The WebGL2RenderingContext.readBuffer() method of the WebGL 2 API selects a color buffer as the source for pixels for subsequent calls to copyTexImage2D, copyTexSubImage2D, copyTexSubImage3D or readPixels.

source

A GLenum specifying a color buffer. Possible values:

gl.BACK

Reads from the back color buffer.

gl.NONE

Reads from no color buffer.

gl.COLOR_ATTACHMENT{0-15}

Reads from one of the 16 color attachment buffers.

None (undefined).

• WebGLRenderingContext.copyTexImage2D()
• WebGLRenderingContext.copyTexSubImage2D()
• WebGL2RenderingContext.copyTexSubImage3D()
• WebGLRenderingContext.readPixels()