Foreshortening produces errors in the perception of angles pictured as on the ground

On the Evaluation of Diverse Vision Systems towards Detecting Human Pose in Collaborative Robot Applications

Tracking human operators working in the vicinity of collaborative robots can improve the design of safety architecture, ergonomics, and the execution of assembly tasks in a human-robot collaboration scenario. Three commercial spatial computation kits were used along with their Software Development Kits that provide various real-time functionalities to track human poses. The paper explored the possibility of combining the capabilities of different hardware systems and software frameworks that may lead to better performance and accuracy in detecting the human pose in collaborative robotic applications. This study assessed their performance in two different human poses at six depth levels, comparing the raw data and noise-reducing filtered data. In addition, a laser measurement device was employed as a ground truth indicator, together with the average Root Mean Square Error as an error metric. The obtained results were analysed and compared in terms of positional accuracy and repeatability, indicating the dependence of the sensors' performance on the tracking distance. A Kalman-based filter was applied to fuse the human skeleton data and then to reconstruct the operator's poses considering their performance in different distance zones. The results indicated that at a distance less than 3 m, Microsoft Azure Kinect demonstrated better tracking performance, followed by Intel RealSense D455 and Stereolabs ZED2, while at ranges higher than 3 m, ZED2 had superior tracking performance.

Gibson and Pictures in Perspective: Reverse the Directions

In his extensive writing about pictures, James J. Gibson offered perspective formulae for square tiles projecting trapezoids onto a picture plane, foreshortening to zero height with distance. I reverse the claim: as distance decreases, the trapezoids increase to infinite height, in marginal distortion, or forelengthening. I also reverse the direction of projection. Usually considered to be incoming, from the distant tile to the picture plane, in reverse—outgoing—the tiles have directions from the center of projection, with implications for haptics and people who are blind. A drawing of a cube illustrates the argument. It is by an adult who is blind. It includes foreshortening, and shows directions of surfaces from a vantage point.

Foreshortening increases apparent angles

The hypothesis that perspective foreshortening leads to errors in the visual perception of angles, was tested in four experiments. An oblique to a z-dimension line was presented (a) on the ground in Experiments 1 and 2, and (b) on a wall in Experiments 3 and 4. Observers judged the acute angle between the oblique and the z-line. Foreshortening increased with the oblique's distance along the z-line and, in Experiments 2 and 4, shorter distances from the eye to the ground or wall. As distance and eye-height vary, so does the target's slant to the line of sight. We argue the apparent angles between the lines increased with foreshortening because vision underestimates the fast rate of foreshortening with elevation compared with the slower rates in azimuth.

Inverse Sideways Perspective on a Road: A Drawing by a Blind Adult

An unusual drawing of a road has two lines converging sideways, from right to left. The left side of the picture is explicitly described as the location of the observer. Also, the fronts of the cars on the road face left, with the largest car on the right. This sideways perspective is novel. In linear perspective, roads running parallel with the picture surface should be drawn with parallel lines. Lines for roads running orthogonal to the picture surface should converge with elevation. The rule for roads is if converging, then upward, and if sideways, then not converging. The sketch is by a blind woman with modest experience in drawing, including perspective. It suggests an intermediate stage of drawing development, with inconsistent use of the observer's vantage point, in keeping with theories of perspective drawing by the blind and sighted of Willats, Kennedy and others.

Pot/Lid Illusion

A new everyday visual size illusion is presented-the Pot/Lid illusion. Observers choose an unduly large lid for a pot. We ask whether the optic slant of the pot brim would increase its apparent size or if vision underestimates the size of tilted lids.