Begin: Work toward obsoleting the 5-arguments Begin() overload. Removed bg_alpha from internal BeginEx(). (3)

imgui.cpp

@@ -4025,25 +4025,41 @@ static ImGuiCol GetWindowBgColorIdxFromFlags(ImGuiWindowFlags flags)
 // Push a new ImGui window to add widgets to.
 // - A default window called "Debug" is automatically stacked at the beginning of every frame so you can use widgets without explicitly calling a Begin/End pair.
 // - Begin/End can be called multiple times during the frame with the same window name to append content.
-// - 'size_on_first_use' for a regular window denote the initial size for first-time creation (no saved data) and isn't that useful. Use SetNextWindowSize() prior to calling Begin() for more flexible window manipulation.
 // - The window name is used as a unique identifier to preserve window information across frames (and save rudimentary information to the .ini file).
 //   You can use the "##" or "###" markers to use the same label with different id, or same id with different label. See documentation at the top of this file.
 // - Return false when window is collapsed, so you can early out in your code. You always need to call ImGui::End() even if false is returned.
 // - Passing 'bool* p_open' displays a Close button on the upper-right corner of the window, the pointed value will be set to false when the button is pressed.
-// - Passing non-zero 'size' is roughly equivalent to calling SetNextWindowSize(size, ImGuiCond_FirstUseEver) prior to calling Begin().
 bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 {
-    return BeginEx(name, p_open, -1.0f, flags);
+    return BeginEx(name, p_open, flags);
 }
 
-bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_use, float bg_alpha, ImGuiWindowFlags flags)
+// FIXME-OBSOLETE: Old Begin() API, avoid calling this version directly!
+bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_use, float bg_alpha_override, ImGuiWindowFlags flags)
 {
+    ImGuiContext& g = *GImGui;
+
+    // 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);
-    return BeginEx(name, p_open, bg_alpha, flags);
+
+    // 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.
+    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 = BeginEx(name, p_open, flags);
+
+    if (bg_alpha_override >= 0.0f)
+        g.Style.Colors[bg_color_idx] = bg_color_backup;
+    return ret;
 }
 
-bool ImGui::BeginEx(const char* name, bool* p_open, float bg_alpha, ImGuiWindowFlags flags)
+bool ImGui::BeginEx(const char* name, bool* p_open, ImGuiWindowFlags flags)
 {
     ImGuiContext& g = *GImGui;
     const ImGuiStyle& style = g.Style;
@@ -4392,13 +4408,8 @@ bool ImGui::BeginEx(const char* name, bool* p_open, float bg_alpha, ImGuiWindowF
             window->BorderSize = (flags & ImGuiWindowFlags_ShowBorders) ? 1.0f : 0.0f;
 
             // Window background, Default Alpha
-            ImGuiCol bg_color_idx = GetWindowBgColorIdxFromFlags(flags);
+            ImU32 bg_col = GetColorU32(GetWindowBgColorIdxFromFlags(flags));
-            ImVec4 bg_color = style.Colors[bg_color_idx]; // We don't use GetColorU32() because bg_alpha is assigned (not multiplied) below
+            window->DrawList->AddRectFilled(window->Pos+ImVec2(0,window->TitleBarHeight()), window->Pos+window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImGuiCorner_All : ImGuiCorner_BotLeft|ImGuiCorner_BotRight);
-            if (bg_alpha >= 0.0f)
-                bg_color.w = bg_alpha;
-            bg_color.w *= style.Alpha;
-            if (bg_color.w > 0.0f)
-                window->DrawList->AddRectFilled(window->Pos+ImVec2(0,window->TitleBarHeight()), window->Pos+window->Size, ColorConvertFloat4ToU32(bg_color), window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImGuiCorner_All : ImGuiCorner_BotLeft|ImGuiCorner_BotRight);
 
             // Title bar
             const bool is_focused = g.NavWindow && window->RootNonPopupWindow == g.NavWindow->RootNonPopupWindow;

imgui_demo.cpp

@@ -2130,7 +2130,7 @@ 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));
+    ImGui::PushStyleColor(ImGuiCol_WindowBg, ImVec4(0.0f, 0.0f, 0.0f, 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)");

imgui_internal.h

@@ -782,7 +782,7 @@ namespace ImGui
     IMGUI_API void          PushItemFlag(ImGuiItemFlags option, bool enabled);
     IMGUI_API void          PopItemFlag();
 
-    IMGUI_API bool          BeginEx(const char* name, bool* p_open, float bg_alpha, ImGuiWindowFlags flags);
+    IMGUI_API bool          BeginEx(const char* name, bool* p_open, ImGuiWindowFlags flags);
 
     IMGUI_API void          OpenPopupEx(ImGuiID id, bool reopen_existing);
     IMGUI_API void          ClosePopup(ImGuiID id);