Columns: Lower overhead on column switches and switching to background channel. (second attempt for 9b3ce49)

Internals: Bits, comments, added ImRect::ToVec4()

docs/CHANGELOG.txt

@@ -56,6 +56,8 @@ Other Changes:
   BeginMenu()/EndMenu() or BeginPopup/EndPopup(). (#3223, #1207) [@rokups]
 - Drag and Drop: Fixed unintended fallback "..." tooltip display during drag operation when
   drag source uses _SourceNoPreviewTooltip flags. (#3160) [@rokups]
+- Columns: Lower overhead on column switches and switching to background channel.
+  Benefits Columns but was primarily made with Tables in mind!
 - ImDrawList: Fixed an issue where draw command merging or primitive unreserve while crossing the
   VtxOffset boundary would lead to draw commands with wrong VtxOffset. (#3129, #3163, #3232, #2591)
   [@thedmd, @Shironekoben, @sergeyn, @ocornut]

imgui.cpp

@@ -4169,7 +4169,12 @@ 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 the
+//   more specialized SetWindowClipRectBeforeSetChannel() to avoid extraneous updates of underlying ImDrawCmds.
 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
@@ -1012,7 +1013,8 @@ struct ImGuiColumns
     float               LineMinY, LineMaxY;
     float               HostCursorPosY;         // Backup of CursorPos at the time of BeginColumns()
     float               HostCursorMaxPosX;      // Backup of CursorMaxPos at the time of BeginColumns()
-    ImRect              HostClipRect;           // Backup of ClipRect at the time of BeginColumns()
+    ImRect              HostInitialClipRect;    // Backup of ClipRect at the time of BeginColumns()
+    ImRect              HostBackupClipRect;     // Backup of ClipRect during PushColumnsBackground()/PopColumnsBackground()
     ImRect              HostWorkRect;           // Backup of WorkRect at the time of BeginColumns()
     ImVector<ImGuiColumnData> Columns;
     ImDrawListSplitter  Splitter;
@@ -1894,6 +1896,7 @@ namespace ImGui
     IMGUI_API bool          IsDragDropPayloadBeingAccepted();
 
     // Internal Columns API (this is not exposed because we will encourage transitioning to the Tables API)
+    IMGUI_API void          SetWindowClipRectBeforeSetChannel(ImGuiWindow* window, const ImRect& clip_rect);
     IMGUI_API void          BeginColumns(const char* str_id, int count, ImGuiColumnsFlags flags = 0); // setup number of columns. use an identifier to distinguish multiple column sets. close with EndColumns().
     IMGUI_API void          EndColumns();                                                             // close columns
     IMGUI_API void          PushColumnClipRect(int column_index);

imgui_widgets.cpp

@@ -7398,6 +7398,7 @@ bool ImGui::TabItemLabelAndCloseButton(ImDrawList* draw_list, const ImRect& bb,
 // [SECTION] Widgets: Columns, BeginColumns, EndColumns, etc.
 // In the current version, Columns are very weak. Needs to be replaced with a more full-featured system.
 //-------------------------------------------------------------------------
+// - SetWindowClipRectBeforeSetChannel() [Internal]
 // - GetColumnIndex()
 // - GetColumnCount()
 // - GetColumnOffset()
@@ -7415,6 +7416,18 @@ bool ImGui::TabItemLabelAndCloseButton(ImDrawList* draw_list, const ImRect& bb,
 // - Columns()
 //-------------------------------------------------------------------------
 
+// [Internal] Small optimization to avoid calls to PopClipRect/SetCurrentChannel/PushClipRect in sequences,
+// they would meddle many times with the underlying ImDrawCmd.
+// Instead, we do a preemptive overwrite of clipping rectangle _without_ altering the command-buffer and let
+// the subsequent single call to SetCurrentChannel() does it things once.
+void ImGui::SetWindowClipRectBeforeSetChannel(ImGuiWindow* window, const ImRect& clip_rect)
+{
+    ImVec4 clip_rect_vec4 = clip_rect.ToVec4();
+    window->ClipRect = clip_rect;
+    window->DrawList->_CmdHeader.ClipRect = clip_rect_vec4;
+    window->DrawList->_ClipRectStack.Data[window->DrawList->_ClipRectStack.Size - 1] = clip_rect_vec4;
+}
+
 int ImGui::GetColumnIndex()
 {
     ImGuiWindow* window = GetCurrentWindowRead();
@@ -7549,11 +7562,11 @@ void ImGui::PushColumnsBackground()
     ImGuiColumns* columns = window->DC.CurrentColumns;
     if (columns->Count == 1)
         return;
+
+    // Optimization: avoid SetCurrentChannel() + PushClipRect()
+    columns->HostBackupClipRect = window->ClipRect;
+    SetWindowClipRectBeforeSetChannel(window, columns->HostInitialClipRect);
     columns->Splitter.SetCurrentChannel(window->DrawList, 0);
-    int cmd_size = window->DrawList->CmdBuffer.Size;
-    PushClipRect(columns->HostClipRect.Min, columns->HostClipRect.Max, false);
-    IM_UNUSED(cmd_size);
-    IM_ASSERT(cmd_size >= window->DrawList->CmdBuffer.Size); // Being in channel 0 this should not have created an ImDrawCmd
 }
 
 void ImGui::PopColumnsBackground()
@@ -7562,8 +7575,10 @@ void ImGui::PopColumnsBackground()
     ImGuiColumns* columns = window->DC.CurrentColumns;
     if (columns->Count == 1)
         return;
+
+    // Optimization: avoid PopClipRect() + SetCurrentChannel()
+    SetWindowClipRectBeforeSetChannel(window, columns->HostBackupClipRect);
     columns->Splitter.SetCurrentChannel(window->DrawList, columns->Current + 1);
-    PopClipRect();
 }
 
 ImGuiColumns* ImGui::FindOrCreateColumns(ImGuiWindow* window, ImGuiID id)
@@ -7611,7 +7626,7 @@ void ImGui::BeginColumns(const char* str_id, int columns_count, ImGuiColumnsFlag
 
     columns->HostCursorPosY = window->DC.CursorPos.y;
     columns->HostCursorMaxPosX = window->DC.CursorMaxPos.x;
-    columns->HostClipRect = window->ClipRect;
+    columns->HostInitialClipRect = window->ClipRect;
     columns->HostWorkRect = window->WorkRect;
 
     // Set state for first column
@@ -7683,25 +7698,31 @@ void ImGui::NextColumn()
         IM_ASSERT(columns->Current == 0);
         return;
     }
+
+    // Next column
+    if (++columns->Current == columns->Count)
+        columns->Current = 0;
+
     PopItemWidth();
-    PopClipRect();
+
+    // Optimization: avoid PopClipRect() + SetCurrentChannel() + PushClipRect()
+    // (which would needlessly attempt to update commands in the wrong channel, then pop or overwrite them),
+    ImGuiColumnData* column = &columns->Columns[columns->Current];
+    SetWindowClipRectBeforeSetChannel(window, column->ClipRect);
+    columns->Splitter.SetCurrentChannel(window->DrawList, columns->Current + 1);
 
     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?
         window->DC.ColumnsOffset.x = GetColumnOffset(columns->Current) - window->DC.Indent.x + column_padding;
-        columns->Splitter.SetCurrentChannel(window->DrawList, columns->Current + 1);
     }
     else
     {
-        // New row/line
-        // Column 0 honor IndentX
+        // New row/line: 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);
@@ -7709,8 +7730,6 @@ void ImGui::NextColumn()
     window->DC.CurrLineSize = ImVec2(0.0f, 0.0f);
     window->DC.CurrLineTextBaseOffset = 0.0f;
 
-    PushColumnClipRect(columns->Current);     // FIXME-COLUMNS: Could it be an overwrite?
-
     // FIXME-COLUMNS: Share code with BeginColumns() - move code on columns setup.
     float offset_0 = GetColumnOffset(columns->Current);
     float offset_1 = GetColumnOffset(columns->Current + 1);