Examples are described of marking specified regions of stored image frame buffer data in an image frame buffer. An imaging system can read the specified regions of the image frame buffer to identify whether the marking has been overwritten or not. The imaging system can thus efficiently identify how much of the image frame buffer has been overwritten with data from a new image frame. Based on this, the imaging system can retrieve partial image frame data from the image frame buffer and can process the partial image frame data, for instance to composite the partial image frame data with virtual content and/or to perform distortion compensation. The processed partial image frame data can be uploaded to a display buffer and displayed by a display, either as-is or once more of the frame is captured and processed. The imaging system can also perform auto-exposure using the partial image frame data.