为什么要支持GDI
先说结论，Windows平台播放渲染这块，一般来说99%以上的机器都是支持D3D的，实现GDI模式绘制，除了为了好的兼容性外，在远程连接的场景下，D3D创建不成功，需要使用GDI模式。
简单来说，Windows平台的RTMP播放器或RTSP播放器，设计如果系统支持D3D，优先D3D，如果检测到不支持D3D，数据回调上来，GDI模式绘制。
在之前的博客，我们提到过：D3D绘制出来的图像效果更细腻，绘制效率也更高，CPU占用相对GDI更低。
上图以1920*1080分辨率、30帧、固定码率（采集屏幕左侧区域）为例，通过大牛直播SDK ( github) 的Windows平台SmartPublisherDemo.exe工具推送到内网nginx服务器，然后分别以D3D模式和GDI模式拉流（播放端缓冲设置为0）。
可以看到：
D3D模式，CPU占用只有2.7%，延迟：249-156 = 93ms；
GDI模式，CPU占用19.5%，延迟249-73 = 176ms。
无论是从延迟和CPU占用上看，D3D模式都占优。
实现思路：
以C++的demo为例：
1. 先检测系统是否支持D3D模式：
if ( NT_ERC_OK == player_api_.IsSupportD3DRender(player_handle_,wrapper_render_wnd_.RenderWnd(), &in_support_d3d_render)){if ( 1 == in_support_d3d_render ){is_support_d3d_render = true;}}2. 如不支持D3D，数据回到上层，做绘制：
if ( is_support_d3d_render ){is_gdi_render_ = false;// 支持d3d绘制的话，就用D3D绘制player_api_.SetRenderWindow(player_handle_, wrapper_render_wnd_.RenderWnd());player_api_.SetRenderScaleMode(player_handle_, btn_check_render_scale_mode_.GetCheck() == BST_CHECKED ? 1 : 0);}else{is_gdi_render_ = true;// 不支持D3D就让播放器吐出数据来，用GDI绘制wrapper_render_wnd_.SetRenderScaleMode(btn_check_render_scale_mode_.GetCheck() == BST_CHECKED ? 1 : 0);player_api_.SetVideoFrameCallBack(player_handle_, NT_SP_E_VIDEO_FRAME_FORMAT_RGB32,GetSafeHwnd(), SM_SDKVideoFrameHandle);}extern "C" NT_VOID NT_CALLBACK SM_SDKVideoFrameHandle(NT_HANDLE handle, NT_PVOID userData, NT_UINT32 status,const NT_SP_VideoFrame* frame)
{/*if (frame != NULL){std::ostringstream ss;ss << "Receive frame time_stamp:" << frame->timestamp_ << "ms" << "\r\n";OutputDebugStringA(ss.str().c_str());}*/if ( frame != NULL ){if ( NT_SP_E_VIDEO_FRAME_FORMAT_RGB32 == frame->format_&& frame->plane0_ != NULL&& frame->stride0_ > 0&& frame->height_ > 0 ){std::unique_ptr<nt_rgb32_image > pImage(new nt_rgb32_image());pImage->size_ = frame->stride0_* frame->height_;pImage->data_ = new NT_BYTE[pImage->size_];memcpy(pImage->data_, frame->plane0_, pImage->size_);pImage->width_ = frame->width_;pImage->height_ = frame->height_;pImage->stride_ = frame->stride0_;HWND hwnd = (HWND)userData;if ( hwnd != NULL && ::IsWindow(hwnd) ){::PostMessage(hwnd, WM_USER_SDK_RGB32_IMAGE, (WPARAM)handle, (LPARAM)pImage.release());}}}
}具体绘制代码：
LRESULT CSmartPlayerDlg::OnSDKRGB32Image(WPARAM wParam, LPARAM lParam)
{nt_rgb32_image* pImage = (nt_rgb32_image*)(lParam);if (pImage == NULL)return S_OK;std::shared_ptr<nt_rgb32_image> sp_image(pImage);if ( is_gdi_render_ ){wrapper_render_wnd_.OnRGB32Image(sp_image);}return S_OK;
}void nt_render_wnd::OnPaint()
{CPaintDC dc(this); // device context for painting// TODO: Add your message handler code here// Do not call CWnd::OnPaint() for painting messagesif ( IsIconic() ){return;}// 先绘制一个黑色的背景CRect rc_client(0, 0, 0, 0);GetClientRect(rc_client);if ( rc_client.IsRectNull()|| rc_client.IsRectEmpty() ){return;}auto mem_dc = ::CreateCompatibleDC(dc.GetSafeHdc());if ( mem_dc == NULL )return;auto mem_bitmap = ::CreateCompatibleBitmap(dc.GetSafeHdc(), rc_client.Width(), rc_client.Height());if ( mem_bitmap == NULL ){::DeleteDC(mem_dc);return;}::SelectObject(mem_dc, mem_bitmap);HBRUSH brush = ::CreateSolidBrush(RGB(0, 0, 0));::FillRect(mem_dc, &rc_client, brush);::DeleteObject(brush);if ( rgb32_image_ ){if ( player_api_.GDIDrawRGB32 != NULL&& player_handle_ != NULL ){auto render_rc = GetRenderRect(rc_client, rgb32_image_->width_, rgb32_image_->height_);player_api_.GDIDrawRGB32(player_handle_, mem_dc,render_rc.left, render_rc.top,render_rc.Width(), render_rc.Height(),0, 0,rgb32_image_->width_, rgb32_image_->height_,rgb32_image_->data_, rgb32_image_->size_,rgb32_image_->width_, rgb32_image_->height_,rgb32_image_->stride_);if (logo_ && logo_->data_ != nullptr){player_api_.GDIDrawARGB(mem_dc,render_logo_left_, render_logo_top_,render_logo_width_, render_logo_height_,0, 0,logo_->width_, logo_->height_,logo_->data_.get(), logo_->stride_,logo_->width_, logo_->height_);}}}::BitBlt(dc.GetSafeHdc(),0, 0,rc_client.Width(), rc_client.Height(),mem_dc,0, 0,SRCCOPY);::DeleteObject(mem_bitmap);::DeleteDC(mem_dc);
}目前来看，不支持D3D的机器少之又少，在环境具备的情况下，优先建议考虑D3D模式绘制，不支持的情况下，同时兼容GDI绘制是个不错的选择。
