[gradient] Improve color accuracy when zoomed with large gradients by…

… only drawing a subset

src/plugins/score-plugin-automation/Color/GradientExecution.cpp

@@ -80,6 +80,7 @@ static ossia::hunter_lab to_ossia_color(QColor c)
 static auto to_ossia_gradient(const Gradient::ProcessModel::gradient_colors& c)
 {
   gradient::grad_type g;
+
   for(auto& e : c)
   {
     g.insert(std::make_pair(e.first, to_ossia_color(e.second)));

src/plugins/score-plugin-automation/Color/GradientView.cpp

@@ -52,24 +52,93 @@ static const QPainterPath& triangle{[] {
 
 void View::paint_impl(QPainter* p) const
 {
+  const auto rect = boundingRect();
+  const auto W = rect.width();
   switch(m_colors.size())
   {
     case 0:
-      p->fillRect(boundingRect(), Qt::black);
+      p->fillRect(rect, Qt::black);
       return;
     case 1:
-      p->fillRect(boundingRect(), m_colors.begin()->second);
+      p->fillRect(rect, m_colors.begin()->second);
       return;
     default:
       break;
   }
+  bool drawFull = false;
 
-  QLinearGradient g{QPointF{0, 0}, QPointF{m_dataWidth, 0.}};
-  for(const auto& col : m_colors)
+  if(auto v = ::getView(*this))
   {
-    g.setColorAt(col.first, col.second);
+    const auto view_left = v->mapToScene(QPoint(0., 0.)).x();
+    const auto view_right = v->mapToScene(QPoint(v->width(), 0.)).x();
+
+    const auto item_left = this->mapToScene(QPointF(0., 0.)).x();
+    const auto item_right = this->mapToScene(QPointF(W, 0.)).x();
+
+    if(item_left >= view_left && item_right <= view_right)
+    {
+      drawFull = true;
+    }
+    else
+    {
+      // Find the index of the first gradient before view_left
+      double view_left_percent = (view_left - item_left) / W;
+      double view_right_percent = (view_right - item_left) / W;
+
+      double left_start = 0.;
+      double right_end = 1.;
+      auto left_it = this->m_colors.lower_bound(view_left_percent);
+      if(left_it != this->m_colors.begin())
+      {
+        --left_it;
+        left_start = left_it->first;
+      }
+
+      auto right_it = this->m_colors.upper_bound(view_right_percent);
+      if(right_it != this->m_colors.end())
+        right_end = right_it->first;
+
+      const double left_item_pos = left_start * W;
+      const double right_item_pos = right_end * W;
+
+      constexpr auto map = [](double input, double input_start, double input_end) {
+        constexpr auto output_start = 0.;
+        constexpr auto output_end = 1.;
+        return output_start
+               + ((output_end - output_start) / (input_end - input_start))
+                     * (input - input_start);
+      };
+
+      // Only fill between the gradient stops visible on screen
+      QLinearGradient g{QPointF{0, 0}, QPointF{1, 0.}};
+      g.setCoordinateMode(QGradient::CoordinateMode::ObjectMode);
+      for(const auto& [pos, col] : m_colors)
+      {
+        if(pos >= left_start && pos <= right_end)
+        {
+          const double mapped_pos = map(pos, left_start, right_end);
+          g.setColorAt(mapped_pos, col);
+        }
+      }
+
+      p->fillRect(
+          QRectF(left_item_pos, 0, right_item_pos - left_item_pos, rect.height()), g);
+    }
+  }
+  else
+  {
+    drawFull = true;
+  }
+
+  if(drawFull)
+  {
+    QLinearGradient g{QPointF{0, 0}, QPointF{m_dataWidth, 0.}};
+    for(const auto& col : m_colors)
+    {
+      g.setColorAt(col.first, col.second);
+    }
+    p->fillRect(rect, g);
   }
-  p->fillRect(boundingRect(), g);
 
   QPen pen(QColor::fromRgba(qRgba(200, 200, 200, 150)), 1);
   QBrush br(Qt::gray);
@@ -96,7 +165,6 @@ void View::mousePressEvent(QGraphicsSceneMouseEvent* event)
   {
     if(std::abs(e.first * m_dataWidth - pos.x()) < 2.)
     {
-
       if(event->button() == Qt::LeftButton)
       {
         if(pos.y() < (side / 1.5))