Function buildShadowAtlas

app/src/Platform/CSD.cpp:123–161

* @brief Builds the (2r+1)^2 9-slice atlas: corner tiles + 1px edge gradients + transparent center. */

 * @brief Builds the (2r+1)^2 9-slice atlas: corner tiles + 1px edge gradients + transparent center.
 */
QImage buildShadowAtlas(int radius)
{
  const int n = 2 * radius + 1;
  QImage atlas(n, n, QImage::Format_ARGB32_Premultiplied);
  atlas.fill(Qt::transparent);

  const QImage corner = generateShadowCorner(radius);
  const qreal inv     = radius > 0 ? 1.0 / radius : 0.0;

  QImage hEdge(1, radius, QImage::Format_ARGB32_Premultiplied);
  hEdge.fill(Qt::transparent);
  for (int y = 0; y < radius; ++y) {
    qreal a = 1.0 - static_cast<qreal>(radius - y - 1) * inv;
    a       = a * a * (3.0 - 2.0 * a) * CSD::ShadowOpacity;
    hEdge.setPixelColor(0, y, QColor(0, 0, 0, qRound(a * 255)));
  }

  QImage vEdge(radius, 1, QImage::Format_ARGB32_Premultiplied);
  vEdge.fill(Qt::transparent);
  for (int x = 0; x < radius; ++x) {
    qreal a = 1.0 - static_cast<qreal>(x) * inv;
    a       = a * a * (3.0 - 2.0 * a) * CSD::ShadowOpacity;
    vEdge.setPixelColor(x, 0, QColor(0, 0, 0, qRound(a * 255)));
  }

  QPainter p(&atlas);
  p.setCompositionMode(QPainter::CompositionMode_Source);
  p.drawImage(0, 0, corner);
  p.drawImage(radius + 1, 0, corner.flipped(Qt::Horizontal));
  p.drawImage(0, radius + 1, corner.flipped(Qt::Vertical));
  p.drawImage(radius + 1, radius + 1, corner.flipped(Qt::Horizontal | Qt::Vertical));
  p.drawImage(radius, 0, hEdge);
  p.drawImage(radius, radius + 1, hEdge.flipped(Qt::Vertical));
  p.drawImage(0, radius, vEdge.flipped(Qt::Horizontal));
  p.drawImage(radius + 1, radius, vEdge);
  p.end();

  return atlas;
}

/**
 * @brief Serves the shadow 9-slice atlas to BorderImage; "image://csdshadow/<radius>".