Backends: SDL: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(). Use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize() when bound to a SDL_Renderer. (#4927)

This is (kind of) an OpenGL-only function, which should be avoided when SDL2 isn't using OpenGL.
The only alternative that is recommended is SDL_GetRendererOutputSize, which limits this fix to the SDL_Renderer backend. Still, I think it's better than nothing.
I say that SDL_GL_GetDrawableSize is "kind of" OpenGL-only because it does technically work even when SDL2 isn't using OpenGL.
It's just that it becomes a shim to SDL_GetWindowSize, which is not suitable for high-DPI usage because it reflects the size of the window in screen coordinates, not actual pixels, so it really should be avoided when not using OpenGL.

backends/imgui_impl_sdl.cpp

@@ -18,6 +18,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2022-02-04: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(), so we can use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize() when bound to a SDL_Renderer.
 //  2022-01-26: Inputs: replaced short-lived io.AddKeyModsEvent() (added two weeks ago)with io.AddKeyEvent() using ImGuiKey_ModXXX flags. Sorry for the confusion.
 //  2021-01-20: Inputs: calling new io.AddKeyAnalogEvent() for gamepad support, instead of writing directly to io.NavInputs[].
 //  2022-01-17: Inputs: calling new io.AddMousePosEvent(), io.AddMouseButtonEvent(), io.AddMouseWheelEvent() API (1.87+).
@@ -79,6 +80,7 @@
 struct ImGui_ImplSDL2_Data
 {
     SDL_Window*     Window;
+    SDL_Renderer*   Renderer;
     Uint64          Time;
     int             MouseButtonsDown;
     SDL_Cursor*     MouseCursors[ImGuiMouseCursor_COUNT];
@@ -299,7 +301,7 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
     return false;
 }
 
-static bool ImGui_ImplSDL2_Init(SDL_Window* window)
+static bool ImGui_ImplSDL2_Init(SDL_Window* window, SDL_Renderer* renderer)
 {
     ImGuiIO& io = ImGui::GetIO();
     IM_ASSERT(io.BackendPlatformUserData == NULL && "Already initialized a platform backend!");
@@ -323,6 +325,7 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window)
     io.BackendFlags |= ImGuiBackendFlags_HasSetMousePos;        // We can honor io.WantSetMousePos requests (optional, rarely used)
 
     bd->Window = window;
+    bd->Renderer = renderer;
     bd->MouseCanUseGlobalState = mouse_can_use_global_state;
 
     io.SetClipboardTextFn = ImGui_ImplSDL2_SetClipboardText;
@@ -365,7 +368,7 @@ static bool ImGui_ImplSDL2_Init(SDL_Window* window)
 bool ImGui_ImplSDL2_InitForOpenGL(SDL_Window* window, void* sdl_gl_context)
 {
     IM_UNUSED(sdl_gl_context); // Viewport branch will need this.
-    return ImGui_ImplSDL2_Init(window);
+    return ImGui_ImplSDL2_Init(window, NULL);
 }
 
 bool ImGui_ImplSDL2_InitForVulkan(SDL_Window* window)
@@ -373,7 +376,7 @@ bool ImGui_ImplSDL2_InitForVulkan(SDL_Window* window)
 #if !SDL_HAS_VULKAN
     IM_ASSERT(0 && "Unsupported");
 #endif
-    return ImGui_ImplSDL2_Init(window);
+    return ImGui_ImplSDL2_Init(window, NULL);
 }
 
 bool ImGui_ImplSDL2_InitForD3D(SDL_Window* window)
@@ -381,17 +384,17 @@ bool ImGui_ImplSDL2_InitForD3D(SDL_Window* window)
 #if !defined(_WIN32)
     IM_ASSERT(0 && "Unsupported");
 #endif
-    return ImGui_ImplSDL2_Init(window);
+    return ImGui_ImplSDL2_Init(window, NULL);
 }
 
 bool ImGui_ImplSDL2_InitForMetal(SDL_Window* window)
 {
-    return ImGui_ImplSDL2_Init(window);
+    return ImGui_ImplSDL2_Init(window, NULL);
 }
 
-bool ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window)
+bool ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window, SDL_Renderer* renderer)
 {
-    return ImGui_ImplSDL2_Init(window);
+    return ImGui_ImplSDL2_Init(window, renderer);
 }
 
 void ImGui_ImplSDL2_Shutdown()
@@ -520,6 +523,9 @@ void ImGui_ImplSDL2_NewFrame()
     SDL_GetWindowSize(bd->Window, &w, &h);
     if (SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_MINIMIZED)
         w = h = 0;
+    if (bd->Renderer != NULL)
+        SDL_GetRendererOutputSize(bd->Renderer, &display_w, &display_h);
+    else
         SDL_GL_GetDrawableSize(bd->Window, &display_w, &display_h);
     io.DisplaySize = ImVec2((float)w, (float)h);
     if (w > 0 && h > 0)

backends/imgui_impl_sdl.h

@@ -19,13 +19,14 @@
 #include "imgui.h"      // IMGUI_IMPL_API
 
 struct SDL_Window;
+struct SDL_Renderer;
 typedef union SDL_Event SDL_Event;
 
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForOpenGL(SDL_Window* window, void* sdl_gl_context);
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForVulkan(SDL_Window* window);
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForD3D(SDL_Window* window);
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForMetal(SDL_Window* window);
-IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window);
+IMGUI_IMPL_API bool     ImGui_ImplSDL2_InitForSDLRenderer(SDL_Window* window, SDL_Renderer* renderer);
 IMGUI_IMPL_API void     ImGui_ImplSDL2_Shutdown();
 IMGUI_IMPL_API void     ImGui_ImplSDL2_NewFrame();
 IMGUI_IMPL_API bool     ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event);

docs/CHANGELOG.txt

@@ -86,6 +86,8 @@ Breaking Changes:
   - If calling ImGui_ImplGlfw_InitXXX with install_callbacks=true: nothing to do. is already done for you.
   - If calling ImGui_ImplGlfw_InitXXX with install_callbacks=false: you WILL NEED to register the GLFW callback
     using glfwSetCursorPosCallback() and forward it to the backend function ImGui_ImplGlfw_CursorPosCallback().
+- Backends: SDL: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(), so backend can call
+  SDL_GetRendererOutputSize() to obtain framebuffer size valid for hi-dpi. (#4927) [@Clownacy]
 - Commented out redirecting functions/enums names that were marked obsolete in 1.69, 1.70, 1.71, 1.72 (March-July 2019)
   - ImGui::SetNextTreeNodeOpen()        -> use ImGui::SetNextItemOpen()
   - ImGui::GetContentRegionAvailWidth() -> use ImGui::GetContentRegionAvail().x

examples/example_sdl_sdlrenderer/main.cpp

@@ -30,7 +30,7 @@ int main(int, char**)
     }
 
     // Setup window
-    SDL_WindowFlags window_flags = (SDL_WindowFlags)(SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI);
+    SDL_WindowFlags window_flags = (SDL_WindowFlags)(SDL_WINDOW_RESIZABLE | SDL_WINDOW_ALLOW_HIGHDPI);
     SDL_Window* window = SDL_CreateWindow("Dear ImGui SDL2+SDL_Renderer example", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1280, 720, window_flags);
 
     // Setup SDL_Renderer instance
@@ -56,7 +56,7 @@ int main(int, char**)
     //ImGui::StyleColorsClassic();
 
     // Setup Platform/Renderer backends
-    ImGui_ImplSDL2_InitForSDLRenderer(window);
+    ImGui_ImplSDL2_InitForSDLRenderer(window, renderer);
     ImGui_ImplSDLRenderer_Init(renderer);
 
     // Load Fonts
@@ -100,7 +100,7 @@ int main(int, char**)
 
         // Start the Dear ImGui frame
         ImGui_ImplSDLRenderer_NewFrame();
-        ImGui_ImplSDL2_NewFrame(window);
+        ImGui_ImplSDL2_NewFrame();
         ImGui::NewFrame();
 
         // 1. Show the big demo window (Most of the sample code is in ImGui::ShowDemoWindow()! You can browse its code to learn more about Dear ImGui!).