plotter/adapters/imgui/imgui_plot.cpp

#include <imgui/imgui_plot.hpp>

#include <imgui.h>

namespace plot::imgui {
namespace {

/// ImGui's button indices are left, right, middle; the controller's enum is the
/// same three, spelled out.
input::Button button_from_index(int index) noexcept {
  switch (index) {
    case 1:
      return input::Button::Right;
    case 2:
      return input::Button::Middle;
    default:
      return input::Button::Left;
  }
}

/// ImGui's modifier booleans as the controller's translatable mask. The bit
/// values are `input::Mod`'s by definition: it is documented as the mask a host
/// translates into, and the GLFW adapter's static_asserts pin the same layout.
u32 mods_from(const ImGuiIO& io) noexcept {
  u32 mods = 0;
  if (io.KeyShift) mods |= static_cast<u32>(input::Mod::Shift);
  if (io.KeyCtrl) mods |= static_cast<u32>(input::Mod::Ctrl);
  if (io.KeyAlt) mods |= static_cast<u32>(input::Mod::Alt);
  if (io.KeySuper) mods |= static_cast<u32>(input::Mod::Super);
  return mods;
}

}  // namespace

void PlotWidget::draw(const char* id, Plot& plot, Vec2 size, Renderer& renderer) {
  ImGuiIO& io = ImGui::GetIO();

  if (!controller_ || plot_ != &plot) {
    // The controller holds a Plot& (and the drag state that goes with it), so a
    // different plot is a different controller; re-binding drops any drag rather
    // than replaying its edges against the new plot.
    controller_.emplace(plot);
    plot_ = &plot;
    reset_pointer();
  }

  // The whole translation below rests on the backend having filled in a sane
  // scale; a zero one would collapse the region and silently draw nothing.
  IM_ASSERT(io.DisplayFramebufferScale.x > 0.0f && io.DisplayFramebufferScale.y > 0.0f);

  // The child is a plot surface, not a scrollable panel: no scrollbar, whatever
  // the item inside it measures. The wheel belongs to the plot while it is
  // hovered, and is taken away from window scrolling by the key owner below.
  const ImGuiWindowFlags child_flags = ImGuiWindowFlags_NoScrollbar;

  // The child is the plot's surface: it must paint nothing of its own over it.
  // ChildBg is transparent in every stock theme, but a host may have pushed a
  // panel colour; forcing it here means the widget can never cover its own plot.
  ImGui::PushStyleColor(ImGuiCol_ChildBg, ImVec4(0.0f, 0.0f, 0.0f, 0.0f));
  const bool shown =
      ImGui::BeginChild(id, ImVec2(size.x, size.y), ImGuiChildFlags_None, child_flags);
  if (!shown) {
    // Clipped away entirely (or collapsed): there is no region to draw into, and
    // no rect to aim a gesture at. EndChild is still owed.
    ImGui::EndChild();
    ImGui::PopStyleColor();
    return;
  }

  // A borderless child has no padding and no border, so the content region is the
  // child's rectangle itself: the pixels the plot draws are exactly the ones the
  // host laid out for it -- if the host leaves them visible. ImGui hands a child
  // the clip rect of an ancestor that cuts it off (and the child's own rect when
  // nothing does, which makes the intersection below a no-op); the widget's GL
  // draws do not go through ImGui's clipping, so a child the hosting window cuts
  // off would otherwise clear and paint over whatever the host drew outside it.
  const ImVec2 origin = ImGui::GetCursorScreenPos();
  const ImVec2 avail = ImGui::GetContentRegionAvail();
  const ImVec2 clip_min = ImGui::GetWindowDrawList()->GetClipRectMin();
  const ImVec2 clip_max = ImGui::GetWindowDrawList()->GetClipRectMax();
  const f32 x0 = origin.x > clip_min.x ? origin.x : clip_min.x;
  const f32 y0 = origin.y > clip_min.y ? origin.y : clip_min.y;
  const f32 x1 = origin.x + avail.x < clip_max.x ? origin.x + avail.x : clip_max.x;
  const f32 y1 = origin.y + avail.y < clip_max.y ? origin.y + avail.y : clip_max.y;
  if (x1 <= x0 || y1 <= y0) {
    // Nothing of the child is visible: no region to draw into, no rect to aim a
    // gesture at, and InvisibleButton wants a non-zero size.
    ImGui::EndChild();
    ImGui::PopStyleColor();
    return;
  }
  const ImVec2 visible{x1 - x0, y1 - y0};

  // An invisible item covering the *visible* region is what makes it *the* hovered
  // thing: `IsItemHovered()` is false while a drag elsewhere in the host is active,
  // so the plot does not pan when the user drags a slider next to it, and
  // `SetItemKeyOwner()` -- ImGui's documented way for a widget to take the wheel
  // away from window scrolling -- needs an item to own it. Both buttons are
  // enabled: the controller's left and right drags are the plot's own, and ImGui
  // only needs to know that the pointer is interacting with this region.
  ImGui::SetCursorScreenPos(ImVec2(x0, y0));
  ImGui::InvisibleButton("surface", visible,
                         ImGuiButtonFlags_MouseButtonLeft | ImGuiButtonFlags_MouseButtonRight |
                             ImGuiButtonFlags_MouseButtonMiddle);
  const bool hovered = ImGui::IsItemHovered();
  ImGui::SetItemKeyOwner(ImGuiKey_MouseWheelY);

  const ImVec2 scale = io.DisplayFramebufferScale;
  const f32 fb_w_f = io.DisplaySize.x * scale.x;
  const f32 fb_h_f = io.DisplaySize.y * scale.y;
  // Points -> framebuffer pixels: the assumption documented on the class. State
  // it in the arithmetic so a reader sees where the mapping enters.
  const Rect region{x0 * scale.x, y0 * scale.y, visible.x * scale.x, visible.y * scale.y};

  if (fb_w_f >= 1.0f && fb_h_f >= 1.0f) {
    // One viewport for the gesture geometry and the frame: the controller
    // forwards it to the plot (and re-lays it out), and `begin_frame` gets the
    // same rect and the whole framebuffer it is a sub-rectangle of.
    controller_->set_viewport(region);
    forward_input(hovered);

    // The backend's framebuffer scale is the display's scale, so the renderer draws
    // its logical style metrics at it: the same assertion above pins it positive,
    // and the chrome is then the same physical size whatever surface the host
    // renders into.
    renderer.style().ui_scale = scale.x;
    const u32 fb_w = static_cast<u32>(fb_w_f + 0.5f);
    const u32 fb_h = static_cast<u32>(fb_h_f + 0.5f);
    renderer.begin_frame(fb_w, fb_h, region);
    renderer.draw(plot);
    renderer.end_frame();
    // `end_frame` leaves the GL state neutral and the viewport at the plot's
    // rect; ImGui's backend sets its own viewport, blend function and so on
    // before its first draw, so the chrome renders correctly after this.
  }

  ImGui::EndChild();
  ImGui::PopStyleColor();
}

void PlotWidget::forward_input(bool hovered) {
  ImGuiIO& io = ImGui::GetIO();
  const ImVec2 scale = io.DisplayFramebufferScale;

  // ImGui reports "the pointer is not on this window" as a sentinel position:
  // imgui_impl_glfw's cursor-enter callback leaves (-FLT_MAX, -FLT_MAX) when the
  // pointer leaves, and ClearInputMouse() does the same on a lost focus. Scaling
  // that and differencing it against the last real position would hand the
  // controller a ~1e35 "movement" for that frame -- and with a press captured over
  // the plot, that movement is a pan, so the range would leave the representable
  // world (and only `f` would bring it back, since a gesture that sets a range also
  // clears auto scale). An invalid position is therefore no position *update*: the
  // last real one stands, the movement is zero, and the button levels below still
  // flow, so the release that ends a drag is never lost.
  const bool pos_valid = ImGui::IsMousePosValid(&io.MousePos);
  const f32 px = pos_valid ? io.MousePos.x * scale.x : prev_x_;
  const f32 py = pos_valid ? io.MousePos.y * scale.y : prev_y_;

  const bool down[3] = {io.MouseDown[0], io.MouseDown[1], io.MouseDown[2]};
  const bool any_down = down[0] || down[1] || down[2];
  const u32 mods = mods_from(io);

  if ((!pos_valid && !have_prev_pos_) || (!hovered && !capturing_)) {
    // Either no position has ever been reported (nothing to aim at, no movement
    // to forward), or the pointer is nowhere near the plot. Remember the levels
    // either way, so re-entering the region does not synthesise an edge out of the
    // state it left with -- and so a re-bound plot, which keeps its state too,
    // cannot fabricate one either.
    prev_down_[0] = down[0];
    prev_down_[1] = down[1];
    prev_down_[2] = down[2];
    if (pos_valid) {
      prev_x_ = px;
      prev_y_ = py;
      have_prev_pos_ = true;
    }
    return;
  }

  // The movement since the previous frame is what a pan consumes; it rides the
  // first event of this frame and nothing else, so one frame's motion can never
  // be counted twice. An invalid position contributes none of it.
  const f32 dx = (pos_valid && have_prev_pos_) ? px - prev_x_ : 0.0f;
  const f32 dy = (pos_valid && have_prev_pos_) ? py - prev_y_ : 0.0f;
  bool delta_pending = true;

  const auto deliver = [&](input::Button button, bool pressed, bool released) {
    input::PointerEvent e;
    e.x = px;
    e.y = py;
    if (delta_pending) {
      e.dx = dx;
      e.dy = dy;
      delta_pending = false;
    }
    e.button = button;
    e.mods = mods;
    e.down = any_down;
    e.pressed = pressed;
    e.released = released;
    controller_->on_pointer(e);
  };

  // ImGui reports levels; the controller wants edges. One event per button whose
  // level changed, which is the press or the release the host's callback would
  // have carried.
  for (int i = 0; i < 3; ++i) {
    if (down[i] == prev_down_[i]) continue;
    deliver(button_from_index(i), down[i], !down[i]);
  }
  if (delta_pending) {
    // No transition this frame: one plain move sample, so a pan in progress
    // advances by exactly the movement since the last one and
    // Controller::hovered_cursor() sees where the pointer is.
    deliver(input::Button::Left, false, false);
  }

  if (hovered && io.MouseWheel != 0.0f) {
    input::WheelEvent e;
    e.x = px;
    e.y = py;
    e.dx = io.MouseWheelH;
    e.dy = io.MouseWheel;  // ImGui's positive y (scroll up) is the zoom-in sign
    e.mods = mods;
    controller_->on_wheel(e);
  }

  // Keyboard gestures, on the same hover gate as the pointer: a key pressed
  // while the plot is under the pointer is aimed at the plot. `+` on most
  // layouts is shift+`=`, and either spelling zooms in, as in the GLFW adapter.
  if (hovered && !io.WantTextInput) {
    const auto key = [&](int code) {
      input::KeyEvent e;
      e.key = code;
      e.mods = mods;
      e.down = true;  // ImGui already edge-detected the press
      controller_->on_key(e);
    };
    if (ImGui::IsKeyPressed(ImGuiKey_Equal, false) ||
        ImGui::IsKeyPressed(ImGuiKey_KeypadAdd, false)) {
      key(input::kKeyZoomIn);
    }
    if (ImGui::IsKeyPressed(ImGuiKey_Minus, false) ||
        ImGui::IsKeyPressed(ImGuiKey_KeypadSubtract, false)) {
      key(input::kKeyZoomOut);
    }
    if (ImGui::IsKeyPressed(ImGuiKey_F, false)) key(input::kKeyFit);
  }

  prev_down_[0] = down[0];
  prev_down_[1] = down[1];
  prev_down_[2] = down[2];
  if (pos_valid) {
    prev_x_ = px;
    prev_y_ = py;
    have_prev_pos_ = true;
  }
  // Capture: a press delivered while the pointer was over the child keeps the
  // release flowing even once the pointer has left it.
  capturing_ = any_down && (capturing_ || hovered);
}

void PlotWidget::reset_pointer() noexcept {
  // The remembered button levels deliberately survive a re-bind, because they are
  // the host's input state and not the plot's: clearing them would make the next
  // frame synthesise a press out of a button that is already down, and a host
  // drawing two plots through one widget re-binds twice a frame. Only the
  // position memory and the capture are dropped, so the new plot's first event
  // carries no movement carried over from the old one.
  prev_x_ = 0.0f;
  prev_y_ = 0.0f;
  have_prev_pos_ = false;
  capturing_ = false;
}

}  // namespace plot::imgui