API: glReadBuffer()/select a color buffer source for pixels :glDrawBuffer()/specify which color buffers are to be drawn into

[References]

1. "glReadBuffer",

2. "glDrawBuffer",

Name

• glReadBuffer, glNamedFramebufferReadBuffer — select a color buffer source for pixels

C Specification

• void glReadBuffer( GLenum mode);
• void glNamedFramebufferReadBuffer( GLuint framebuffer,GLenum mode);

Parameters

framebuffer

• Specifies the name of the framebuffer object for glNamedFramebufferReadBuffer function.

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.

________________________

Name

• glDrawBuffer, glNamedFramebufferDrawBuffer — specify which color buffers are to be drawn into

C Specification

• void glDrawBuffer( GLenum buf);
• void glNamedFramebufferDrawBuffer( GLuint framebuffer,GLenum buf);

Parameters

framebuffer

• Specifies the name of the framebuffer object for glNamedFramebufferDrawBuffer function. 
• Must be zero or the name of a framebuffer object.

buf

• For default framebuffer, the argument specifies up to four color buffers to be drawn into. Symbolic constants GL_NONE, GL_FRONT_LEFT, GL_FRONT_RIGHT,GL_BACK_LEFT, GL_BACK_RIGHT, GL_FRONT, GL_BACK, GL_LEFT, GL_RIGHT, and GL_FRONT_AND_BACK are accepted. 
• The initial value is GL_FRONT for single-buffered contexts, and 
• GL_BACK for double-buffered contexts.
•  For framebuffer objects, GL_COLOR_ATTACHMENT$m$ and GL_NONE enums are accepted, where$m$ is a value between 0 and GL_MAX_COLOR_ATTACHMENTS.

Description

When colors are written to the frame buffer, they are written into the color buffers specified by glDrawBuffer. One of the following values can be used for default framebuffer:

• GL_NONE No color buffers are written.
• GL_FRONT_LEFT Only the front left color buffer is written.
• GL_FRONT_RIGHT Only the front right color buffer is written.
• GL_BACK_LEFT Only the back left color buffer is written.
• GL_BACK_RIGHT Only the back right color buffer is written.
• GL_FRONT Only the front left and front right color buffers are written. If there is no front right color buffer, only the front left color buffer is written.
• GL_BACK  Only the back left and back right color buffers are written. If there is no back right color buffer, only the back left color buffer is written.
• GL_LEFT  Only the front left and back left color buffers are written. If there is no back left color buffer, only the front left color buffer is written.
• GL_RIGHT Only the front right and back right color buffers are written. If there is no back right color buffer, only the front right color buffer is written.
• GL_FRONT_AND_BACK All the front and back color buffers (front left, front right, back left, back right) are written. If there are no back color buffers, only the front left and front right color buffers are written. If there are no right color buffers, only the front left and back left color buffers are written. If there are no right or back color buffers, only the front left color buffer is written.

• If more than one color buffer is selected for drawing, then blending or logical operations are computed and applied independently for each color buffer and can produce different results in each buffer.

• Monoscopic contexts include only left buffers, and stereoscopic contexts include both left and right buffers. Likewise, single-buffered contexts include only frontbuffers, and double-buffered contexts include both front and back buffers. The context is selected at GL initialization.

• For framebuffer objects, GL_COLOR_ATTACHMENT$m$ and GL_NONE enums are accepted, where $m$ is a value between 0 and GL_MAX_COLOR_ATTACHMENTS. glDrawBuffer will set the draw buffer for fragment colors other than zero to GL_NONE.