Current chasms between applications implemented with different user interface toolkits make it difficult to implement and explore potentially important interaction techniques in new and existing applications, limiting the progress and impact of human-computer interaction research. We examine an approach based in the single most common characteristic of all graphical user interface toolkits, that they ultimately paint pixels to a display. We present Prefab, a system for implementing advanced behaviors through the reverse engineering of the pixels in graphical interfaces. Informed by how user interface toolkits paint interfaces, Prefab features a separation of the modeling of widget layout from the recognition of widget appearance. We validate Prefab in implementations of three applications: target-aware pointing techniques, Phosphor transitions, and Side Views parameter spectrums. Working only from pixels, we demonstrate a single implementation of these enhancements in complex existing applications created in different user interface toolkits running on different windowing systems.