High-precision camera distortion measurements with a "calibration harp"

TL;DR

This paper introduces a high-precision camera distortion measurement method using a specially designed 'calibration harp' with tightly stretched strings, achieving 2/100 pixel accuracy and improving calibration reliability and automation.

Contribution

It presents a novel 'calibration harp' device and algorithms for automatic, high-precision lens distortion measurement, surpassing traditional flat pattern methods.

Findings

Achieved 2/100 pixel measurement precision.

Enabled objective evaluation of distortion correction algorithms.

Improved calibration reliability and automation.

Abstract

This paper addresses the high precision measurement of the distortion of a digital camera from photographs. Traditionally, this distortion is measured from photographs of a flat pattern which contains aligned elements. Nevertheless, it is nearly impossible to fabricate a very flat pattern and to validate its flatness. This fact limits the attainable measurable precisions. In contrast, it is much easier to obtain physically very precise straight lines by tightly stretching good quality strings on a frame. Taking literally "plumb-line methods", we built a "calibration harp" instead of the classic flat patterns to obtain a high precision measurement tool, demonstrably reaching 2/100 pixel precisions. The harp is complemented with the algorithms computing automatically from harp photographs two different and complementary lens distortion measurements. The precision of the method is evaluated on images corrected by state-of-the-art distortion correction algorithms, and by popular software. Three applications are shown: first an objective and reliable measurement of the result of any distortion correction. Second, the harp permits to control state-of-the art global camera calibration algorithms: It permits to select the right distortion model, thus avoiding internal compensation errors inherent to these methods. Third, the method replaces manual procedures in other distortion correction methods, makes them fully automatic, and increases their reliability and precision.