Make a non-linear fader for dB use. Not perfect yet, but will do.

[nageru] / nonlinear_fader.cpp

diff --git a/nonlinear_fader.cpp b/nonlinear_fader.cpp
new file mode 100644 (file)
index 0000000..7c26434
--- /dev/null
+++ b/nonlinear_fader.cpp
@@ -0,0 +1,150 @@
+#include <assert.h>
+#include <math.h>
+#include <stdio.h>
+#include <vector>
+#include <utility>
+
+#include <QResizeEvent>
+#include <QPainter>
+#include <QStyle>
+#include <QStyleOptionSlider>
+
+#include "nonlinear_fader.h"
+
+using namespace std;
+
+namespace {
+
+vector<pair<double, double>> fader_control_points = {
+       // The main area is from +6 to -12 dB (18 dB), and we use half the slider range for it.
+       { 6.0, 0.0 },
+       { -12.0, 0.5 },
+
+       // -12 to -21 is half the range (9 dB). Halve.
+       { -21.0, 0.625 },
+
+       // -21 to -30 (9 dB) gets the same range as the previous one.
+       { -30.0, 0.75 },
+
+       // -30 to -48 (18 dB) gets half of half.
+       { -48.0, 0.875 },
+
+       // -48 to -84 (36 dB) gets half of half of half.
+       { -84.0, 1.0 },
+};
+
+double slider_fraction_to_db(double db)
+{
+       if (db <= fader_control_points[0].second) {
+               return fader_control_points[0].first;
+       }
+       if (db >= fader_control_points.back().second) {
+               return fader_control_points.back().first;
+       }
+       for (unsigned i = 1; i < fader_control_points.size(); ++i) {
+               const double x0 = fader_control_points[i - 1].second;
+               const double x1 = fader_control_points[i].second;
+               const double y0 = fader_control_points[i - 1].first;
+               const double y1 = fader_control_points[i].first;
+               if (db >= x0 && db <= x1) {
+                       const double t = (db - x0) / (x1 - x0);
+                       return y0 + t * (y1 - y0);
+               }
+       }
+       assert(false);
+}
+
+double db_to_slider_fraction(double x)
+{
+       if (x >= fader_control_points[0].first) {
+               return fader_control_points[0].second;
+       }
+       if (x <= fader_control_points.back().first) {
+               return fader_control_points.back().second;
+       }
+       for (unsigned i = 1; i < fader_control_points.size(); ++i) {
+               const double x0 = fader_control_points[i].first;
+               const double x1 = fader_control_points[i - 1].first;
+               const double y0 = fader_control_points[i].second;
+               const double y1 = fader_control_points[i - 1].second;
+               if (x >= x0 && x <= x1) {
+                       const double t = (x - x0) / (x1 - x0);
+                       return y0 + t * (y1 - y0);
+               }
+       }
+       assert(false);
+}
+
+}  // namespace
+
+NonLinearFader::NonLinearFader(QWidget *parent)
+       : QSlider(parent)
+{
+       update_slider_position();
+}
+
+void NonLinearFader::setDbValue(double db)
+{
+       db_value = db;
+       update_slider_position();
+       emit dbValueChanged(db);
+}
+
+void NonLinearFader::paintEvent(QPaintEvent *event)
+{
+       QStyleOptionSlider opt;
+       this->initStyleOption(&opt);
+       QRect gr = this->style()->subControlRect(QStyle::CC_Slider, &opt, QStyle::SC_SliderGroove, this);
+       QRect sr = this->style()->subControlRect(QStyle::CC_Slider, &opt, QStyle::SC_SliderHandle, this);
+
+       // FIXME: Where does the slider_length / 2 come from? I can't really find it
+       // in the Qt code, but it seems to match up with reality.
+       int slider_length = sr.height();
+       int slider_min = gr.top() + (slider_length / 2);
+       int slider_max = gr.bottom() + (slider_length / 2) - slider_length + 1;
+
+       QPainter p(this);
+
+       // Draw some ticks every 6 dB.
+       // FIXME: Find a way to make the slider wider, so that we have more space for tickmarks
+       // and some dB numbering.
+       int x_margin = 5;
+       p.setPen(Qt::darkGray);
+       for (int db = -84; db <= 6; db += 6) {
+               int y = slider_min + lrint(db_to_slider_fraction(db) * (slider_max - slider_min));
+               p.drawLine(QPoint(0, y), QPoint(gr.left() - x_margin, y));
+               p.drawLine(QPoint(gr.right() + x_margin, y), QPoint(width() - 1, y));
+       }
+
+       QSlider::paintEvent(event);
+}
+
+void NonLinearFader::resizeEvent(QResizeEvent *event)
+{
+       QSlider::resizeEvent(event);
+       inhibit_updates = true;
+       setRange(0, event->size().height() - 1);
+       inhibit_updates = false;
+       update_slider_position();
+}
+
+void NonLinearFader::sliderChange(SliderChange change)
+{
+       QSlider::sliderChange(change);
+       if (change == QAbstractSlider::SliderValueChange && !inhibit_updates) {
+               if (value() == 0) {
+                       db_value = -HUGE_VAL;
+               } else {
+                       double frac = 1.0 - value() / (height() - 1.0);
+                       db_value = slider_fraction_to_db(frac);
+               }
+               emit dbValueChanged(db_value);
+       }
+}
+
+void NonLinearFader::update_slider_position()
+{
+       inhibit_updates = true;
+       setValue(lrint((1.0 - db_to_slider_fraction(db_value)) * (height() - 1.0)));
+       inhibit_updates = false;
+}