EGL: Does a FBO get created when creating a pBuffer surface? - opengl-es-2.0

Question

I am working with EGL on an ARM GPU, and I am using a pbuffer to do off screen rendering. I follow the standard procedures as described in the documentation to set everything up:
EGLDisplay display;
EGLConfig config;
EGLContext context;
EGLSurface surface;
EGLint num_config;
// assume I allocated both attrib lists somewhere
attribute_list[0] = EGL_SURFACE_TYPE;
attribute_list[1] = EGL_PBUFFER_BIT;
attribute_list[2] = EGL_RENDERABLE_TYPE;
attribute_list[3] = EGL_OPENGL_ES2_BIT;
attribute_list[4] = EGL_OPENGL_RED_SIZE;
attribute_list[5] = 8;
attribute_list[6] = EGL_OPENGL_GREEN_SIZE;
attribute_list[7] = 8;
attribute_list[8] = EGL_OPENGL_BLUE_SIZE;
attribute_list[9] = 8;
attribute_list[9] = EGL_OPENGL_ALPHA_SIZE;
attribute_list[10] = 8;
attribute_list[11] = EGL_OPENGL_DEPTH_SIZE;
attribute_list[12] = 8;
attribute_list[13] = EGL_NONE;
pbuffer_attribs[0] = EGL_WIDTH;
pbuffer_attribs[1] = 512;
pbuffer_attribs[2] = EGL_HEIGHT;
pbuffer_attribs[3] = 512;
pbuffer_attribs[4] = EGL_NONE;
/* get an EGL display connection */
display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
/* initialize the EGL display connection */
eglInitialize(display, NULL, NULL);
/* get an appropriate EGL frame buffer configuration */
eglChooseConfig(display, attribute_list, &config, 1, &num_config);
/* create an EGL rendering context */
context = eglCreateContext(display, config, EGL_NO_CONTEXT, NULL);
/* create the surface */
surface = eglCreatePbufferSurface(display, config, pbuffer_attribs);
/* connect the context to the surface */
eglMakeCurrent(display, surface, surface, context);
After this, my reads and writes should be associated with this offscreen pBuffer, correct? Does this pBuffer have a FBO which is distinct from the default framebuffer associated with it? The issue I am running into is I get a GL_FRAMEBUFFER_UNDEFINED error when I try to glReadPixels. This error happens when:
GL_FRAMEBUFFER_UNDEFINED is returned if target is the default framebuffer, but the default framebuffer does not exist.
My reading of this error is I am rendering to the default FBO and not to the pBuffer FBO. Is this interpretation correct? If so, what else do I need to do so I can read and write to the pBuffer FBO?

Answer 1

If the above sequence completes successfully (without errors), then, yes, the offscreen pBuffer becomes the default framebuffer for the OpenGL ES context and all reads and writes will be associated with the pBuffer (unless a non-default FBO is bound).
It's worth checking that eglGetError() returns EGL_SUCCESS after each EGL call. The following part of your code listing looks suspicious:
attribute_list[9] = 8;
attribute_list[9] = EGL_OPENGL_ALPHA_SIZE;