desktopCapture: scale the cursor image according to screen scale factor on OSX

webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm

Index: webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
diff --git a/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm b/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
index 73d5e028577225796cd38e35e580810cac02ba8b..af63bfdbb4f418baf65382877b40278f7f73ceda 100644
--- a/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
+++ b/webrtc/modules/desktop_capture/mouse_cursor_monitor_mac.mm
@@ -30,21 +30,26 @@
 namespace webrtc {
 
 namespace {
-// Paint the image, so that we can get a bitmap representation compatible with
-// current context. For example, in the retina display, we are going to get an
-// image with same visual size but underlying pixel size conforms to the retina
-// setting.
-NSImage* PaintInCurrentContext(NSImage* source) {
-  NSSize size = [source size];
-  NSImage* new_image = [[NSImage alloc] initWithSize:size];
-  [new_image lockFocus];
-  NSRect frame = NSMakeRect(0, 0, size.width, size.height);
-  [source drawInRect:frame
-            fromRect:frame
-           operation:NSCompositeCopy
-            fraction:1.0];
-  [new_image unlockFocus];
-  return new_image;
+CGImageRef CreateScaledCGImage(CGImageRef image, int width, int height) {
+  // Create context, keeping original image properties.
+  CGColorSpaceRef colorspace = CGImageGetColorSpace(image);
+  CGContextRef context = CGBitmapContextCreate(nullptr,
+                                               width,
+                                               height,
+                                               CGImageGetBitsPerComponent(image),
+                                               width * DesktopFrame::kBytesPerPixel,
+                                               colorspace,
+                                               CGImageGetBitmapInfo(image));
+
+  if (!context) return nil;
+
+  // Draw image to context, resizing it.
+  CGContextDrawImage(context, CGRectMake(0, 0, width, height), image);
+  // Extract resulting image from context.
+  CGImageRef imgRef = CGBitmapContextCreateImage(context);
+  CGContextRelease(context);
+
+  return imgRef;
 }
 }  // namespace
 
@@ -72,7 +77,7 @@ class MouseCursorMonitorMac : public MouseCursorMonitor {
   ScreenId screen_id_;
   Callback* callback_;
   Mode mode_;
-  std::unique_ptr<MouseCursor> last_cursor_;
+  __strong NSImage* last_cursor_;
   rtc::scoped_refptr<FullScreenChromeWindowDetector>
       full_screen_chrome_window_detector_;
 };
@@ -253,10 +258,9 @@ void MouseCursorMonitorMac::CaptureImage(float scale) {
   NSImage* nsimage = [nscursor image];
   NSSize nssize = [nsimage size];  // DIP size
 
-  // For retina screen, we need to paint the cursor in current graphic context
-  // to get retina representation.
-  if (scale != 1.0)
-    nsimage = PaintInCurrentContext(nsimage);
+  // No need to caputre cursor image if it's unchanged since last capture.
+  if ([[nsimage TIFFRepresentation] isEqual:[last_cursor_ TIFFRepresentation]]) return;
+  last_cursor_ = nsimage;
 
   DesktopSize size(round(nssize.width * scale),
                    round(nssize.height * scale));  // Pixel size
@@ -271,30 +275,33 @@ void MouseCursorMonitorMac::CaptureImage(float scale) {
   if (!cg_image)
     return;
 
+  // Before 10.12, OSX may report 1X cursor on Retina screen. (See
+  // crbug.com/632995.) After 10.12, OSX may report 2X cursor on non-Retina
+  // screen. (See crbug.com/671436.) So scaling the cursor if needed.
+  CGImageRef scaled_cg_image = nil;
+  if (CGImageGetWidth(cg_image) != static_cast<size_t>(size.width())) {
+    scaled_cg_image = CreateScaledCGImage(cg_image, size.width(), size.height());
+    if (scaled_cg_image != nil) {
+      cg_image = scaled_cg_image;
+    }
+  }
   if (CGImageGetBitsPerPixel(cg_image) != DesktopFrame::kBytesPerPixel * 8 ||
       CGImageGetWidth(cg_image) != static_cast<size_t>(size.width()) ||
       CGImageGetBitsPerComponent(cg_image) != 8) {
+    if (scaled_cg_image != nil) CGImageRelease(scaled_cg_image);
     return;
   }
 
   CGDataProviderRef provider = CGImageGetDataProvider(cg_image);
   CFDataRef image_data_ref = CGDataProviderCopyData(provider);
-  if (image_data_ref == NULL)
+  if (image_data_ref == NULL) {
+    if (scaled_cg_image != nil) CGImageRelease(scaled_cg_image);
     return;
+  }
 
   const uint8_t* src_data =
       reinterpret_cast<const uint8_t*>(CFDataGetBytePtr(image_data_ref));
 
-  // Compare the cursor with the previous one.
-  if (last_cursor_.get() &&
-      last_cursor_->image()->size().equals(size) &&
-      last_cursor_->hotspot().equals(hotspot) &&
-      memcmp(last_cursor_->image()->data(), src_data,
-             last_cursor_->image()->stride() * size.height()) == 0) {
-    CFRelease(image_data_ref);
-    return;
-  }
-
   // Create a MouseCursor that describes the cursor and pass it to
   // the client.
   std::unique_ptr<DesktopFrame> image(
@@ -304,10 +311,10 @@ void MouseCursorMonitorMac::CaptureImage(float scale) {
   image->CopyPixelsFrom(src_data, src_stride, DesktopRect::MakeSize(size));
 
   CFRelease(image_data_ref);
+  if (scaled_cg_image != nil) CGImageRelease(scaled_cg_image);
 
   std::unique_ptr<MouseCursor> cursor(
       new MouseCursor(image.release(), hotspot));
-  last_cursor_.reset(MouseCursor::CopyOf(*cursor));
 
   callback_->OnMouseCursor(cursor.release());
 }