An intelligent system detects triangles in digital photographic images, including portrait photography. The method extracts a set of filtered line segments as candidate triangle sides and/or objects as candidate triangle vertices. A modified RANSAC algorithm is utilized to fit triangles onto the set of line segments and/or vertices. Two metrics may then be used evaluate the fitted triangles. Those with high fitting scores are considered as detected triangles. The system can accurately locate preeminent triangles in photographs without any knowledge about the camera parameters or lens choices. The invention can also help amateurs gain a deeper understanding and inspirations from professional photographic works.