diff --git a/README.md b/README.md
index dc3d44e..8e20b5a 100644
--- a/README.md
+++ b/README.md
@@ -40,3 +40,10 @@
 - Reboot the device immediately and run the test again. The test should save the
   contents of pmsg buffer into /data/misc/recovery/inject.txt. Test will pass if
   this file has expected contents.
+
+`ResourceTest` validates whether the png files are qualified as background text
+image under recovery.
+
+    1. `adb sync data` to make sure the test-dir has the images to test.
+    2. The test will automatically pickup and verify all `_text.png` files in
+       the test dir.
diff --git a/minui/resources.cpp b/minui/resources.cpp
index e6909f2..726c627 100644
--- a/minui/resources.cpp
+++ b/minui/resources.cpp
@@ -374,7 +374,9 @@
 // This function tests if a locale string stored in PNG (prefix) matches
 // the locale string provided by the system (locale).
 bool matches_locale(const char* prefix, const char* locale) {
-    if (locale == NULL) return false;
+    if (locale == nullptr) {
+        return false;
+    }
 
     // Return true if the whole string of prefix matches the top part of
     // locale. For instance, prefix == "en" matches locale == "en_US";
diff --git a/tests/Android.mk b/tests/Android.mk
index 1203817..1dbd2b6 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -49,10 +49,34 @@
 LOCAL_CFLAGS := -Werror
 LOCAL_MODULE := recovery_manual_test
 LOCAL_ADDITIONAL_DEPENDENCIES := $(LOCAL_PATH)/Android.mk
-LOCAL_STATIC_LIBRARIES := libbase
+LOCAL_STATIC_LIBRARIES := \
+    libminui \
+    libbase
 
 LOCAL_SRC_FILES := manual/recovery_test.cpp
-LOCAL_SHARED_LIBRARIES := liblog
+LOCAL_SHARED_LIBRARIES := \
+    liblog \
+    libpng
+
+resource_files := $(call find-files-in-subdirs, bootable/recovery, \
+    "*_text.png", \
+    res-mdpi/images \
+    res-hdpi/images \
+    res-xhdpi/images \
+    res-xxhdpi/images \
+    res-xxxhdpi/images \
+    )
+
+# The resource image files that will go to $OUT/data/nativetest/recovery.
+testimage_out_path := $(TARGET_OUT_DATA)/nativetest/recovery
+GEN := $(addprefix $(testimage_out_path)/, $(resource_files))
+
+$(GEN): PRIVATE_PATH := $(LOCAL_PATH)
+$(GEN): PRIVATE_CUSTOM_TOOL = cp $< $@
+$(GEN): $(testimage_out_path)/% : bootable/recovery/%
+	$(transform-generated-source)
+LOCAL_GENERATED_SOURCES += $(GEN)
+
 include $(BUILD_NATIVE_TEST)
 
 # Component tests
diff --git a/tests/manual/recovery_test.cpp b/tests/manual/recovery_test.cpp
index e73cb15..d36dd33 100644
--- a/tests/manual/recovery_test.cpp
+++ b/tests/manual/recovery_test.cpp
@@ -14,19 +14,27 @@
  * limitations under the License.
  */
 
+#include <dirent.h>
 #include <string.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #include <string>
+#include <vector>
 
 #include <android-base/file.h>
+#include <android-base/strings.h>
 #include <android/log.h>
 #include <gtest/gtest.h>
+#include <png.h>
 #include <private/android_logger.h>
 
+#include "minui/minui.h"
+
 static const std::string myFilename = "/data/misc/recovery/inject.txt";
 static const std::string myContent = "Hello World\nWelcome to my recovery\n";
+static const std::string kLocale = "zu";
+static const std::string kResourceTestDir = "/data/nativetest/recovery/";
 
 // Failure is expected on systems that do not deliver either the
 // recovery-persist or recovery-refresh executables. Tests also require
@@ -84,3 +92,133 @@
   }
   EXPECT_EQ(0, unlink(myFilename.c_str()));
 }
+
+std::vector<std::string> image_dir {
+  "res-mdpi/images/",
+  "res-hdpi/images/",
+  "res-xhdpi/images/",
+  "res-xxhdpi/images/",
+  "res-xxxhdpi/images/"
+};
+
+static int png_filter(const dirent* de) {
+  if (de->d_type != DT_REG || !android::base::EndsWith(de->d_name, "_text.png")) {
+    return 0;
+  }
+  return 1;
+}
+
+// Find out all png files to test under /data/nativetest/recovery/.
+static std::vector<std::string> add_files() {
+  std::vector<std::string> files;
+  for (const std::string& str : image_dir) {
+    std::string dir_path = kResourceTestDir + str;
+    dirent** namelist;
+    int n = scandir(dir_path.c_str(), &namelist, png_filter, alphasort);
+    if (n == -1) {
+      printf("Failed to scan dir %s: %s\n", kResourceTestDir.c_str(), strerror(errno));
+      return files;
+    }
+    if (n == 0) {
+      printf("No file is added for test in %s\n", kResourceTestDir.c_str());
+    }
+
+    while (n--) {
+      std::string file_path = dir_path + namelist[n]->d_name;
+      files.push_back(file_path);
+      free(namelist[n]);
+    }
+    free(namelist);
+  }
+  return files;
+}
+
+class ResourceTest : public testing::TestWithParam<std::string> {
+ public:
+  static std::vector<std::string> png_list;
+
+  // Parse a png file and test if it's qualified for the background text image
+  // under recovery.
+  void SetUp() override {
+    std::string file_path = GetParam();
+    fp = fopen(file_path.c_str(), "rb");
+    ASSERT_NE(nullptr, fp);
+
+    unsigned char header[8];
+    size_t bytesRead = fread(header, 1, sizeof(header), fp);
+    ASSERT_EQ(sizeof(header), bytesRead);
+    ASSERT_EQ(0, png_sig_cmp(header, 0, sizeof(header)));
+
+    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
+    ASSERT_NE(nullptr, png_ptr);
+
+    info_ptr = png_create_info_struct(png_ptr);
+    ASSERT_NE(nullptr, info_ptr);
+
+    png_init_io(png_ptr, fp);
+    png_set_sig_bytes(png_ptr, sizeof(header));
+    png_read_info(png_ptr, info_ptr);
+
+    int color_type, bit_depth;
+    png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type, nullptr, nullptr,
+                 nullptr);
+    ASSERT_EQ(PNG_COLOR_TYPE_GRAY, color_type) << "Recovery expects grayscale PNG file.";
+    ASSERT_LT(static_cast<png_uint_32>(5), width);
+    ASSERT_LT(static_cast<png_uint_32>(0), height);
+    if (bit_depth <= 8) {
+      // 1-, 2-, 4-, or 8-bit gray images: expand to 8-bit gray.
+      png_set_expand_gray_1_2_4_to_8(png_ptr);
+    }
+
+    png_byte channels = png_get_channels(png_ptr, info_ptr);
+    ASSERT_EQ(1, channels) << "Recovery background text images expects 1-channel PNG file.";
+  }
+
+  void TearDown() override {
+    if (png_ptr != nullptr && info_ptr != nullptr) {
+      png_destroy_read_struct(&png_ptr, &info_ptr, nullptr);
+    }
+
+    if (fp != nullptr) {
+      fclose(fp);
+    }
+  }
+
+ protected:
+  png_structp png_ptr;
+  png_infop info_ptr;
+  png_uint_32 width, height;
+
+  FILE* fp;
+};
+
+std::vector<std::string> ResourceTest::png_list = add_files();
+
+TEST_P(ResourceTest, ValidateLocale) {
+  std::vector<unsigned char> row(width);
+  for (png_uint_32 y = 0; y < height; ++y) {
+    png_read_row(png_ptr, row.data(), nullptr);
+    int w = (row[1] << 8) | row[0];
+    int h = (row[3] << 8) | row[2];
+    int len = row[4];
+    EXPECT_LT(0, w);
+    EXPECT_LT(0, h);
+    EXPECT_LT(0, len) << "Locale string should be non-empty.";
+    EXPECT_NE(0, row[5]) << "Locale string is missing.";
+
+    ASSERT_GT(height, y + 1 + h) << "Locale: " << kLocale << " is not found in the file.";
+    char* loc = reinterpret_cast<char*>(&row[5]);
+    if (matches_locale(loc, kLocale.c_str())) {
+      EXPECT_TRUE(android::base::StartsWith(loc, kLocale.c_str()));
+      break;
+    } else {
+      for (int i = 0; i < h; ++i, ++y) {
+        png_read_row(png_ptr, row.data(), nullptr);
+      }
+    }
+  }
+}
+
+INSTANTIATE_TEST_CASE_P(BackgroundTextValidation, ResourceTest,
+                        ::testing::ValuesIn(ResourceTest::png_list.cbegin(),
+                                            ResourceTest::png_list.cend()));
diff --git a/tests/unit/locale_test.cpp b/tests/unit/locale_test.cpp
index 0e515f8..f732350 100644
--- a/tests/unit/locale_test.cpp
+++ b/tests/unit/locale_test.cpp
@@ -26,4 +26,7 @@
     EXPECT_TRUE(matches_locale("en", "en_GB"));
     EXPECT_FALSE(matches_locale("en_GB", "en"));
     EXPECT_FALSE(matches_locale("en_GB", "en_US"));
+    EXPECT_FALSE(matches_locale("en_US", ""));
+    // Empty locale prefix in the PNG file will match the input locale.
+    EXPECT_TRUE(matches_locale("", "en_US"));
 }