Improve spectrogram time alignment

Currently, SpectrogramPlot::getLine() uses the sample parameter to
determine the index of the first sample that is used to calculate
the FFT for a particular spectrogram line. Since the FFT is in some
sense an average of fftSize samples, this causes features (such as
the start and end of packet bursts) to appear somewhat sooner in
the spectrogram compared to their actual locations in the IQ file.

This improves the time alignment of the spectrogram plot by making
sample refer to the middle sample of the FFT (so the samples used
to compute the FFT start at sample - fftSize / 2.

For the beginning of the file we need to make an exception, because
if we try to fetch samples before the beginning of the file, then
inputSource->getSamples() returns nullptr. SpectrogramPlot::getLine()
handles this gracefully, but an ugly red bar appears at the beginning
of the file when the FFT size and zoom are large. To solve this, we
cheat and force the FFT to start at the beginning of the file. To
be more precise we could pad the beginning with zeros instead.

Signed-off-by: Daniel Estévez <daniel@destevez.net>

src/spectrogramplot.cpp

@@ -26,6 +26,7 @@
 #include <QPixmapCache>
 #include <QRect>
 #include <liquid/liquid.h>
+#include <algorithm>
 #include <functional>
 #include <cstdlib>
 #include "util.h"
@@ -286,7 +287,13 @@ float* SpectrogramPlot::getFFTTile(size_t tile)
 void SpectrogramPlot::getLine(float *dest, size_t sample)
 {
     if (inputSource && fft) {
-        auto buffer = inputSource->getSamples(sample, fftSize);
+	// Make sample be the midpoint of the FFT, unless this takes us
+	// past the beginning of the inputSource (if we remove the
+	// std::max(·, 0), then an ugly red bar appears at the beginning
+	// of the spectrogram with large zooms and FFT sizes).
+	const auto first_sample = std::max(static_cast<ssize_t>(sample) - fftSize / 2,
+					   static_cast<ssize_t>(0));
+        auto buffer = inputSource->getSamples(first_sample, fftSize);
         if (buffer == nullptr) {
             auto neg_infinity = -1 * std::numeric_limits<float>::infinity();
             for (int i = 0; i < fftSize; i++, dest++)