In feedback mode, each primitive to be rasterized generates a block of values that is copied into the feedback array. Supply this array with glFeedbackBuffer, which you must call before putting OpenGL into feedback mode. Each block of values begins with a code indicating the primitive type, followed by values that describe the primitive's vertices and associated data. Entries are also written for bitmaps and pixel rectangles. Values are not guaranteed to be written into the feedback array until you call glRenderMode to take OpenGL out of feedback mode. You can use glPassThrough to supply a marker that is returned in feedback mode as if it were a primitive.