diff --git a/core/fxge/BUILD.gn b/core/fxge/BUILD.gn
index 9dfce17..044f723 100644
--- a/core/fxge/BUILD.gn
+++ b/core/fxge/BUILD.gn
@@ -213,6 +213,7 @@
     "cfx_folderfontinfo_unittest.cpp",
     "cfx_fontmapper_unittest.cpp",
     "dib/cfx_cmyk_to_srgb_unittest.cpp",
+    "dib/cfx_dibbase_unittest.cpp",
     "dib/cfx_dibitmap_unittest.cpp",
     "dib/cstretchengine_unittest.cpp",
     "fx_font_unittest.cpp",
diff --git a/core/fxge/dib/cfx_dibbase_unittest.cpp b/core/fxge/dib/cfx_dibbase_unittest.cpp
new file mode 100644
index 0000000..46b68e5
--- /dev/null
+++ b/core/fxge/dib/cfx_dibbase_unittest.cpp
@@ -0,0 +1,163 @@
+// Copyright 2020 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/fxge/dib/cfx_dibbase.h"
+
+#include "core/fxcrt/fx_coordinates.h"
+#include "core/fxge/dib/cfx_dibitmap.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+struct Input {
+  CFX_Point src_top_left;
+  CFX_Size src_size;
+  CFX_Point dest_top_left;
+  CFX_Size overlap_size;
+};
+
+struct Output {
+  CFX_Point src_top_left;
+  CFX_Point dest_top_left;
+  CFX_Size overlap_size;
+};
+
+void RunOverlapRectTest(const CFX_DIBitmap* bitmap,
+                        const Input& input,
+                        const Output* expected_output) {
+  // Initialize in-out parameters.
+  int src_left = input.src_top_left.x;
+  int src_top = input.src_top_left.y;
+  int dest_left = input.dest_top_left.x;
+  int dest_top = input.dest_top_left.y;
+  int overlap_width = input.overlap_size.width;
+  int overlap_height = input.overlap_size.height;
+
+  bool success = bitmap->GetOverlapRect(
+      dest_left, dest_top, overlap_width, overlap_height, input.src_size.width,
+      input.src_size.height, src_left, src_top,
+      /*pClipRgn=*/nullptr);
+  if (success == !expected_output) {
+    ADD_FAILURE();
+    return;
+  }
+
+  if (expected_output) {
+    EXPECT_EQ(expected_output->src_top_left.x, src_left);
+    EXPECT_EQ(expected_output->src_top_left.y, src_top);
+    EXPECT_EQ(expected_output->dest_top_left.x, dest_left);
+    EXPECT_EQ(expected_output->dest_top_left.y, dest_top);
+    EXPECT_EQ(expected_output->overlap_size.width, overlap_width);
+    EXPECT_EQ(expected_output->overlap_size.height, overlap_height);
+  }
+}
+
+}  // namespace
+
+TEST(CFX_DIBBaseTest, GetOverlapRectTrivialOverlap) {
+  auto bitmap = pdfium::MakeRetain<CFX_DIBitmap>();
+  EXPECT_TRUE(bitmap->Create(400, 300, FXDIB_Format::k1bppRgb));
+
+  const Input kInput = {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+                        /*dest_top_left=*/{0, 0},
+                        /*overlap_size=*/{400, 300}};
+  const Output kExpectedOutput = {/*src_top_left=*/{0, 0},
+                                  /*dest_top_left=*/{0, 0},
+                                  /*overlap_size=*/{400, 300}};
+  RunOverlapRectTest(bitmap.Get(), kInput, &kExpectedOutput);
+}
+
+TEST(CFX_DIBBaseTest, GetOverlapRectOverlapNoLimit) {
+  auto bitmap = pdfium::MakeRetain<CFX_DIBitmap>();
+  EXPECT_TRUE(bitmap->Create(400, 300, FXDIB_Format::k1bppRgb));
+
+  const Input kInput = {/*src_top_left=*/{35, 41}, /*src_size=*/{400, 300},
+                        /*dest_top_left=*/{123, 137},
+                        /*overlap_size=*/{200, 100}};
+  const Output kExpectedOutput = {/*src_top_left=*/{35, 41},
+                                  /*dest_top_left=*/{123, 137},
+                                  /*overlap_size=*/{200, 100}};
+  RunOverlapRectTest(bitmap.Get(), kInput, &kExpectedOutput);
+}
+
+TEST(CFX_DIBBaseTest, GetOverlapRectOverlapLimitedBySource) {
+  auto bitmap = pdfium::MakeRetain<CFX_DIBitmap>();
+  EXPECT_TRUE(bitmap->Create(400, 300, FXDIB_Format::k1bppRgb));
+
+  const Input kInput = {/*src_top_left=*/{141, 154}, /*src_size=*/{400, 300},
+                        /*dest_top_left=*/{35, 41},
+                        /*overlap_size=*/{270, 160}};
+  const Output kExpectedOutput = {/*src_top_left=*/{141, 154},
+                                  /*dest_top_left=*/{35, 41},
+                                  /*overlap_size=*/{259, 146}};
+  RunOverlapRectTest(bitmap.Get(), kInput, &kExpectedOutput);
+}
+
+TEST(CFX_DIBBaseTest, GetOverlapRectOverlapLimitedByDestination) {
+  auto bitmap = pdfium::MakeRetain<CFX_DIBitmap>();
+  EXPECT_TRUE(bitmap->Create(400, 300, FXDIB_Format::k1bppRgb));
+
+  const Input kInput = {/*src_top_left=*/{35, 41}, /*src_size=*/{400, 300},
+                        /*dest_top_left=*/{123, 137},
+                        /*overlap_size=*/{280, 170}};
+  const Output kExpectedOutput = {/*src_top_left=*/{35, 41},
+                                  /*dest_top_left=*/{123, 137},
+                                  /*overlap_size=*/{277, 163}};
+  RunOverlapRectTest(bitmap.Get(), kInput, &kExpectedOutput);
+}
+
+TEST(CFX_DIBBaseTest, GetOverlapRectBadInputs) {
+  auto bitmap = pdfium::MakeRetain<CFX_DIBitmap>();
+  EXPECT_TRUE(bitmap->Create(400, 300, FXDIB_Format::k1bppRgb));
+
+  const Input kEmptyInputs[] = {
+      // Empty source rect.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{0, 0},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{400, 300}},
+      // Empty overlap size.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{0, 0}},
+      // Source out of bounds on x-axis.
+      {/*src_top_left=*/{-400, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{400, 300}},
+  };
+  for (const Input& input : kEmptyInputs)
+    RunOverlapRectTest(bitmap.Get(), input, /*expected_output=*/nullptr);
+
+  const Input kOutOfBoundInputs[] = {
+      // Source out of bounds on x-axis.
+      {/*src_top_left=*/{400, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{400, 300}},
+      // Source out of bounds on y-axis.
+      {/*src_top_left=*/{0, 300}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{400, 300}},
+      // Source out of bounds on y-axis.
+      {/*src_top_left=*/{0, -300}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 0},
+       /*overlap_size=*/{400, 300}},
+      // Destination out of bounds on x-axis.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{-400, 0},
+       /*overlap_size=*/{400, 300}},
+      // Destination out of bounds on x-axis.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{400, 0},
+       /*overlap_size=*/{400, 300}},
+      // Destination out of bounds on y-axis.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, -300},
+       /*overlap_size=*/{400, 300}},
+      // Destination out of bounds on y-axis.
+      {/*src_top_left=*/{0, 0}, /*src_size=*/{400, 300},
+       /*dest_top_left=*/{0, 300},
+       /*overlap_size=*/{400, 300}},
+  };
+  for (const Input& input : kOutOfBoundInputs)
+    RunOverlapRectTest(bitmap.Get(), input, /*expected_output=*/nullptr);
+}