The following patch adds support for reading display DPI from Android devices.

Please ignore the additional lines at the beginning of the patch.

The trustworthiness of the values depend on the manufacturer of the device, but over time this has improved.

I did not know how to fill the "diagonal DPI", so I put the "densityDPI" value in FWIW.

-------------------------------------------------------------------------------------------------


--- SDL_snapshot/src/core/android/SDL_android.c	2016-10-02 09:27:31.000000000 +0200
+++ SDL_patches/src/core/android/SDL_android.c	2016-10-02 11:49:15.985195517 +0200
@@ -421,6 +421,32 @@
     return result;
 }
 
+void Android_JNI_HintCallback(void *userdata, const char *name, const char *oldValue, const char *newValue) {
+    JNIEnv *env = Android_JNI_GetEnv();
+
+    jobject callback = (jobject)userdata;
+   jclass cls = (*env)->GetObjectClass(env, callback);
+    jmethodID method = (*env)->GetMethodID(env, cls, "callback", "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)V");
+
+    jstring javaName     = (*env)->NewStringUTF(env, name);
+    jstring javaOldValue = (*env)->NewStringUTF(env, oldValue);
+    jstring javaNewValue = (*env)->NewStringUTF(env, newValue);
+
+    (*env)->CallVoidMethod(env, callback, method, javaName, javaOldValue, javaNewValue);
+
+    (*env)->DeleteLocalRef(env, javaName);
+    (*env)->DeleteLocalRef(env, javaOldValue);
+    (*env)->DeleteLocalRef(env, javaNewValue);
+}
+
+void Java_org_libsdl_app_SDLActivity_nativeAddHintCallback(JNIEnv* env, jclass cls, jstring name, jobject callback) {
+    const char *utfname = (*env)->GetStringUTFChars(env, name, NULL);
+
+    SDL_AddHintCallback(utfname, Android_JNI_HintCallback, (*env)->NewGlobalRef(env, callback));
+
+    (*env)->ReleaseStringUTFChars(env, name, utfname);    
+}
+
 /*******************************************************************************
              Functions called by SDL into Java
 *******************************************************************************/
@@ -1770,6 +1796,63 @@
     return mActivityClass;
 }
 
+int Android_JNI_GetDisplayMetrics(float *density, int *densityDpi, float *xdpi, float *ydpi)
+{
+    jmethodID mid;
+    jobject   context;
+    jclass    contextClass;
+    jobject   resources;
+    jclass    resourcesClass;
+    jobject   displayMetrics;
+    jclass    displayMetricsClass;
+    jfieldID  IDdensity;
+    jfieldID  IDdensityDpi;
+    jfieldID  IDxdpi;
+    jfieldID  IDydpi;
+
+    struct LocalReferenceHolder refs = LocalReferenceHolder_Setup(__FUNCTION__);
+
+    JNIEnv *env = Android_JNI_GetEnv();
+
+    if (!LocalReferenceHolder_Init(&refs, env)) {
+        LocalReferenceHolder_Cleanup(&refs);
+        return -1;
+    }
+
+    /* context = SDLActivity.getContext(); */
+    mid = (*env)->GetStaticMethodID(env, mActivityClass,
+            "getContext","()Landroid/content/Context;");
+    context = (*env)->CallStaticObjectMethod(env, mActivityClass, mid);
+    contextClass = (*env)->GetObjectClass(env, context);
+
+    mid = (*env)->GetMethodID(env, contextClass,
+    		"getResources", "()Landroid/content/res/Resources;");
+    resources = (*env)->CallObjectMethod(env, context, mid);
+
+    resourcesClass = (*env)->GetObjectClass(env, resources);
+    mid = (*env)->GetMethodID(env, resourcesClass,
+    		"getDisplayMetrics", "()Landroid/util/DisplayMetrics;");
+    displayMetrics = (*env)->CallObjectMethod(env, resources, mid);
+
+    displayMetricsClass = (*env)->GetObjectClass(env, displayMetrics);
+
+    IDdensity    = (*env)->GetFieldID(env, displayMetricsClass, "density", "F");
+    IDdensityDpi = (*env)->GetFieldID(env, displayMetricsClass, "densityDpi", "I");
+    IDxdpi       = (*env)->GetFieldID(env, displayMetricsClass, "xdpi", "F");
+    IDydpi       = (*env)->GetFieldID(env, displayMetricsClass, "ydpi", "F");
+
+    *density    = (float) ((*env)->GetFloatField  (env, displayMetrics, IDdensity));
+    *densityDpi = (int)   ((*env)->GetIntField  (env, displayMetrics, IDdensityDpi));
+    *xdpi       = (float) ((*env)->GetFloatField(env, displayMetrics, IDxdpi));
+    *ydpi       = (float) ((*env)->GetFloatField(env, displayMetrics, IDydpi));
+
+    __android_log_print(ANDROID_LOG_INFO, "SDL", "SDL display density: dens=%.2f dpi=%d x=%.2f y=%.2f!", *density, *densityDpi, *xdpi, *ydpi);
+
+    LocalReferenceHolder_Cleanup(&refs);
+
+    return 0;
+}
+
 #endif /* __ANDROID__ */
 
 /* vi: set ts=4 sw=4 expandtab: */
diff -aur SDL_snapshot/src/core/android/SDL_android.h SDL_patches/src/core/android/SDL_android.h
--- SDL_snapshot/src/core/android/SDL_android.h	2016-10-02 09:27:31.000000000 +0200
+++ SDL_patches/src/core/android/SDL_android.h	2016-10-02 11:50:39.825776984 +0200
@@ -69,6 +69,7 @@
 
 /* Video */
 void Android_JNI_SuspendScreenSaver(SDL_bool suspend);
+int Android_JNI_GetDisplayMetrics(float *density, int *densityDpi, float *xdpi, float *ydpi);
 
 /* Touch support */
 int Android_JNI_GetTouchDeviceIds(int **ids);

--- SDL_snapshot/src/video/android/SDL_androidvideo.c	2016-10-02 09:27:31.000000000 +0200
+++ SDL_patches/src/video/android/SDL_androidvideo.c	2016-10-02 12:12:25.658772321 +0200
@@ -45,6 +45,7 @@
 /* Initialization/Query functions */
 static int Android_VideoInit(_THIS);
 static void Android_VideoQuit(_THIS);
+static int Android_GetDisplayDPI(_THIS, SDL_VideoDisplay * display, float * ddpi, float * hdpi, float * vdpi);
 
 #include "../SDL_egl_c.h"
 /* GL functions (SDL_androidgl.c) */
@@ -122,6 +123,7 @@
     device->SetWindowTitle = Android_SetWindowTitle;
     device->DestroyWindow = Android_DestroyWindow;
     device->GetWindowWMInfo = Android_GetWindowWMInfo;
+    device->GetDisplayDPI = Android_GetDisplayDPI;
 
     device->free = Android_DeleteDevice;
 
@@ -223,6 +225,21 @@
     }
 }
 
+static int Android_GetDisplayDPI(_THIS, SDL_VideoDisplay * display, float * ddpi, float * hdpi, float * vdpi)
+{
+	float density;
+	int densityDpi;
+
+	if (Android_JNI_GetDisplayMetrics(&density, &densityDpi, hdpi, vdpi) == -1)
+	{
+		return -1;
+	}
+
+    *ddpi = (float) densityDpi;
+
+    return 0;
+}
+
 #endif /* SDL_VIDEO_DRIVER_ANDROID */
 
 /* vi: set ts=4 sw=4 expandtab: */