2015-11-21 19:36:47 -07:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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<title>Mesa EGL</title>
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<link rel="stylesheet" type="text/css" href="mesa.css">
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</head>
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<body>
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<div class="header">
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<h1>The Mesa 3D Graphics Library</h1>
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</div>
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<iframe src="contents.html"></iframe>
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<div class="content">
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<h1>Mesa EGL</h1>
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<p>The current version of EGL in Mesa implements EGL 1.4. More information
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about EGL can be found at
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<a href="https://www.khronos.org/egl/">
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https://www.khronos.org/egl/</a>.</p>
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<p>The Mesa's implementation of EGL uses a driver architecture. The main
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library (<code>libEGL</code>) is window system neutral. It provides the EGL
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API entry points and helper functions for use by the drivers. Drivers are
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dynamically loaded by the main library and most of the EGL API calls are
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directly dispatched to the drivers.</p>
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<p>The driver in use decides the window system to support.</p>
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<h2>Build EGL</h2>
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<ol>
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<li>
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<p>Run <code>configure</code> with the desired client APIs and enable
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the driver for your hardware. For example</p>
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<pre>
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$ ./configure --enable-gles1 --enable-gles2 \
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--with-dri-drivers=... \
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--with-gallium-drivers=...
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</pre>
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<p>The main library and OpenGL is enabled by default. The first two options
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above enables <a href="opengles.html">OpenGL ES 1.x and 2.x</a>. The last two
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options enables the listed classic and Gallium drivers respectively.</p>
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</li>
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<li>Build and install Mesa as usual.</li>
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</ol>
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<p>In the given example, it will build and install <code>libEGL</code>,
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<code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>, and one
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or more EGL drivers.</p>
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<h3>Configure Options</h3>
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<p>There are several options that control the build of EGL at configuration
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time</p>
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<dl>
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<dt><code>--enable-egl</code></dt>
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<dd>
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<p>By default, EGL is enabled. When disabled, the main library and the drivers
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will not be built.</p>
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</dd>
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<dt><code>--with-egl-driver-dir</code></dt>
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<dd>
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<p>The directory EGL drivers should be installed to. If not specified, EGL
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drivers will be installed to <code>${libdir}/egl</code>.</p>
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</dd>
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<dt><code>--with-platforms</code></dt>
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<dd>
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<p>List the platforms (window systems) to support. Its argument is a comma
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separated string such as <code>--with-platforms=x11,drm</code>. It decides
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the platforms a driver may support. The first listed platform is also used by
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the main library to decide the native platform.</p>
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<p>The available platforms are <code>x11</code>, <code>drm</code>,
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<code>wayland</code>, <code>surfaceless</code>, <code>android</code>,
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and <code>haiku</code>.
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The <code>android</code> platform can either be built as a system
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component, part of AOSP, using <code>Android.mk</code> files, or
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cross-compiled using appropriate <code>configure</code> options.
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The <code>haiku</code> platform can only be built with SCons.
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Unless for special needs, the build system should
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select the right platforms automatically.</p>
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</dd>
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<dt><code>--enable-gles1</code></dt>
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<dt><code>--enable-gles2</code></dt>
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<dd>
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<p>These options enable OpenGL ES support in OpenGL. The result is one big
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internal library that supports multiple APIs.</p>
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</dd>
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<dt><code>--enable-shared-glapi</code></dt>
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<dd>
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<p>By default, <code>libGL</code> has its own copy of <code>libglapi</code>.
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This options makes <code>libGL</code> use the shared <code>libglapi</code>. This
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is required if applications mix OpenGL and OpenGL ES.</p>
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</dd>
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</dl>
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<h2>Use EGL</h2>
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<h3>Demos</h3>
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<p>There are demos for the client APIs supported by EGL. They can be found in
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mesa/demos repository.</p>
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<h3>Environment Variables</h3>
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<p>There are several environment variables that control the behavior of EGL at
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runtime</p>
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<dl>
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<dt><code>EGL_PLATFORM</code></dt>
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<dd>
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<p>This variable specifies the native platform. The valid values are the same
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as those for <code>--with-platforms</code>. When the variable is not set,
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the main library uses the first platform listed in
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<code>--with-platforms</code> as the native platform.</p>
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<p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
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create displays for non-native platforms. These extensions are usually used by
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applications that support non-native platforms. Setting this variable is
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probably required only for some of the demos found in mesa/demo repository.</p>
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</dd>
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<dt><code>EGL_LOG_LEVEL</code></dt>
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<dd>
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<p>This changes the log level of the main library and the drivers. The valid
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values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
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<code>fatal</code>.</p>
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</dd>
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</dl>
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<h2>EGL Drivers</h2>
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<dl>
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<dt><code>egl_dri2</code></dt>
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<dd>
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<p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
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It functions as a DRI driver loader. For <code>x11</code> support, it talks to
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the X server directly using (XCB-)DRI2 protocol.</p>
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<p>This driver can share DRI drivers with <code>libGL</code>.</p>
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</dd>
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</dl>
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<h2>Packaging</h2>
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<p>The ABI between the main library and its drivers are not stable. Nor is
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there a plan to stabilize it at the moment.</p>
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<h2>Developers</h2>
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<p>The sources of the main library and drivers can be found at
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<code>src/egl/</code>.</p>
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<h3>Lifetime of Display Resources</h3>
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<p>Contexts and surfaces are examples of display resources. They might live
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longer than the display that creates them.</p>
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<p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
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display resources should be destroyed. Similarly, when a thread is released
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through <code>eglReleaseThread</code>, all current display resources should be
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released. Another way to destroy or release resources is through functions
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such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
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<p>When a resource that is current to some thread is destroyed, the resource
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should not be destroyed immediately. EGL requires the resource to live until
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it is no longer current. A driver usually calls
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<code>eglIs<Resource>Bound</code> to check if a resource is bound
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(current) to any thread in the destroy callbacks. If it is still bound, the
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resource is not destroyed.</p>
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<p>The main library will mark destroyed current resources as unlinked. In a
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driver's <code>MakeCurrent</code> callback,
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<code>eglIs<Resource>Linked</code> can then be called to check if a newly
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released resource is linked to a display. If it is not, the last reference to
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the resource is removed and the driver should destroy the resource. But it
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should be careful here because <code>MakeCurrent</code> might be called with an
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uninitialized display.</p>
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<p>This is the only mechanism provided by the main library to help manage the
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resources. The drivers are responsible to the correct behavior as defined by
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EGL.</p>
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<h3><code>EGL_RENDER_BUFFER</code></h3>
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<p>In EGL, the color buffer a context should try to render to is decided by the
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binding surface. It should try to render to the front buffer if the binding
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surface has <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_SINGLE_BUFFER</code>; If the same context is later bound to a
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surface with <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_BACK_BUFFER</code>, the context should try to render to the back
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buffer. However, the context is allowed to make the final decision as to which
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color buffer it wants to or is able to render to.</p>
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<p>For pbuffer surfaces, the render buffer is always
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<code>EGL_BACK_BUFFER</code>. And for pixmap surfaces, the render buffer is
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always <code>EGL_SINGLE_BUFFER</code>. Unlike window surfaces, EGL spec
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requires their <code>EGL_RENDER_BUFFER</code> values to be honored. As a
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result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
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<code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
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config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
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pbuffer surfaces.</p>
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<p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
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single-buffered, in that <code>eglSwapBuffers</code> has no effect on them. It
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is desirable that a driver allocates a private color buffer for each pbuffer
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surface created. If the window system the driver supports has native pbuffers,
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or if the native pixmaps have more than one color buffers, the driver should
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carefully attach the native color buffers to the EGL surfaces, re-route them if
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required.</p>
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<p>There is no defined behavior as to, for example, how
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<code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>. Right
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now, it is desired that the draw buffer in a client API be fixed for pixmap and
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pbuffer surfaces. Therefore, the driver is responsible to guarantee that the
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client API renders to the specified render buffer for pixmap and pbuffer
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surfaces.</p>
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<h3><code>EGLDisplay</code> Mutex</h3>
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The <code>EGLDisplay</code> will be locked before calling any of the dispatch
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functions (well, except for GetProcAddress which does not take an
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<code>EGLDisplay</code>). This guarantees that the same dispatch function will
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not be called with the sample display at the same time. If a driver has access
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to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
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should as well lock the display before using it.
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</div>
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</body>
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</html>
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