blob: 25677e986858f7cce6062083f6f50afe31638bc1 [file] [log] [blame]
* Copyright (c) 2020 The WebRTC project authors. All Rights Reserved.
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
#include "modules/desktop_capture/win/window_capturer_win_gdi.h"
#include <cmath>
#include <map>
#include <memory>
#include <utility>
#include <vector>
#include "modules/desktop_capture/cropped_desktop_frame.h"
#include "modules/desktop_capture/desktop_capture_metrics_helper.h"
#include "modules/desktop_capture/desktop_capture_types.h"
#include "modules/desktop_capture/desktop_capturer.h"
#include "modules/desktop_capture/desktop_frame_win.h"
#include "modules/desktop_capture/win/screen_capture_utils.h"
#include "modules/desktop_capture/win/selected_window_context.h"
#include "rtc_base/arraysize.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
#include "rtc_base/string_utils.h"
#include "rtc_base/time_utils.h"
#include "rtc_base/trace_event.h"
#include "rtc_base/win32.h"
#include "system_wrappers/include/metrics.h"
namespace webrtc {
// Used to pass input/output data during the EnumWindows call to collect
// owned/pop-up windows that should be captured.
struct OwnedWindowCollectorContext : public SelectedWindowContext {
OwnedWindowCollectorContext(HWND selected_window,
DesktopRect selected_window_rect,
WindowCaptureHelperWin* window_capture_helper,
std::vector<HWND>* owned_windows)
: SelectedWindowContext(selected_window,
owned_windows(owned_windows) {}
std::vector<HWND>* owned_windows;
// Called via EnumWindows for each root window; adds owned/pop-up windows that
// should be captured to a vector it's passed.
BOOL CALLBACK OwnedWindowCollector(HWND hwnd, LPARAM param) {
OwnedWindowCollectorContext* context =
if (hwnd == context->selected_window()) {
// Windows are enumerated in top-down z-order, so we can stop enumerating
// upon reaching the selected window.
return FALSE;
// Skip windows that aren't visible pop-up windows.
if (!(GetWindowLong(hwnd, GWL_STYLE) & WS_POPUP) ||
hwnd)) {
return TRUE;
// Owned windows that intersect the selected window should be captured.
if (context->IsWindowOwnedBySelectedWindow(hwnd) &&
context->IsWindowOverlappingSelectedWindow(hwnd)) {
// Skip windows that draw shadows around menus. These "SysShadow" windows
// would otherwise be captured as solid black bars with no transparency
// gradient (since this capturer doesn't detect / respect variations in the
// window alpha channel). Any other semi-transparent owned windows will be
// captured fully-opaque. This seems preferable to excluding them (at least
// when they have content aside from a solid fill color / visual adornment;
// e.g. some tooltips have the transparent style set).
if (GetWindowLong(hwnd, GWL_EXSTYLE) & WS_EX_TRANSPARENT) {
const WCHAR kSysShadow[] = L"SysShadow";
const size_t kClassLength = arraysize(kSysShadow);
WCHAR class_name[kClassLength];
const int class_name_length =
GetClassNameW(hwnd, class_name, kClassLength);
if (class_name_length == kClassLength - 1 &&
wcscmp(class_name, kSysShadow) == 0) {
return TRUE;
return TRUE;
bool enumerate_current_process_windows)
: enumerate_current_process_windows_(enumerate_current_process_windows) {}
WindowCapturerWinGdi::~WindowCapturerWinGdi() {}
bool WindowCapturerWinGdi::GetSourceList(SourceList* sources) {
if (!window_capture_helper_.EnumerateCapturableWindows(
sources, enumerate_current_process_windows_))
return false;
std::map<HWND, DesktopSize> new_map;
for (const auto& item : *sources) {
HWND hwnd = reinterpret_cast<HWND>(;
new_map[hwnd] = window_size_map_[hwnd];
return true;
bool WindowCapturerWinGdi::SelectSource(SourceId id) {
HWND window = reinterpret_cast<HWND>(id);
if (!IsWindowValidAndVisible(window))
return false;
window_ = window;
// When a window is not in the map, window_size_map_[window] will create an
// item with DesktopSize (0, 0).
previous_size_ = window_size_map_[window];
return true;
bool WindowCapturerWinGdi::FocusOnSelectedSource() {
if (!window_)
return false;
if (!IsWindowValidAndVisible(window_))
return false;
return BringWindowToTop(window_) && SetForegroundWindow(window_);
bool WindowCapturerWinGdi::IsOccluded(const DesktopVector& pos) {
DesktopVector sys_pos = pos.add(GetFullscreenRect().top_left());
HWND hwnd =
return hwnd != window_ &&
std::find(owned_windows_.begin(), owned_windows_.end(), hwnd) ==
void WindowCapturerWinGdi::Start(Callback* callback) {
callback_ = callback;
void WindowCapturerWinGdi::CaptureFrame() {
int64_t capture_start_time_nanos = rtc::TimeNanos();
CaptureResults results = CaptureFrame(/*capture_owned_windows*/ true);
if (results.frame) {
int capture_time_ms = (rtc::TimeNanos() - capture_start_time_nanos) /
callback_->OnCaptureResult(results.result, std::move(results.frame));
WindowCapturerWinGdi::CaptureResults WindowCapturerWinGdi::CaptureFrame(
bool capture_owned_windows) {
TRACE_EVENT0("webrtc", "WindowCapturerWinGdi::CaptureFrame");
if (!window_) {
RTC_LOG(LS_ERROR) << "Window hasn't been selected: " << GetLastError();
return {Result::ERROR_PERMANENT, nullptr};
// Stop capturing if the window has been closed.
if (!IsWindow(window_)) {
RTC_LOG(LS_ERROR) << "Target window has been closed.";
return {Result::ERROR_PERMANENT, nullptr};
// Determine the window region excluding any resize border, and including
// any visible border if capturing an owned window / dialog. (Don't include
// any visible border for the selected window for consistency with
// CroppingWindowCapturerWin, which would expose a bit of the background
// through the partially-transparent border.)
const bool avoid_cropping_border = !capture_owned_windows;
DesktopRect cropped_rect;
DesktopRect original_rect;
if (!GetCroppedWindowRect(window_, avoid_cropping_border, &cropped_rect,
&original_rect)) {
RTC_LOG(LS_WARNING) << "Failed to get drawable window area: "
<< GetLastError();
return {Result::ERROR_TEMPORARY, nullptr};
// Return a 1x1 black frame if the window is minimized or invisible on current
// desktop, to match behavior on mace. Window can be temporarily invisible
// during the transition of full screen mode on/off.
if (original_rect.is_empty() ||
!window_capture_helper_.IsWindowVisibleOnCurrentDesktop(window_)) {
std::unique_ptr<DesktopFrame> frame(
new BasicDesktopFrame(DesktopSize(1, 1)));
previous_size_ = frame->size();
window_size_map_[window_] = previous_size_;
return {Result::SUCCESS, std::move(frame)};
HDC window_dc = GetWindowDC(window_);
if (!window_dc) {
RTC_LOG(LS_WARNING) << "Failed to get window DC: " << GetLastError();
return {Result::ERROR_TEMPORARY, nullptr};
DesktopRect unscaled_cropped_rect = cropped_rect;
double horizontal_scale = 1.0;
double vertical_scale = 1.0;
DesktopSize window_dc_size;
if (GetDcSize(window_dc, &window_dc_size)) {
// The |window_dc_size| is used to detect the scaling of the original
// window. If the application does not support high-DPI settings, it will
// be scaled by Windows according to the scaling setting.
// So the size of the |window_dc|, i.e. the bitmap we can retrieve from
// PrintWindow() or BitBlt() function, will be smaller than
// |original_rect| and |cropped_rect|. Part of the captured desktop frame
// will be black. See
// bug for
// details.
// If |window_dc_size| is smaller than |window_rect|, let's resize both
// |original_rect| and |cropped_rect| according to the scaling factor.
// This will adjust the width and height of the two rects.
horizontal_scale =
static_cast<double>(window_dc_size.width()) / original_rect.width();
vertical_scale =
static_cast<double>(window_dc_size.height()) / original_rect.height();
original_rect.Scale(horizontal_scale, vertical_scale);
cropped_rect.Scale(horizontal_scale, vertical_scale);
// Translate |cropped_rect| to the left so that its position within
// |original_rect| remains accurate after scaling.
// See for more info.
int translate_left = static_cast<int>(std::round(
(cropped_rect.left() - original_rect.left()) * (horizontal_scale - 1)));
int translate_top = static_cast<int>(std::round(
( - * (vertical_scale - 1)));
cropped_rect.Translate(translate_left, translate_top);
std::unique_ptr<DesktopFrameWin> frame(
DesktopFrameWin::Create(original_rect.size(), nullptr, window_dc));
if (!frame.get()) {
RTC_LOG(LS_WARNING) << "Failed to create frame.";
ReleaseDC(window_, window_dc);
return {Result::ERROR_TEMPORARY, nullptr};
HDC mem_dc = CreateCompatibleDC(window_dc);
HGDIOBJ previous_object = SelectObject(mem_dc, frame->bitmap());
BOOL result = FALSE;
// When desktop composition (Aero) is enabled each window is rendered to a
// private buffer allowing BitBlt() to get the window content even if the
// window is occluded. PrintWindow() is slower but lets rendering the window
// contents to an off-screen device context when Aero is not available.
// PrintWindow() is not supported by some applications.
// If Aero is enabled, we prefer BitBlt() because it's faster and avoids
// window flickering. Otherwise, we prefer PrintWindow() because BitBlt() may
// render occluding windows on top of the desired window.
// When composition is enabled the DC returned by GetWindowDC() doesn't always
// have window frame rendered correctly. Windows renders it only once and then
// caches the result between captures. We hack it around by calling
// PrintWindow() whenever window size changes, including the first time of
// capturing - it somehow affects what we get from BitBlt() on the subsequent
// captures.
// For Windows 8.1 and later, we want to always use PrintWindow when the
// cropping screen capturer falls back to the window capturer. I.e.
// on Windows 8.1 and later, PrintWindow is only used when the window is
// occluded. When the window is not occluded, it is much faster to capture
// the screen and to crop it to the window position and size.
if (rtc::IsWindows8OrLater()) {
// Special flag that makes PrintWindow to work on Windows 8.1 and later.
// Indeed certain apps (e.g. those using DirectComposition rendering) can't
// be captured using BitBlt or PrintWindow without this flag. Note that on
// Windows 8.0 this flag is not supported so the block below will fallback
// to the other call to PrintWindow. It seems to be very tricky to detect
// Windows 8.0 vs 8.1 so a try/fallback is more approriate here.
result = PrintWindow(window_, mem_dc, flags);
if (!result && (!window_capture_helper_.IsAeroEnabled() ||
!previous_size_.equals(frame->size()))) {
result = PrintWindow(window_, mem_dc, 0);
// Aero is enabled or PrintWindow() failed, use BitBlt.
if (!result) {
result = BitBlt(mem_dc, 0, 0, frame->size().width(), frame->size().height(),
window_dc, 0, 0, SRCCOPY);
SelectObject(mem_dc, previous_object);
ReleaseDC(window_, window_dc);
previous_size_ = frame->size();
window_size_map_[window_] = previous_size_;
if (!result) {
RTC_LOG(LS_ERROR) << "Both PrintWindow() and BitBlt() failed.";
return {Result::ERROR_TEMPORARY, nullptr};
// Rect for the data is relative to the first pixel of the frame.
std::unique_ptr<DesktopFrame> cropped_frame =
CreateCroppedDesktopFrame(std::move(frame), cropped_rect);
if (capture_owned_windows) {
// If any owned/pop-up windows overlap the selected window, capture them
// and copy/composite their contents into the frame.
OwnedWindowCollectorContext context(window_, unscaled_cropped_rect,
if (context.IsSelectedWindowValid()) {
EnumWindows(OwnedWindowCollector, reinterpret_cast<LPARAM>(&context));
if (!owned_windows_.empty()) {
if (!owned_window_capturer_) {
owned_window_capturer_ = std::make_unique<WindowCapturerWinGdi>(
// Owned windows are stored in top-down z-order, so this iterates in
// reverse to capture / draw them in bottom-up z-order
for (auto it = owned_windows_.rbegin(); it != owned_windows_.rend();
it++) {
HWND hwnd = *it;
if (owned_window_capturer_->SelectSource(
reinterpret_cast<SourceId>(hwnd))) {
CaptureResults results = owned_window_capturer_->CaptureFrame(
/*capture_owned_windows*/ false);
if (results.result != DesktopCapturer::Result::SUCCESS) {
// Simply log any error capturing an owned/pop-up window without
// bubbling it up to the caller (an expected error here is that
// the owned/pop-up window was closed; any unexpected errors won't
// fail the outer capture).
RTC_LOG(LS_INFO) << "Capturing owned window failed (previous "
"error/warning pertained to that)";
} else {
// Copy / composite the captured frame into the outer frame. This
// may no-op if they no longer intersect (if the owned window was
// moved outside the owner bounds since scheduled for capture.)
*results.frame, horizontal_scale, vertical_scale);
return {Result::SUCCESS, std::move(cropped_frame)};
// static
std::unique_ptr<DesktopCapturer> WindowCapturerWinGdi::CreateRawWindowCapturer(
const DesktopCaptureOptions& options) {
return std::unique_ptr<DesktopCapturer>(
new WindowCapturerWinGdi(options.enumerate_current_process_windows()));
} // namespace webrtc