Columns: Lower overhead on column switches and switching to backgroun…

…d channel (some stress tests in debug builds went 3->2 ms). (#125)

This change benefits Columns but was primarily made with Tables in mind.

docs/CHANGELOG.txt

@@ -46,6 +46,8 @@ Other Changes:
  flag was also set, and _OpenOnArrow is frequently set along with _OpenOnDoubleClick).
 - TreeNode: Fixed bug where dragging a payload over a TreeNode() with either _OpenOnDoubleClick
  or _OpenOnArrow would open the node. (#143)
+- Columns: Lower overhead on column switches and switching to background channel (some of our stress
+ tests in debug builds went 3->2 ms). Benefits Columns but was primarily made with Tables in mind!
 - Style: Added style.TabMinWidthForUnselectedCloseButton settings.
  Set to 0.0f (default) to always make a close button appear on hover (same as Chrome, VS).
  Set to FLT_MAX to only display a close button when selected (merely hovering is not enough).

imgui.cpp

@@ -4167,7 +4167,12 @@ static void SetupDrawData(ImVector<ImDrawList*>* draw_lists, ImDrawData* draw_da
  }
 }
 
-// When using this function it is sane to ensure that float are perfectly rounded to integer values, to that e.g. (int)(max.x-min.x) in user's render produce correct result.
+// Push a clipping rectangle for both ImGui logic (hit-testing etc.) and low-level ImDrawList rendering.
+// - When using this function it is sane to ensure that float are perfectly rounded to integer values,
+// so that e.g. (int)(max.x-min.x) in user's render produce correct result.
+// - If the code here changes, may need to update code of functions like NextColumn() and PushColumnClipRect():
+// some frequently called functions which to modify both channels and clipping simultaneously tend to use a more
+// specialized code path to added extraneous updates of the underlying ImDrawCmd.
 void ImGui::PushClipRect(const ImVec2& clip_rect_min, const ImVec2& clip_rect_max, bool intersect_with_current_clip_rect)
 {
  ImGuiWindow* window = GetCurrentWindow();

imgui_internal.h

@@ -435,6 +435,7 @@ struct IMGUI_API ImRect
  void ClipWithFull(const ImRect& r) { Min = ImClamp(Min, r.Min, r.Max); Max = ImClamp(Max, r.Min, r.Max); } // Full version, ensure both points are fully clipped.
  void Floor() { Min.x = IM_FLOOR(Min.x); Min.y = IM_FLOOR(Min.y); Max.x = IM_FLOOR(Max.x); Max.y = IM_FLOOR(Max.y); }
  bool IsInverted() const { return Min.x > Max.x || Min.y > Max.y; }
+ ImVec4 ToVec4() const { return ImVec4(Min.x, Min.y, Max.x, Max.y); }
 };
 
 // Helper: ImBitArray

imgui_widgets.cpp

@@ -7549,6 +7549,11 @@ void ImGui::PushColumnsBackground()
  ImGuiColumns* columns = window->DC.CurrentColumns;
  if (columns->Count == 1)
  return;
+
+ // Set cmd header ahead to avoid SetCurrentChannel+PushClipRect doing an unnecessary AddDrawCmd/Pop
+ //if (window->DrawList->Flags & ImDrawListFlags_Debug) IMGUI_DEBUG_LOG("PushColumnsBackground()\n");
+ window->DrawList->_CmdHeader.ClipRect = columns->HostClipRect.ToVec4();
+
  columns->Splitter.SetCurrentChannel(window->DrawList, 0);
  int cmd_size = window->DrawList->CmdBuffer.Size;
  PushClipRect(columns->HostClipRect.Min, columns->HostClipRect.Max, false);
@@ -7562,6 +7567,12 @@ void ImGui::PopColumnsBackground()
  ImGuiColumns* columns = window->DC.CurrentColumns;
  if (columns->Count == 1)
  return;
+
+ // Set cmd header ahead to avoid SetCurrentChannel+PushClipRect doing an unnecessary AddDrawCmd/Pop
+ //if (window->DrawList->Flags & ImDrawListFlags_Debug) IMGUI_DEBUG_LOG("PopColumnsBackground()\n");
+ ImVec4 pop_clip_rect = window->DrawList->_ClipRectStack.Data[window->DrawList->_ClipRectStack.Size - 2];
+ window->DrawList->_CmdHeader.ClipRect = pop_clip_rect;
+
  columns->Splitter.SetCurrentChannel(window->DrawList, columns->Current + 1);
  PopClipRect();
 }
@@ -7684,11 +7695,22 @@ void ImGui::NextColumn()
  return;
  }
  PopItemWidth();
- PopClipRect();
+
+ // Next column
+ if (++columns->Current == columns->Count)
+ columns->Current = 0;
+
+ // As a small optimization, to avoid doing PopClipRect() + SetCurrentChannel() + PushClipRect()
+ // (which would needlessly attempt to update commands in the wrong channel, then pop or overwrite them),
+ // We use a shortcut: we override ClipRect in window and drawlist's CmdHeader + SetCurrentChannel().
+ ImGuiColumnData* column = &columns->Columns[columns->Current];
+ window->ClipRect = column->ClipRect;
+ window->DrawList->_CmdHeader.ClipRect = column->ClipRect.ToVec4();
+ //PopClipRect();
 
  const float column_padding = g.Style.ItemSpacing.x;
  columns->LineMaxY = ImMax(columns->LineMaxY, window->DC.CursorPos.y);
- if (++columns->Current < columns->Count)
+ if (columns->Current > 0)
  {
  // Columns 1+ ignore IndentX (by canceling it out)
  // FIXME-COLUMNS: Unnecessary, could be locked?
@@ -7701,15 +7723,14 @@ void ImGui::NextColumn()
  // Column 0 honor IndentX
  window->DC.ColumnsOffset.x = ImMax(column_padding - window->WindowPadding.x, 0.0f);
  columns->Splitter.SetCurrentChannel(window->DrawList, 1);
- columns->Current = 0;
  columns->LineMinY = columns->LineMaxY;
  }
  window->DC.CursorPos.x = IM_FLOOR(window->Pos.x + window->DC.Indent.x + window->DC.ColumnsOffset.x);
  window->DC.CursorPos.y = columns->LineMinY;
  window->DC.CurrLineSize = ImVec2(0.0f, 0.0f);
  window->DC.CurrLineTextBaseOffset = 0.0f;
 
- PushColumnClipRect(columns->Current); // FIXME-COLUMNS: Could it be an overwrite?
+ //PushColumnClipRect(columns->Current);
 
  // FIXME-COLUMNS: Share code with BeginColumns() - move code on columns setup.
  float offset_0 = GetColumnOffset(columns->Current);