Optical sensing is provided for anti-spoofing of transparent spoof overlay material using pixel spreading detection. Some embodiments operate in context of an under-display optical fingerprint sensor integrated into an electronic device. An optical scanning system can direct illumination incident on a sensing region, which is redirected, by specular reflection off of a contact surface, onto an optical sensor in a predictable manner. When an object (e.g., finger) is in direct contact with a contact surface, the predictable specular correspondence is maintained. However, when a transparent spoof material is placed between the object and the contact surface, the spoof material produces additional specular and non-specular reflection, resulting in pixel spreading at the optical sensor. Thus, presence of the spoof material can be detected by detecting such pixel spreading. Some embodiments use illumination masking to help facilitate the pixel spreading detection.