A movable camera travels along an inspection path for optically inspecting an inspection surface of an object for defect detection. In planning the inspection path, a set of viewpoints on the inspection surface is generated. Each viewpoint is associated with a patch, which is a largest area of the inspection surface within a field of view (FOV) of the camera when the camera is located over the viewpoint for capturing an image of FOV. An effective region of the patch is advantageously predicted by a neural network according to a three-dimensional geometric characterization of the patch such that the predicted effective region is valid for defect detection based on the captured image. A valid area is one whose corresponding area on the captured image is not blurred and is neither underexposed nor overexposed. The inspection path is determined according to respective effective regions associated with the set of viewpoints.