Method ComputeViewboxMatrix

src/svg/node/SVGNode.cpp:146–190

https://www.w3.org/TR/SVG11/coords.html#PreserveAspectRatioAttribute

// https://www.w3.org/TR/SVG11/coords.html#PreserveAspectRatioAttribute
Matrix SVGNode::ComputeViewboxMatrix(const Rect& viewBox, const Rect& viewPort,
                                     SVGPreserveAspectRatio PreAspectRatio) {
  if (viewBox.isEmpty() || viewPort.isEmpty()) {
    return Matrix::MakeScale(0, 0);
  }

  auto computeScale = [&]() -> Point {
    const auto scaleX = viewPort.width() / viewBox.width();
    const auto scaleY = viewPort.height() / viewBox.height();

    if (PreAspectRatio.align == SVGPreserveAspectRatio::Align::None) {
      // none -> anisotropic scaling, regardless of fScale
      return {scaleX, scaleY};
    }

    // isotropic scaling
    const auto s = PreAspectRatio.scale == SVGPreserveAspectRatio::Scale::Meet
                       ? std::min(scaleX, scaleY)
                       : std::max(scaleX, scaleY);
    return {s, s};
  };

  auto computeTrans = [&](const Point& scale) -> Point {
    static constexpr float alignCoeffs[] = {
        0.0f,  // Min
        0.5f,  // Mid
        1.0f   // Max
    };

    const size_t x_coeff = static_cast<int>(PreAspectRatio.align) >> 0 & 0x03;
    const size_t y_coeff = static_cast<int>(PreAspectRatio.align) >> 2 & 0x03;

    const auto tx = -viewBox.x() * scale.x;
    const auto ty = -viewBox.y() * scale.y;
    const auto dx = viewPort.width() - viewBox.width() * scale.x;
    const auto dy = viewPort.height() - viewBox.height() * scale.y;

    return {tx + dx * alignCoeffs[x_coeff], ty + dy * alignCoeffs[y_coeff]};
  };

  auto scale = computeScale();
  auto transform = computeTrans(scale);

  return Matrix::MakeTrans(transform.x, transform.y) * Matrix::MakeScale(scale.x, scale.y);
}
}  // namespace tgfx