chromecast: Tighten up scale factors used for automatic scaling

Rather than allowing arbitrary scales, only allow integer scales and 1.5
(for 1080p). This means we won't apply strange scales that may not work
well when using a non-standard resolution. There's no change for standard
TV resolutions.

Bug: b/117782357
Test: cast_graphics_unittests

Change-Id: Ied44492232fbe7bf61d0d48e99ab5a6038f28031

Reviewed-on: https://chromium-review.googlesource.com/c/1344904
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609853}

chromecast/browser/cast_display_configurator.cc

@@ -14,6 +14,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "chromecast/base/cast_features.h"
 #include "chromecast/browser/cast_touch_device_manager.h"
+#include "chromecast/graphics/cast_display_util.h"
 #include "chromecast/graphics/cast_screen.h"
 #include "chromecast/public/graphics_properties_shlib.h"
 #include "ui/display/types/display_snapshot.h"
@@ -68,17 +69,6 @@ display::Display::Rotation GetRotationFromCommandLine() {
     return display::Display::ROTATE_0;
 }
 
-float GetDeviceScaleFactor(gfx::Size display_resolution) {
-  // TODO(spang): Look into tightening up the allowed scale factors here
-  // rather than allowing all scales >= 1.f
-  int smaller_dimension =
-      std::min(display_resolution.width(), display_resolution.height());
-  float ratio = smaller_dimension / 720.f;
-  if (ratio < 1.f)
-    return 1.f;
-  return ratio;
-}
-
 gfx::Rect GetScreenBounds(const gfx::Size& size_in_pixels,
                           display::Display::Rotation rotation) {
   switch (rotation) {

chromecast/graphics/BUILD.gn

@@ -43,6 +43,8 @@ cast_source_set("graphics") {
       "accessibility/magnification_controller.h",
       "accessibility/partial_magnification_controller.cc",
       "accessibility/partial_magnification_controller.h",
+      "cast_display_util.cc",
+      "cast_display_util.h",
       "cast_focus_client_aura.cc",
       "cast_focus_client_aura.h",
       "cast_touch_event_gate.cc",
@@ -116,6 +118,7 @@ if (use_aura && !is_cast_audio_only) {
     sources = [
       "accessibility/accessibility_focus_ring_controller_unittest.cc",
       "accessibility/partial_magnification_controller_unittest.cc",
+      "cast_display_util_unittest.cc",
       "cast_focus_client_aura_test.cc",
       "cast_touch_event_gate_test.cc",
       "cast_views_test.cc",

chromecast/graphics/cast_display_util.cc

+// Copyright 2018 The Chromium 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 "chromecast/graphics/cast_display_util.h"
+
+#include <math.h>
+#include <algorithm>
+
+namespace chromecast {
+
+float GetDeviceScaleFactor(const gfx::Size& display_resolution) {
+  int smaller_dimension =
+      std::min(display_resolution.width(), display_resolution.height());
+  float ratio = smaller_dimension / 720.f;
+  if (ratio >= 2.f)
+    return floorf(ratio);
+  if (ratio >= 1.5f)
+    return 1.5f;
+  return 1.f;
+}
+
+}  // namespace chromecast

chromecast/graphics/cast_display_util.h

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMECAST_GRAPHICS_CAST_DISPLAY_UTIL_H_
+#define CHROMECAST_GRAPHICS_CAST_DISPLAY_UTIL_H_
+
+#include "ui/gfx/geometry/size.h"
+
+namespace chromecast {
+
+// Computes the default scale factor for display with given resolution.
+//
+// Cast applications target 720p; larger resolutions will be scaled up so
+// that content fills the screen.
+float GetDeviceScaleFactor(const gfx::Size& display_resolution);
+
+}  // namespace chromecast
+
+#endif  // CHROMECAST_GRAPHICS_CAST_DISPLAY_UTIL_H_

chromecast/graphics/cast_display_util_unittest.cc

+// Copyright 2018 The Chromium 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 "chromecast/graphics/cast_display_util.h"
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace chromecast {
+
+TEST(CastDisplayScaleTest, StandardResolutions_16x9) {
+  const gfx::Size kResolution720p(1280, 720);
+  const gfx::Size kResolution1080p(1920, 1080);
+  const gfx::Size kResolution1440p(2560, 1440);
+  const gfx::Size kResolution2160p(3840, 2160);
+  const gfx::Size kResolution4320p(7680, 4320);
+
+  EXPECT_EQ(1.f, GetDeviceScaleFactor(kResolution720p));
+  EXPECT_EQ(1.5f, GetDeviceScaleFactor(kResolution1080p));
+  EXPECT_EQ(2.f, GetDeviceScaleFactor(kResolution1440p));
+  EXPECT_EQ(3.f, GetDeviceScaleFactor(kResolution2160p));
+  EXPECT_EQ(6.f, GetDeviceScaleFactor(kResolution4320p));
+}
+
+TEST(CastDisplayScaleTest, NonstandardResolutions) {
+  const gfx::Size kResolutionHomeHub(600, 1024);
+  EXPECT_EQ(1.f, GetDeviceScaleFactor(kResolutionHomeHub));
+}
+
+}  // namespace chromecast