Added SetNextWindowBgAlpha() helper. (#1567) particularly helpul with the marking of the old 5-parameters version of Begin() as obsolete.

imgui.cpp

@@ -4803,8 +4803,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         }
         else
         {
-            // Window background, Default Alpha
+            // Window background
             ImU32 bg_col = GetColorU32(GetWindowBgColorIdxFromFlags(flags));
+            if (g.NextWindowData.BgAlphaCond != 0)
+                bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(g.NextWindowData.BgAlphaVal) << IM_COL32_A_SHIFT);
             window->DrawList->AddRectFilled(window->Pos+ImVec2(0,window->TitleBarHeight()), window->Pos+window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImDrawCornerFlags_All : ImDrawCornerFlags_Bot);
 
             // Title bar
@@ -5010,22 +5012,12 @@ bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_us
 {
     // Old API feature: we could pass the initial window size as a parameter, however this was very misleading because in most cases it would only affect the window when it didn't have storage in the .ini file.
     if (size_on_first_use.x != 0.0f || size_on_first_use.y != 0.0f)
-        SetNextWindowSize(size_on_first_use, ImGuiCond_FirstUseEver);
+        ImGui::SetNextWindowSize(size_on_first_use, ImGuiCond_FirstUseEver);
 
-    // Old API feature: we could override the window background alpha with a parameter. This is actually tricky to reproduce manually because: 
-    // (1) there are multiple variants of WindowBg (popup, tooltip, etc.) and (2) you can't call PushStyleColor before Begin and PopStyleColor just after Begin() because of how CheckStackSizes() behave.
-    // The user-side solution is to do backup = GetStyleColorVec4(ImGuiCol_xxxBG), PushStyleColor(ImGuiCol_xxxBg), Begin, PushStyleColor(ImGuiCol_xxxBg, backup), [...], PopStyleColor(), End(); PopStyleColor() - which is super awkward.
-    // The alpha override was rarely used but for now we'll leave the Begin() variant around for a bit. We may either lift the constraint on CheckStackSizes() either add a SetNextWindowBgAlpha() helper that does it magically.
-    ImGuiContext& g = *GImGui;
-    const ImGuiCol bg_color_idx = GetWindowBgColorIdxFromFlags(flags);
-    const ImVec4 bg_color_backup = g.Style.Colors[bg_color_idx];
-    if (bg_alpha_override >= 0.0f)
-        g.Style.Colors[bg_color_idx].w = bg_alpha_override;
-
-    bool ret = Begin(name, p_open, flags);
+    // Old API feature: override the window background alpha with a parameter.
+    ImGui::SetNextWindowBgAlpha(bg_alpha_override);
 
-    if (bg_alpha_override >= 0.0f)
-        g.Style.Colors[bg_color_idx] = bg_color_backup;
+    bool ret = ImGui::Begin(name, p_open, flags);
     return ret;
 }
 #endif // IMGUI_DISABLE_OBSOLETE_FUNCTIONS
@@ -5803,7 +5795,14 @@ void ImGui::SetNextWindowCollapsed(bool collapsed, ImGuiCond cond)
 void ImGui::SetNextWindowFocus()
 {
     ImGuiContext& g = *GImGui;
-    g.NextWindowData.FocusCond = ImGuiCond_Always;
+    g.NextWindowData.FocusCond = ImGuiCond_Always;   // Using a Cond member for consistency (may transition all of them to single flag set for fast Clear() op)
+}
+
+void ImGui::SetNextWindowBgAlpha(float alpha)
+{
+    ImGuiContext& g = *GImGui;
+    g.NextWindowData.BgAlphaVal = alpha;
+    g.NextWindowData.BgAlphaCond = ImGuiCond_Always; // Using a Cond member for consistency (may transition all of them to single flag set for fast Clear() op)
 }
 
 // In window space (not screen space!)

imgui.h

@@ -171,6 +171,7 @@ namespace ImGui
     IMGUI_API void          SetNextWindowContentSize(const ImVec2& size);                       // set next window content size (~ enforce the range of scrollbars). not including window decorations (title bar, menu bar, etc.). set an axis to 0.0f to leave it automatic. call before Begin()
     IMGUI_API void          SetNextWindowCollapsed(bool collapsed, ImGuiCond cond = 0);         // set next window collapsed state. call before Begin()
     IMGUI_API void          SetNextWindowFocus();                                               // set next window to be focused / front-most. call before Begin()
+    IMGUI_API void          SetNextWindowBgAlpha(float alpha);                                  // set next window background color alpha. helper to easily modify ImGuiCol_WindowBg/ChildBg/PopupBg.
     IMGUI_API void          SetWindowPos(const ImVec2& pos, ImGuiCond cond = 0);                // (not recommended) set current window position - call within Begin()/End(). prefer using SetNextWindowPos(), as this may incur tearing and side-effects.
     IMGUI_API void          SetWindowSize(const ImVec2& size, ImGuiCond cond = 0);              // (not recommended) set current window size - call within Begin()/End(). set to ImVec2(0,0) to force an auto-fit. prefer using SetNextWindowSize(), as this may incur tearing and minor side-effects.    
     IMGUI_API void          SetWindowCollapsed(bool collapsed, ImGuiCond cond = 0);             // (not recommended) set current window collapsed state. prefer using SetNextWindowCollapsed().

imgui_demo.cpp

@@ -2398,7 +2398,7 @@ static void ShowExampleAppFixedOverlay(bool* p_open)
     ImVec2 window_pos = ImVec2((corner & 1) ? ImGui::GetIO().DisplaySize.x - DISTANCE : DISTANCE, (corner & 2) ? ImGui::GetIO().DisplaySize.y - DISTANCE : DISTANCE);
     ImVec2 window_pos_pivot = ImVec2((corner & 1) ? 1.0f : 0.0f, (corner & 2) ? 1.0f : 0.0f);
     ImGui::SetNextWindowPos(window_pos, ImGuiCond_Always, window_pos_pivot);
-    ImGui::PushStyleColor(ImGuiCol_WindowBg, ImVec4(0.0f, 0.0f, 0.0f, 0.3f)); // Transparent background
+    ImGui::SetNextWindowBgAlpha(0.3f); // Transparent background
     if (ImGui::Begin("Example: Fixed Overlay", p_open, ImGuiWindowFlags_NoTitleBar|ImGuiWindowFlags_NoResize|ImGuiWindowFlags_AlwaysAutoResize|ImGuiWindowFlags_NoMove|ImGuiWindowFlags_NoSavedSettings))
     {
         ImGui::Text("Simple overlay\nin the corner of the screen.\n(right-click to change position)");
@@ -2415,7 +2415,6 @@ static void ShowExampleAppFixedOverlay(bool* p_open)
         }
         ImGui::End();
     }
-    ImGui::PopStyleColor();
 }
 
 // Demonstrate using "##" and "###" in identifiers to manipulate ID generation.

imgui_internal.h

@@ -467,6 +467,7 @@ struct ImGuiNextWindowData
     ImGuiCond               CollapsedCond;
     ImGuiCond               SizeConstraintCond;
     ImGuiCond               FocusCond;
+    ImGuiCond               BgAlphaCond;
     ImVec2                  PosVal;
     ImVec2                  PosPivotVal;
     ImVec2                  SizeVal;
@@ -475,21 +476,23 @@ struct ImGuiNextWindowData
     ImRect                  SizeConstraintRect;                 // Valid if 'SetNextWindowSizeConstraint' is true
     ImGuiSizeCallback       SizeCallback;
     void*                   SizeCallbackUserData;
+    float                   BgAlphaVal;
 
     ImGuiNextWindowData()
     {
-        PosCond = SizeCond = ContentSizeCond = CollapsedCond = SizeConstraintCond = FocusCond = 0;
+        PosCond = SizeCond = ContentSizeCond = CollapsedCond = SizeConstraintCond = FocusCond = BgAlphaCond = 0;
         PosVal = PosPivotVal = SizeVal = ImVec2(0.0f, 0.0f);
         ContentSizeVal = ImVec2(0.0f, 0.0f);
         CollapsedVal = false;
         SizeConstraintRect = ImRect();
         SizeCallback = NULL;
         SizeCallbackUserData = NULL;
+        BgAlphaVal = FLT_MAX;
     }
 
     void    Clear()
     {
-        PosCond = SizeCond = ContentSizeCond = CollapsedCond = SizeConstraintCond = FocusCond = 0;
+        PosCond = SizeCond = ContentSizeCond = CollapsedCond = SizeConstraintCond = FocusCond = BgAlphaCond = 0;
     }
 };