refs/heads/master@{#378081}
huningxin · Feb 27, 2016 · ef6f6ae · ef6f6ae
1 parent 455d762
commit ef6f6ae
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Showing 4 changed files with 76 additions and 8 deletions.
diff --git a/third_party/WebKit/LayoutTests/webaudio/realtimeanalyser-zero-expected.txt b/third_party/WebKit/LayoutTests/webaudio/realtimeanalyser-zero-expected.txt
@@ -0,0 +1,10 @@
+Test AnalyserNode getFloatFrequencyData With Zero-Valued Input
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS getFloatFrequencyData() with zero-valued input contains only the constant -Infinity.
+PASS successfullyParsed is true
+
+TEST COMPLETE
+
diff --git a/third_party/WebKit/LayoutTests/webaudio/realtimeanalyser-zero.html b/third_party/WebKit/LayoutTests/webaudio/realtimeanalyser-zero.html
@@ -0,0 +1,63 @@
+<!doctype html>
+<html>
+  <head>
+    <script src="../resources/js-test.js"></script>
+    <script src="resources/compatibility.js"></script>
+    <script src="resources/audio-testing.js"></script>
+    <title>Test getFloatFrequencyData With Zero Inputs</title>
+  </head>
+
+  <body>
+    <script>
+      description("Test AnalyserNode getFloatFrequencyData With Zero-Valued Input");
+      window.jsTestIsAsync = true;
+
+      var sampleRate = 48000;
+
+      // Render enough data to run the test.  
+      var renderFrames = 2*1024;
+      var renderDuration = renderFrames / sampleRate;
+
+      var audit = Audit.createTaskRunner();
+
+
+      // Test that getFloatFrequencyData returns -Infinity when the input is all-zeroes.
+      audit.defineTask("zero input", function (done) {
+        var context = new OfflineAudioContext(1, renderFrames, sampleRate);
+
+        // Constant source of 0's.
+        var source = context.createBufferSource();
+        source.buffer = createConstantBuffer(context, 1, 0);
+        source.loop = true;
+
+        // Create analyser and use some non-default minDecibels value.
+        var analyser = context.createAnalyser();
+        analyser.minDecibels = -123;
+
+        source.connect(analyser);
+        analyser.connect(context.destination);
+
+        source.start();
+
+        // Suspend after some number of frames and grab the float frequency data. 
+        context.suspend(1024 / sampleRate).then(function () {
+          var f = new Float32Array(analyser.frequencyBinCount);
+          analyser.getFloatFrequencyData(f);
+
+          Should("getFloatFrequencyData() with zero-valued input", f)
+            .beConstantValueOf(-Infinity);
+        }).then(context.resume.bind(context));
+
+        context.startRendering().then(done);
+      });
+
+      audit.defineTask("finish", function (done) {
+        finishJSTest();
+        done();
+      });
+
+      audit.runTasks();
+
+    </script>
+  </body>
+</html>
diff --git a/third_party/WebKit/Source/modules/webaudio/RealtimeAnalyser.cpp b/third_party/WebKit/Source/modules/webaudio/RealtimeAnalyser.cpp
@@ -188,7 +188,6 @@ void RealtimeAnalyser::doFFTAnalysis()
 void RealtimeAnalyser::convertFloatToDb(DOMFloat32Array* destinationArray)
 {
     // Convert from linear magnitude to floating-point decibels.
-    const double minDecibels = m_minDecibels;
     unsigned sourceLength = magnitudeBuffer().size();
     size_t len = std::min(sourceLength, destinationArray->length());
     if (len > 0) {
@@ -197,7 +196,7 @@ void RealtimeAnalyser::convertFloatToDb(DOMFloat32Array* destinationArray)
 
         for (unsigned i = 0; i < len; ++i) {
             float linearValue = source[i];
-            double dbMag = !linearValue ? minDecibels : AudioUtilities::linearToDecibels(linearValue);
+            double dbMag = AudioUtilities::linearToDecibels(linearValue);
             destination[i] = float(dbMag);
         }
     }
@@ -234,7 +233,7 @@ void RealtimeAnalyser::convertToByteData(DOMUint8Array* destinationArray)
 
         for (unsigned i = 0; i < len; ++i) {
             float linearValue = source[i];
-            double dbMag = !linearValue ? minDecibels : AudioUtilities::linearToDecibels(linearValue);
+            double dbMag = AudioUtilities::linearToDecibels(linearValue);
 
             // The range m_minDecibels to m_maxDecibels will be scaled to byte values from 0 to UCHAR_MAX.
             double scaledValue = UCHAR_MAX * (dbMag - minDecibels) * rangeScaleFactor;

diff --git a/third_party/WebKit/Source/platform/audio/AudioUtilities.cpp b/third_party/WebKit/Source/platform/audio/AudioUtilities.cpp
@@ -37,11 +37,7 @@ float decibelsToLinear(float decibels)
 
 float linearToDecibels(float linear)
 {
-    // It's not possible to calculate decibels for a zero linear value since it would be -Inf.
-    // -1000.0 dB represents a very tiny linear value in case we ever reach this case.
-    ASSERT(linear);
-    if (!linear)
-        return -1000;
+    ASSERT(linear >= 0);
 
     return 20 * log10f(linear);
 }