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该函数主要是确定颜色缓冲区的来源（不会影响到深度、模板等缓冲区的读取），这里的设置将会影响到glReadPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D, glCopyTexSubImage3D的读取结果。

函数名字

Name

为像素选择颜色缓冲区源

glReadBuffer, glNamedFramebufferReadBuffer — select a color buffer source for pixels

函数原型

C Specification
void glReadBuffer(     GLenum mode);
 
void glNamedFramebufferReadBuffer(     GLuint framebuffer,
      GLenum mode);
参数
Parameters

帧缓冲区名

framebuffer

    Specifies the name of the framebuffer object for glNamedFramebufferReadBuffer function.

指定颜色缓冲区
mode

    Specifies a color buffer. Accepted values are GL_FRONT_LEFT, GL_FRONT_RIGHT, GL_BACK_LEFT, GL_BACK_RIGHT, GL_FRONT, GL_BACK, GL_LEFT, GL_RIGHT, and the constants GL_COLOR_ATTACHMENTi.

Description

glReadBuffer specifies a color buffer as the source for subsequent glReadPixels, glCopyTexImage1D, glCopyTexImage2D, glCopyTexSubImage1D, glCopyTexSubImage2D, and glCopyTexSubImage3D commands. mode accepts one of twelve or more predefined values. In a fully configured system, GL_FRONT, GL_LEFT, and GL_FRONT_LEFT all name the front left buffer, GL_FRONT_RIGHT and GL_RIGHT name the front right buffer, and GL_BACK_LEFT and GL_BACK name the back left buffer. Further more, the constants GL_COLOR_ATTACHMENTi may be used to indicate the ith color attachment where i ranges from zero to the value of GL_MAX_COLOR_ATTACHMENTS minus one.

Nonstereo double-buffered configurations have only a front left and a back left buffer. Single-buffered configurations have a front left and a front right buffer if stereo, and only a front left buffer if nonstereo. It is an error to specify a nonexistent buffer to glReadBuffer.

mode is initially GL_FRONT in single-buffered configurations and GL_BACK in double-buffered configurations.

For glReadBuffer, the target framebuffer object is that bound to GL_READ_FRAMEBUFFER. For glNamedFramebufferReadBuffer, framebuffer must either be zero or the name of the target framebuffer object. If framebuffer is zero, then the default read framebuffer is affected.
Errors

GL_INVALID_ENUM is generated if mode is not one of the twelve (or more) accepted values.

GL_INVALID_OPERATION is generated if mode specifies a buffer that does not exist.

GL_INVALID_OPERATION is generated by glNamedFramebufferReadBuffer if framebuffer is not zero or the name of an existing framebuffer object.
Associated Gets

glGet with argument GL_READ_BUFFER
Version Support
    OpenGL Version
Function / Feature Name     2.0     2.1     3.0     3.1     3.2     3.3     4.0     4.1     4.2     4.3     4.4     4.5
glNamedFramebufferReadBuffer     -     -     -     -     -     -     -     -     -     -     -     ✔
glReadBuffer     ✔     ✔     ✔     ✔     ✔     ✔     ✔     ✔     ✔     ✔     ✔     ✔
 

glReadBuffer(GL_BACK);//读取后端窗口
glReadBuffer(GL_FRONT);//读取前端窗口