Android: Use scaledDensity as backbuffer scale (for imgui)

Source/Android/jni/MainAndroid.cpp

@@ -52,6 +52,7 @@
 #include "VideoCommon/RenderBase.h"
 #include "VideoCommon/VideoBackendBase.h"
 
+#include "../../Core/Common/WindowSystemInfo.h"
 #include "jni/AndroidCommon/AndroidCommon.h"
 #include "jni/AndroidCommon/IDCache.h"
 #include "jni/ButtonManager.h"
@@ -589,7 +590,52 @@ JNIEXPORT void JNICALL Java_org_dolphinemu_dolphinemu_NativeLibrary_ReloadWiimot
   Wiimote::LoadConfig();
 }
 
-static void Run(const std::vector<std::string>& paths, bool first_open,
+// Returns the scale factor for imgui rendering.
+// Based on the scaledDensity of the device's display metrics.
+static float GetRenderSurfaceScale(JNIEnv* env)
+{
+  // NativeLibrary emulation_activity = NativeLibrary.getEmulationActivity();
+  jclass native_library_class = env->FindClass("org/dolphinemu/dolphinemu/NativeLibrary");
+  jmethodID get_emulation_activity_method =
+      env->GetStaticMethodID(native_library_class, "getEmulationActivity",
+                             "()Lorg/dolphinemu/dolphinemu/activities/EmulationActivity;");
+  jobject emulation_activity =
+      env->CallStaticObjectMethod(native_library_class, get_emulation_activity_method);
+
+  // WindowManager window_manager = emulation_activity.getWindowManager();
+  jmethodID get_window_manager_method =
+      env->GetMethodID(env->GetObjectClass(emulation_activity), "getWindowManager",
+                       "()Landroid/view/WindowManager;");
+  jobject window_manager = env->CallObjectMethod(emulation_activity, get_window_manager_method);
+
+  // Display display = window_manager.getDisplay();
+  jmethodID get_display_method = env->GetMethodID(env->GetObjectClass(window_manager),
+                                                  "getDefaultDisplay", "()Landroid/view/Display;");
+  jobject display = env->CallObjectMethod(window_manager, get_display_method);
+
+  // DisplayMetrics metrics = new DisplayMetrics();
+  jclass display_metrics_class = env->FindClass("android/util/DisplayMetrics");
+  jmethodID display_metrics_constructor = env->GetMethodID(display_metrics_class, "<init>", "()V");
+  jobject metrics = env->NewObject(display_metrics_class, display_metrics_constructor);
+
+  // display.getMetrics(metrics);
+  jmethodID get_metrics_method = env->GetMethodID(env->GetObjectClass(display), "getMetrics",
+                                                  "(Landroid/util/DisplayMetrics;)V");
+  env->CallVoidMethod(display, get_metrics_method, metrics);
+
+  // float scaled_density = metrics.scaledDensity;
+  jfieldID scaled_density_field =
+      env->GetFieldID(env->GetObjectClass(metrics), "scaledDensity", "F");
+  float scaled_density = env->GetFloatField(metrics, scaled_density_field);
+  __android_log_print(ANDROID_LOG_INFO, DOLPHIN_TAG, "Using %f for render surface scale.",
+                      scaled_density);
+
+  // cleanup
+  env->DeleteLocalRef(metrics);
+  return scaled_density;
+}
+
+static void Run(JNIEnv* env, const std::vector<std::string>& paths, bool first_open,
                 std::optional<std::string> savestate_path = {}, bool delete_savestate = false)
 {
   ASSERT(!paths.empty());
@@ -614,6 +660,7 @@ static void Run(const std::vector<std::string>& paths, bool first_open,
   std::unique_ptr<BootParameters> boot = BootParameters::GenerateFromFile(paths, savestate_path);
   boot->delete_savestate = delete_savestate;
   WindowSystemInfo wsi(WindowSystemType::Android, nullptr, s_surf);
+  wsi.render_surface_scale = GetRenderSurfaceScale(env);
   if (BootManager::BootCore(std::move(boot), wsi))
   {
     ButtonManager::Init(SConfig::GetInstance().GetGameID());
@@ -650,14 +697,14 @@ static void Run(const std::vector<std::string>& paths, bool first_open,
 JNIEXPORT void JNICALL Java_org_dolphinemu_dolphinemu_NativeLibrary_Run___3Ljava_lang_String_2Z(
     JNIEnv* env, jobject obj, jobjectArray jPaths, jboolean jfirstOpen)
 {
-  Run(JStringArrayToVector(env, jPaths), jfirstOpen);
+  Run(env, JStringArrayToVector(env, jPaths), jfirstOpen);
 }
 
 JNIEXPORT void JNICALL
 Java_org_dolphinemu_dolphinemu_NativeLibrary_Run___3Ljava_lang_String_2Ljava_lang_String_2Z(
     JNIEnv* env, jobject obj, jobjectArray jPaths, jstring jSavestate, jboolean jDeleteSavestate)
 {
-  Run(JStringArrayToVector(env, jPaths), false, GetJString(env, jSavestate), jDeleteSavestate);
+  Run(env, JStringArrayToVector(env, jPaths), false, GetJString(env, jSavestate), jDeleteSavestate);
 }
 
 JNIEXPORT void JNICALL Java_org_dolphinemu_dolphinemu_NativeLibrary_ChangeDisc(JNIEnv* env,