DirectX based screen capturer logic

webrtc/modules/desktop_capture/desktop_geometry.h

Index: webrtc/modules/desktop_capture/desktop_geometry.h
diff --git a/webrtc/modules/desktop_capture/desktop_geometry.h b/webrtc/modules/desktop_capture/desktop_geometry.h
index 047eeec3d9c8fd89c13f29347a0a67b77b2096da..b9a912758e98418edb71589a8ebd2f51b121030b 100644
--- a/webrtc/modules/desktop_capture/desktop_geometry.h
+++ b/webrtc/modules/desktop_capture/desktop_geometry.h
@@ -105,6 +105,13 @@ class DesktopRect {
   int32_t width() const { return right_ - left_; }
   int32_t height() const { return bottom_ - top_; }
 
+  // Create a new DesktopRect instance to represent a rectangle which is
+  // |padding| pixels larger than current instance in each direction. This is
+  // for VideoEncoderVpx and ScreenCapturerWinDirectx.

[Sergey Ulanov, 2016/04/14 23:10:42]

Don't need this sentence (it will expire quickly after we start using this
function in other places)

[Hzj_jie, 2016/04/15 19:42:17]

Done.

+  // Note, left and top of the new instance won't be less than zero, and right
+  // and bottom wont't be larger than the desktop size.

[Sergey Ulanov, 2016/04/14 23:10:42]

So it looks like this function currently does two things: it expands the rect
and then crops it to fit the screen. I don't think it should be responsible for
both. We already have IntersectWith() to crop a rect, so you can achieve the
same effect using this expression:
  rect.Expand(padding);
  rect.IntersectWith(DesktopRect::MakeSize(size));

[Hzj_jie, 2016/04/15 19:42:17]

Done.

+  DesktopRect Expand(uint32_t padding, const DesktopSize& size) const;

[Sergey Ulanov, 2016/04/14 23:10:42]

Other functions in this class mutate the rect in-place. For consistency I think
we want to do the same thing here.

[Sergey Ulanov, 2016/04/14 23:10:42]

nit: Move this function below, after other mutators.

[Sergey Ulanov, 2016/04/14 23:10:42]

padding should be int.
From https://google.github.io/styleguide/cppguide.html#Integer_Types :
  document that a variable is non-negative using assertions. Don't use an
unsigned type.

[Hzj_jie, 2016/04/15 19:42:17]

Considering this is a simple class, mutable eventually make the code even
longer. Such as,

DesktopRect new_rect = existing_rect.Expand(100);

will become

DesktopRect new_rect = existing_rect;
new_rect.Expand(100);

I remember lots of Java simple classes are immutable, since copying a set of
pointers are actually cheap, which is the exactly similar scenario as here.

But I agree keeping consistency is also important.

[Hzj_jie, 2016/04/15 19:42:17]

Done.

[Hzj_jie, 2016/04/15 19:42:17]

Done.

+
   DesktopVector top_left() const { return DesktopVector(left_, top_); }
   DesktopSize size() const { return DesktopSize(width(), height()); }