LFP Viewer support for low sampling rates

Plugins/LfpViewer/DisplayBuffer.cpp

@@ -27,6 +27,7 @@ namespace LfpViewer
 {
 
 #define BUFFER_LENGTH_S 1.0f
+#define MIN_BUFFER_SAMPLES 1000
 
 DisplayBuffer::DisplayBuffer (int id_, String name_, float sampleRate_) : id (id_), name (name_), sampleRate (sampleRate_), isNeeded (true)
 {
@@ -104,7 +105,7 @@ void DisplayBuffer::addChannel (
 void DisplayBuffer::update()
 {
     if (numChannels != previousSize)
-        setSize (numChannels + 1, int (sampleRate * BUFFER_LENGTH_S));
+        setSize (numChannels + 1, std::max (int (sampleRate * BUFFER_LENGTH_S), MIN_BUFFER_SAMPLES));
 
     clear();
 

Plugins/LfpViewer/LfpDisplayCanvas.cpp

@@ -1357,6 +1357,11 @@ void LfpDisplaySplitter::updateScreenBuffer()
 
             int sampleNumber = 0;
 
+            // For ratio < 1: save the display-buffer start index so we can
+            // compute each pixel's sample position with a direct multiply
+            // (drift-free) instead of accumulating subSampleOffset in a loop.
+            const int dbiStart = dbi;
+
             if (pixelsToFill > 0 && pixelsToFill < 1000000)
             {
                 float i;
@@ -1404,39 +1409,41 @@ void LfpDisplaySplitter::updateScreenBuffer()
 
                         if (ratio < 1.0f) // less than one sample per pixel
                         {
+                            // Drift-free interpolation: compute sample position
+                            // directly from pixel index rather than accumulating
+                            // subSampleOffset, which accumulates rounding errors,
+                            // causing the displayed sample positions to drift from
+                            // the true timeline.
+                            float samplePos = float (i) * ratio;
+                            int sampleStep = int (samplePos);
+                            float alpha = samplePos - float (sampleStep);
+
+                            int curDbi = (dbiStart + sampleStep) % displayBufferSize;
+                            int lastIndex = (curDbi - 1 + displayBufferSize) % displayBufferSize;
+
                             if (isEventChannel)
                             {
-                                eventWritePtr[sbi] = displayData[dbi];
+                                eventWritePtr[sbi] = displayData[curDbi];
                             }
                             else
                             {
-                                const float alpha = subSampleOffset;
-                                const float invAlpha = 1.0f - alpha;
-
-                                int lastIndex = dbi - 1;
-
-                                if (lastIndex < 0)
-                                {
-                                    lastIndex = displayBufferSize - 1;
+                                // Skip the very first pixel if the display buffer has
+                                // never been written (dbiStart == 0 means no previous
+                                // sample exists to interpolate from).
+                                if (dbiStart == 0 && sampleStep == 0)
                                     continue;
-                                }
 
                                 const float val0 = displayData[lastIndex];
-                                const float val1 = displayData[dbi];
-                                const float val = invAlpha * val0 + alpha * val1;
-
-                                meanWritePtr[sbi] += val;
-                                minWritePtr[sbi] += val;
-                                maxWritePtr[sbi] += val;
-                            }
+                                const float val1 = displayData[curDbi];
+                                const float val = (1.0f - alpha) * val0 + alpha * val1;
 
-                            subSampleOffset += ratio;
+                                // Span min..max across consecutive pixels so the plotter
+                                // draws connected line segments at low sample rates.
+                                const float prevVal = (sbi > 0) ? meanReadPtr[sbi - 1] : val;
 
-                            if (subSampleOffset > 1.0f) // go to next pixel
-                            {
-                                subSampleOffset -= 1.0f;
-                                dbi += 1;
-                                dbi %= displayBufferSize;
+                                meanWritePtr[sbi] += val;
+                                minWritePtr[sbi] += jmin (val, prevVal);
+                                maxWritePtr[sbi] += jmax (val, prevVal);
                             }
                         }
                         else
@@ -1621,10 +1628,12 @@ void LfpDisplaySplitter::updateScreenBuffer()
                     } // !isPaused
                 }
 
-                if (ratio > 1.0f)
-                    leftOverSamples.set (channel, pixelsToFill - i); // +(pixelsToFill - (i - 1)) * ratio);
-                else
-                    leftOverSamples.set (channel, subSampleOffset - 1.0f);
+                // Same formula for both ratio branches: after the loop, `i` is
+                // the first value that failed (i < pixelsToFill), so
+                // pixelsToFill - i is the signed fractional carry. For ratio < 1
+                // the loop runs ceil(pixelsToFill) iterations and the carry is
+                // <= 0, which subtracts from the next call and prevents drift.
+                leftOverSamples.set (channel, pixelsToFill - i);
 
                 //std::cout << "Setting channel " << channel << " sbi to " << sbi << std::endl;
                 screenBufferIndex.set (channel, sbi);

Plugins/LfpViewer/Tests/LfpDisplayNodeTests.cpp

@@ -380,3 +380,122 @@ TEST_F (LfpDisplayNodeTests, DataIntegrityTest)
 
     processor->stopAcquisition();
 }
+
+class LfpDisplayNodeLowRateTests : public LfpDisplayNodeTests
+{
+protected:
+    void SetUp() override
+    {
+        sampleRate = 10.0f; // Override before base SetUp reads it
+        LfpDisplayNodeTests::SetUp();
+    }
+};
+
+// Checks if low sampling rate signals receive a sufficient buffer legnth
+TEST_F (LfpDisplayNodeLowRateTests, LowRateDisplayBufferHasMinimumSize)
+{
+    Array<LfpViewer::DisplayBuffer*> displayBuffers = processor->getDisplayBuffers();
+    ASSERT_GT (displayBuffers.size(), 0);
+    EXPECT_GE (displayBuffers[0]->getNumSamples(), 1000);
+}
+
+// Checks that low sampling rate signals with numSamples / pixels < 1.0 still display.
+TEST_F (LfpDisplayNodeLowRateTests, LowRateSignalIsNotFlatLine)
+{
+    const int canvasX = 600;
+    const int canvasY = 800;
+    const int numSamples = 20;
+
+    std::unique_ptr<LfpViewer::LfpDisplayCanvas> canvas =
+        std::make_unique<LfpViewer::LfpDisplayCanvas> (processor, LfpViewer::SplitLayouts::SINGLE, false);
+    canvas->updateSettings();
+    canvas->setSize (canvasX, canvasY);
+    canvas->resized();
+    canvas->setVisible (true);
+    setExpectedImageParameters (canvas.get());
+
+    processor->startAcquisition();
+    canvas->beginAnimation();
+
+    auto inputBuffer = createBufferSinusoidal (1, numChannels, numSamples, 100);
+    writeBlock (inputBuffer);
+    canvas->refreshState();
+
+    Rectangle<int> canvasSnapshot (x, y, width, height);
+    Image canvasImage = canvas->createComponentSnapshot (canvasSnapshot);
+
+    const int firstChannelHeight = height / numChannels;
+    const int midlineRow = firstChannelHeight / 2;
+    const int midlineBand = 4; // tolerance for midline so zero-buffer flat line cannot pass
+
+    bool hasOffMidlineSignal = false;
+    for (int px = 0; px < width && !hasOffMidlineSignal; px++)
+    {
+        for (int py = 0; py < firstChannelHeight && !hasOffMidlineSignal; py++)
+        {
+            if (std::abs (py - midlineRow) <= midlineBand)
+                continue;
+            if (canvasImage.getPixelAt (px, py) == channelColours[0])
+                hasOffMidlineSignal = true;
+        }
+    }
+
+    EXPECT_TRUE (hasOffMidlineSignal)
+        << "No off-midline signal found: canvas likely rendered a zero-valued flat line";
+
+    processor->stopAcquisition();
+}
+
+// A connected trace produces signal in almost every pixel column; isolated dots produce
+// signal in only ~numSamples columns. The midline band is excluded so a zero-buffer
+// flat line scores zero columns.
+TEST_F (LfpDisplayNodeLowRateTests, LowRateTraceIsConnected)
+{
+    const int canvasX = 600;
+    const int canvasY = 800;
+    const int numSamples = 20;
+
+    std::unique_ptr<LfpViewer::LfpDisplayCanvas> canvas =
+        std::make_unique<LfpViewer::LfpDisplayCanvas> (processor, LfpViewer::SplitLayouts::SINGLE, false);
+    canvas->updateSettings();
+    canvas->setSize (canvasX, canvasY);
+    canvas->resized();
+    canvas->setVisible (true);
+    setExpectedImageParameters (canvas.get());
+
+    processor->startAcquisition();
+    canvas->beginAnimation();
+
+    auto inputBuffer = createBufferSinusoidal (1, numChannels, numSamples, 100);
+    writeBlock (inputBuffer);
+    canvas->refreshState();
+
+    Rectangle<int> canvasSnapshot (x, y, width, height);
+    Image canvasImage = canvas->createComponentSnapshot (canvasSnapshot);
+
+    const int firstChannelHeight = height / numChannels;
+    const int midlineRow = firstChannelHeight / 2;
+    const int midlineBand = 4; // tolerance for midline so zero-buffer flat line cannot pass
+
+    int signalColumns = 0;
+    for (int px = 0; px < width; px++)
+    {
+        for (int py = 0; py < firstChannelHeight; py++)
+        {
+            if (std::abs (py - midlineRow) <= midlineBand)
+                continue;
+            if (canvasImage.getPixelAt (px, py) == channelColours[0])
+            {
+                signalColumns++;
+                break;
+            }
+        }
+    }
+
+    // With connected rendering almost every column has off-midline signal.
+    EXPECT_GT (signalColumns, numSamples * 5)
+        << "Trace appears disjointed: " << signalColumns
+        << " off-midline signal columns, expected > " << numSamples * 5;
+
+    processor->stopAcquisition();
+}