Compare EGL code to DXGI and Direct3D

Artigo
10/13/2015

[This article is for Windows 8.x and Windows Phone 8.x developers writing Windows Runtime apps. If you’re developing for Windows 10, see the latest documentation]

The DirectX Graphics Interface (DXGI) and several Direct3D APIs serve the same role as EGL. This topic helps you understand DXGI and Direct3D 11 from the perspective of EGL.

DXGI and Direct3D, like EGL, provide methods to configure graphics resources, obtain a rendering context for your shaders to draw into, and to display the results in a window. However, DXGI and Direct3D have quite a few more options, and require more effort to set up correctly when porting from EGL.

Note This guidance is based off the Khronos Group's open specification for EGL 1.4, found here: Khronos Native Platform Graphics Interface (EGL Version 1.4 - April 6, 2011) [PDF]. All OpenGL ES 2.0 code is provided in C/C++ syntax as compiled on Windows 8 with Visual Studio 2012. Differences in syntax specific to other platforms and development languages are not covered in this guidance.

How does DXGI and Direct3D compare?

The big advantage of EGL over DXGI and Direct3D is that it is relatively simple to start drawing to a window surface. This is because OpenGL ES 2.0—and therefore EGL—is a specification implemented by multiple platform providers, whereas DXGI and Direct3D are a single reference that hardware vendor drivers must conform to. This means that Microsoft must implement a set of APIs that enable the broadest possible set of vendor features, rather than focusing on a functional subset offered by a specific vendor, or by combining vendor-specific setup commands into simpler APIs. On the other hand, Direct3D provides a single set of APIs that cover a very broad range of graphics hardware platforms and feature levels, and offer more flexibility for developers experienced with the platform.

Like EGL, DXGI and Direct3D provide APIs for the following behaviors:

Obtaining, and reading and writing to a frame buffer (called a "swap chain" in DXGI).
Associating the frame buffer with a UI window.
Obtaining and configuring rendering contexts in which to draw.
Issuing commands to the graphics pipeline for a specific rendering context.
Creating and managing shader resources, and associating them with a rendering content.
Rendering to specific render targets (such as textures).
Updating the window's display surface with the results of rendering with the graphics resources.

To see the basic Direct3D process for configuring the graphics pipeline, check out the Direct3D Windows Store app template in Microsoft Visual Studio. The base rendering class in it provides a good baseline for setting up the Direct3D 11 graphics infrastructure and configuring basic resources on it, as well as supporting Windows Store app features such as screen rotation.

EGL has very few APIs relative to Direct3D 11, and navigating the latter can be a challenge if you aren't familiar with the naming and jargon particular to the platform. Here's a simple overview to help you get oriented.

First, review the basic EGL object to Direct3D interface mapping:

EGL abstraction	Similar Direct3D representation
EGLDisplay	In Direct3D (for Windows Store apps), the display handle is obtained through the Windows::UI::CoreWindow API (or the ICoreWindowInterop interface that exposes the HWND). The adapter and hardware configuration are set with the IDXGIAdapter and IDXGIDevice1 COM interfaces, respectively.
EGLSurface	In Direct3D, the buffers and other window resources (visible or offscreen) are created and configured by specific DXGI interfaces, including IDXGIFactory2 (a factory pattern implementation used to acquire DXGI resources such as theIDXGISwapChain1 (display buffers). The ID3D11Device1 that represents the graphics device and its resources, is acquired with D3D11Device::CreateDevice. For render targets, use the ID3D11RenderTargetView interface.
EGLContext	In Direct3D, you configure and issue commands to the graphics pipeline with the ID3D11DeviceContext1 interface.
EGLConfig	In Direct3D 11, you create and configure graphics resources such as a buffers, textures, stencils and shaders with methods on the ID3D11Device1 interface.

Now, here's the most basic process for setting up a simple graphics display in a Windows Store app, resources and context in DXGI and Direct3D for a Windows Store app.

Obtain a handle to the CoreWindow object for the app's core UI thread by calling CoreWindow::GetForCurrentThread.
For Windows Store apps, acquire a swap chain from the IDXGIAdapter2 with IDXGIFactory2::CreateSwapChainForCoreWindow, and pass it the CoreWindow reference you obtained in step 1. You will get an IDXGISwapChain1 instance in return. Scope it to your renderer object and its rendering thread.
Obtain ID3D11Device1 and ID3D11DeviceContext1 instances by calling the D3D11Device::CreateDevice method. Scope them to your renderer object as well.
Create shaders, textures, and other resources using methods on your renderer's ID3D11Device1 object.
Define buffers, run shaders and manage the pipeline stages using methods on your renderer's ID3D11DeviceContext1 object.
When the pipeline has executed and a frame is drawn to the back buffer, present it to the screen with IDXGISwapChain1::Present1.

To examine this process in more detail, review Getting started with DirectX graphics. The rest of this article covers many of the common steps for basic graphics pipeline setup and management.

Note Windows Desktop apps have different APIs for obtaining a Direct3D swap chain, such as D3D11Device::CreateDeviceAndSwapChain, and do not use a CoreWindow object.

Obtaining a window for display

In this example, eglGetDisplay is passed an HWND for a window resource specific to the Microsoft Windows platform. Other platforms, such as Apple's iOS (Cocoa) and Google's Android, have different handles or references to window resources, and may have different calling syntax altogether. After obtaining a display, you initialize it, set the preferred configuration, and create a surface with a back buffer you can draw into.

Obtaining a display and configuring it with EGL..

// Obtain an EGL display object.
EGLDisplay display = eglGetDisplay(GetDC(hWnd));
if (display == EGL_NO_DISPLAY)
{
  return EGL_FALSE;
}

// Initialize the display
if (!eglInitialize(display, &majorVersion, &minorVersion))
{
  return EGL_FALSE;
}

// Obtain the display configs
if (!eglGetConfigs(display, NULL, 0, &numConfigs))
{
  return EGL_FALSE;
}

// Choose the display config
if (!eglChooseConfig(display, attribList, &config, 1, &numConfigs))
{
  return EGL_FALSE;
}

// Create a surface
surface = eglCreateWindowSurface(display, config, (EGLNativeWindowType)hWnd, NULL);
if (surface == EGL_NO_SURFACE)
{
  return EGL_FALSE;
}

In Direct3D, a Windows Store app's main window is represented by the CoreWindow object, which can be obtained from the app object by calling CoreWindow::GetForCurrentThread as part of the initialization process of the "view provider" you construct for Direct3D. (If you are using Direct3D-XAML interop, you use the XAML framework's view provider.) The process for creating a Direct3D view provider is covered in How to set up your app to display a view.

Obtaining a CoreWindow for Direct3D.

CoreWindow::GetForCurrentThread();

Once the CoreWindow reference is obtained, the window must be activated, which executes the Run method of your main object and begins window event processing. After that, create an ID3D11Device1 and an ID3D11DeviceContext1, and use them to get the underlying IDXGIDevice1 and IDXGIAdapter so you can obtain an IDXGIFactory2 object to create a swap chain resource based on your DXGI_SWAP_CHAIN_DESC1 configuration.

Configuring and setting the DXGI swap chain on the CoreWindow for Direct3D.

// Called when the CoreWindow object is created (or re-created).
void SimpleDirect3DApp::SetWindow(CoreWindow^ window)
{
  // Register event handlers with the CoreWindow object.
  // ...

  // Obtain your ID3D11Device1 and ID3D11DeviceContext1 objects
  // In this example, m_d3dDevice contains the scoped ID3D11Device1 object
  // ...

  ComPtr<IDXGIDevice1>  dxgiDevice;
  // Get the underlying DXGI device of the Direct3D device.
  m_d3dDevice.As(&dxgiDevice);

  ComPtr<IDXGIAdapter> dxgiAdapter;
  dxgiDevice->GetAdapter(&dxgiAdapter);

  ComPtr<IDXGIFactory2> dxgiFactory;
  dxgiAdapter->GetParent(
    __uuidof(IDXGIFactory2), 
    &dxgiFactory);

  DXGI_SWAP_CHAIN_DESC1 swapChainDesc = {0};
  swapChainDesc.Width = static_cast<UINT>(m_d3dRenderTargetSize.Width); // Match the size of the window.
  swapChainDesc.Height = static_cast<UINT>(m_d3dRenderTargetSize.Height);
  swapChainDesc.Format = DXGI_FORMAT_B8G8R8A8_UNORM; // This is the most common swap chain format.
  swapChainDesc.Stereo = false;
  swapChainDesc.SampleDesc.Count = 1; // Don't use multi-sampling.
  swapChainDesc.SampleDesc.Quality = 0;
  swapChainDesc.BufferUsage = DXGI_USAGE_RENDER_TARGET_OUTPUT;
  swapChainDesc.BufferCount = 2; // Use double-buffering to minimize latency.
  swapChainDesc.SwapEffect = DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL; // All Windows Store apps must use this SwapEffect.
  swapChainDesc.Flags = 0;

  // ...

  Windows::UI::Core::CoreWindow^ window = m_window.Get();
  dxgiFactory->CreateSwapChainForCoreWindow(
    m_d3dDevice.Get(),
    reinterpret_cast<IUnknown*>(window),
    &swapChainDesc,
    nullptr, // Allow on all displays.
    &m_swapChainCoreWindow);
}

Call the IDXGISwapChain1::Present1 method after you prepare a frame in order to display it.

Note that in Direct3D 11, there isn't an abstraction identical to EGLSurface. (There is IDXGISurface1, but it is used differently.) The closest conceptual approximation is the ID3D11RenderTargetView object that we use to assign a texture (ID3D11Texture2D) as the back buffer that our shader pipeline will draw into.

Setting up the back buffer for the swap chain in Direct3D 11

ComPtr<ID3D11RenderTargetView>    m_d3dRenderTargetViewWin; // scoped to renderer object

// ...

ComPtr<ID3D11Texture2D> backBuffer2;
    
m_swapChainCoreWindow->GetBuffer(0, IID_PPV_ARGS(&backBuffer2));

m_d3dDevice->CreateRenderTargetView(
  backBuffer2.Get(),
  nullptr,
    &m_d3dRenderTargetViewWin);

A good practice is to call this code whenever the window is created or changes size. During rendering, set the render target view with ID3D11DeviceContext1::OMSetRenderTargets before setting up any other subresources like vertex buffers or shaders.

// Set the render target for the draw operation.
m_d3dContext->OMSetRenderTargets(
        1,
        d3dRenderTargetView.GetAddressOf(),
        nullptr);

Creating a rendering context

In EGL 1.4, a "display" represents a set of window resources. Typically, you configure a "surface" for the display by supplying a set of attributes to the display object and getting a surface in return. You create a context for displaying the contents of the surface by creating that context and binding it to the surface and the display.

The call flow usually looks similar to this:

Call eglGetDisplay with the handle to a display or window resource and obtain a display object.
Initialize the display with eglInitialize.
Obtain the available display configuration and select one with eglGetConfigs and eglChooseConfig.
Create a window surface with eglCreateWindowSurface.
Create a display context for drawing with eglCreateContext.
Bind the display context to the display and the surface with eglMakeCurrent.

n the previous section, we created the EGLDisplay and the EGLSurface, and now we use the EGLDisplay to create a context and associate that context with the display, using the configured EGLSurface to parameterize the output.

Obtaining a rendering context with EGL 1.4

// Configure your EGLDisplay and obtain an EGLSurface here ...
// ...

// Create a drawing context from the EGLDisplay
context = eglCreateContext(display, config, EGL_NO_CONTEXT, contextAttribs);
if (context == EGL_NO_CONTEXT)
{
  return EGL_FALSE;
}   
   
// Make the context current
if (!eglMakeCurrent(display, surface, surface, context))
{
  return EGL_FALSE;
}

A rendering context in Direct3D 11 is represented by an ID3D11Device1 object, which represents the adapter and allows you to create Direct3D resources such as buffers and shaders; and by the ID3D11DeviceContext1 object, which allows you to manage the graphics pipeline and execute the shaders.

Be aware of Direct3D feature levels! These are used to support older Direct3D hardware platforms, from DirectX 9.1 to DirectX 11. Many platforms that use low power graphics hardware, such as tablets, only have access to DirectX 9.1 features, and older supported graphics hardware could be from 9.1 through 11. For more information on Direct3D feature levels, read Developing for different Direct3D feature levels.

Creating a rendering context with DXGI and Direct3D


// ... 

UINT creationFlags = D3D11_CREATE_DEVICE_BGRA_SUPPORT;
ComPtr<IDXGIDevice> dxgiDevice;

D3D_FEATURE_LEVEL featureLevels[] = 
{
        D3D_FEATURE_LEVEL_11_1,
        D3D_FEATURE_LEVEL_11_0,
        D3D_FEATURE_LEVEL_10_1,
        D3D_FEATURE_LEVEL_10_0,
        D3D_FEATURE_LEVEL_9_3,
        D3D_FEATURE_LEVEL_9_2,
        D3D_FEATURE_LEVEL_9_1
};

// Create the Direct3D 11 API device object and a corresponding context.
ComPtr<ID3D11Device> device;
ComPtr<ID3D11DeviceContext> d3dContext;

D3D11CreateDevice(
  nullptr, // Specify nullptr to use the default adapter.
  D3D_DRIVER_TYPE_HARDWARE,
  nullptr,
  creationFlags, // Set set debug and Direct2D compatibility flags.
  featureLevels, // List of feature levels this app can support.
  ARRAYSIZE(featureLevels),
  D3D11_SDK_VERSION, // Always set this to D3D11_SDK_VERSION for Windows Store apps.
  &device, // Returns the Direct3D device created.
  &m_featureLevel, // Returns feature level of device created.
  &d3dContext // Returns the device immediate context.
);

Drawing into a texture or pixmap resource

To draw into a texture with OpenGL ES 2.0, configure a pixel buffer, or PBuffer. After you successfully a configure and create an EGLSurface for it you can supply it with a rendering context and execute the shader pipeline to draw into the texture.

Draw into a pixel buffer with OpenGL ES 2.0

// Create a pixel buffer surface to draw into
EGLConfig pBufConfig;
EGLint totalpBufAttrs;

const EGLint pBufConfigAttrs[] =
{
    // Configure the pBuffer here...
};
 
eglChooseConfig(eglDsplay, pBufConfigAttrs, &pBufConfig, 1, &totalpBufAttrs);
EGLSurface pBuffer = eglCreatePbufferSurface(eglDisplay, pBufConfig, EGL_TEXTURE_RGBA);

In Direct3D 11, you create an ID3D11Texture2D resource and makei it a render target. Configure the render target using D3D11_RENDER_TARGET_VIEW_DESC. When you call the ID3D11DeviceContext::Draw method(or a similar Draw* operation on the device context) using this render target, the results are drawn into a texture.

Draw into a texture with Direct3D 11

ComPtr<ID3D11Texture2D> renderTarget1;

D3D11_RENDER_TARGET_VIEW_DESC renderTargetDesc = {0};
// Configure renderTargetDesc here ...

m_d3dDevice->CreateRenderTargetView(
  renderTarget1.Get(),
  nullptr,
  &m_d3dRenderTargetViewWin);

// Later, in your render loop...

// Set the render target for the draw operation.
m_d3dContext->OMSetRenderTargets(
        1,
        d3dRenderTargetView.GetAddressOf(),
        nullptr);

This texture can be passed to a shader if it is associated with an ID3D11ShaderResourceView.

Drawing to the screen

Once you have used your EGLContext to configure your buffers and update your data, you run the shaders bound to it and draw the results to the back buffer with glDrawElements. You display the back buffer by calling eglSwapBuffers.

Open GL ES 2.0: Drawing to the screen.

glDrawElements(GL_TRIANGLES, renderer->numIndices, GL_UNSIGNED_INT, 0);

eglSwapBuffers(drawContext->eglDisplay, drawContext->eglSurface);

In Direct3D 11, you configure your buffers and bind shaders with your IDXGISwapChain::Present1. Then you call one of the ID3D11DeviceContext1::Draw* methods to run the shaders and draw the results to a render target configured as the back buffer for the swap chain. After that, you simply present the back buffer to the display by calling IDXGISwapChain::Present1.

Direct3D 11: Drawing to the screen.


m_d3dContext->DrawIndexed(
        m_indexCount,
        0,
        0);

// ...

m_swapChainCoreWindow->Present1(1, 0, &parameters);

Releasing graphics resources

In EGL, you release the window resources by passing the EGLDisplay to eglTerminate.

Terminating a display with EGL 1.4

EGLBoolean eglTerminate(eglDisplay);

In a Windows Store app, you can close the CoreWindow with CoreWindow::Close, although this can only be used for secondary UI windows. The primary UI thread and its associated CoreWindow cannot be closed; rather, they are expired by the operating system. However, when a secondary CoreWindow is closed, the CoreWindow::Closed event is raised.

API Reference mapping for EGL to Direct3D 11

EGL API	Similar Direct3D 11 API or behavior
eglBindAPI	N/A.
eglBindTexImage	Call ID3D11Device::CreateTexture2D to set a 2D texture.
eglChooseConfig	Direct3D does not supply a set of default frame buffer configurations. The swap chain's configuration
eglCopyBuffers	To copy a buffer data, call ID3D11DeviceContext::CopyStructureCount. To copy a resource, call ID3DDeviceCOntext::CopyResource.
eglCreateContext	Create a Direct3D device context by calling D3D11CreateDevice, which returns both a handle to a Direct3D device and a default Direct3D immediate context (ID3D11DeviceContext1 object). You can also create a Direct3D deferred context by calling ID3D11Device2::CreateDeferredContext on the returned ID3D11Device1 object.
eglCreatePbufferFromClientBuffer	All buffers are read and written as a Direct3D subresource, such as an ID3D11Texture2D. Copy from one to another compatible subresource type with a methods such as ID3D11DeviceContext1:CopyResource.
eglCreatePbufferSurface	To create a Direct3D device with no swap chain, call the static D3D11CreateDevice method. For a Direct3D render target view, call ID3D11Device::CreateRenderTargetView.
eglCreatePixmapSurface	To create a Direct3D device with no swap chain, call the static D3D11CreateDevice method. For a Direct3D render target view, call ID3D11Device::CreateRenderTargetView.
eglCreateWindowSurface	Ontain an IDXGISwapChain1 (for the display buffers) and an ID3D11Device1 (a virtual interface for the graphics device and its resources). Use the ID3D11Device1 to define an ID3D11RenderTargetView that you can use to create the frame buffer you supply to the IDXGISwapChain1.
eglDestroyContext	N/A. Use ID3D11DeviceContext::DiscardView1 to get rid of a render target view. To close the parent ID3D11DeviceContext1, set the instance to null and wait for the platform to reclaim its resources. You cannot destroy the device context directly.
eglDestroySurface	N/A. Graphics resources are cleaned up when the Windows Store app's CoreWindow is closed by the platform.
eglGetCurrentDisplay	Call CoreWindow::GetForCurrentThread to get a reference to the current main app window.
eglGetCurrentSurface	This is the current ID3D11RenderTargetView. Typically, this is scoped to your renderer object.
eglGetError	Errors are obtained as HRESULTs returned by most methods on DirectX interfaces. If the method does not return an HRESULT, call GetLastError. To convert a system error into an HRESULT value, use the HRESULT_FROM_WIN32 macro.
eglInitialize	Call CoreWindow::GetForCurrentThread to get a reference to the current main app window.
eglMakeCurrent	Set a render target for drawing on the current context with ID3D11DeviceContext1::OMSetRenderTargets.
eglQueryContext	N/A. However, you may acquire rendering targets from an ID3D11Device1 instance, as well as some configuration data. (See the link for the list of available methods.)
eglQuerySurface	N/A. However, you may acquire data about viewports and the current graphics hardware from methods on an ID3D11Device1 instance. (See the link for the list of available methods.)
eglReleaseTexImage	N/A.
eglReleaseThread	For general GPU multithreading, read Multithreading.
eglSurfaceAttrib	Use D3D11_RENDER_TARGET_VIEW_DESC to configure a Direct3D render target view,
eglSwapBuffers	Use IDXGISwapChain1::Present1.
eglSwapInterval	See IDXGISwapChain1.
eglTerminate	The CoreWindow used to display the output of the graphics pipeline is managed by the operating system.
eglWaitClient	For shared surfaces, use IDXGIKeyedMutex. For general GPU multithreading, read Multithreading.
eglWaitGL	For shared surfaces, use IDXGIKeyedMutex. For general GPU multithreading, read Multithreading.
eglWaitNative	For shared surfaces, use IDXGIKeyedMutex. For general GPU multithreading, read Multithreading.

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