opengl 多线程渲染中的问题

eglMakeCurrent详解

最新推荐文章于 2025-08-06 11:16:41 发布

原创最新推荐文章于 2025-08-06 11:16:41 发布 · 8.2k 阅读

3 ·

CC 4.0 BY-SA版权

文章标签：

#opengl

opengl 专栏收录该内容

9 篇文章

订阅专栏

本文详细介绍了EGL API中的eglMakeCurrent函数，该函数用于将渲染上下文绑定到绘图和读取表面。文章深入探讨了函数参数的意义及使用场景，并解释了如何确保正确配置上下文和表面。

Name

eglMakeCurrent — attach an EGL rendering context to EGL surfaces

把EGL的渲染上下文贴到EGL的表面上

C Specification

`EGLBoolean eglMakeCurrent(`	EGLDisplay `display`,
	EGLSurface `draw`,
	EGLSurface `read`,
	EGLContext `context)`;

Parameters

display

Specifies the EGL display connection.

draw

Specifies the EGL draw surface.

read

Specifies the EGL read surface.

context

Specifies the EGL rendering context to be attached to the surfaces.

Description

eglMakeCurrent binds context to the current rendering thread and to the draw and read surfaces. draw is used for all GL operations except for any pixel data read back (glReadPixels, glCopyTexImage2D, and glCopyTexSubImage2D), which is taken from the frame buffer values of read.

If the calling thread has already a current rendering context, that context is flushed and marked as no longer current.

The first time that context is made current, the viewport and scissor dimensions are set to the size of the draw surface. The viewport and scissor are not modified whencontext is subsequently made current.

To release the current context without assigning a new one, call eglMakeCurrent with draw and read set to EGL_NO_SURFACE and context set to EGL_NO_CONTEXT.

Use eglGetCurrentContext, eglGetCurrentDisplay, and eglGetCurrentSurface to query the current rendering context and associated display connection and surfaces.

首先第一

在 eglMakeCurrent函数中，有下面这段检查，意思是说，要不然read draw buffer都为空，要不然就都不能为空。

eglapi.c

if (!draw_surf || !read_surf) {
      /* surfaces may be NULL if surfaceless */
      if (!disp->Extensions.KHR_surfaceless_context)
         RETURN_EGL_ERROR(disp, EGL_BAD_SURFACE, EGL_FALSE);

      if ((!draw_surf && draw != EGL_NO_SURFACE) ||
          (!read_surf && read != EGL_NO_SURFACE))
         RETURN_EGL_ERROR(disp, EGL_BAD_SURFACE, EGL_FALSE);
      if (draw_surf || read_surf)
         RETURN_EGL_ERROR(disp, EGL_BAD_MATCH, EGL_FALSE);
   }

然后在eglContext.c中有以下代码

这里ctx表示着当前需要绑定的context，然后draw表示需要读的surface，那么可以看出surface也必须是已经绑定了本个线程的context

里面有一句话。假如draw或者read的surface已经在别的线程被绑定了，那么这里会产生一个错误，那就是EGL_BAD_ACESS

/*
* The spec says
*
* "If ctx is current to some other thread, or if either draw or read are
* bound to contexts in another thread, an EGL_BAD_ACCESS error is
* generated."and at most one context may be bound to a particular surface at a given
* time"
*/

  if (draw && draw->CurrentContext && draw->CurrentContext != ctx) {
      if (draw->CurrentContext->Binding != t ||
          draw->CurrentContext->ClientAPI != ctx->ClientAPI)
         return _eglError(EGL_BAD_ACCESS, "eglMakeCurrent");
   }