Photometric stereo is a technique used to recover the shapes of objects by analyzing their reflectance properties. However, it can be challenging to accurately model the bidirectional reflectance distribution function (BRDF) of certain materials, such as plastics or spray paints. In such cases, a calibration-based approach can be employed to overcome this limitation and apply photometric stereo to objects with unknown shapes.
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The Calibration Process
To perform calibration-based photometric stereo, a calibration object made of the same material as the object of interest is created. Typically, a sphere is used as the calibration object. Both the calibration object and the object of interest are illuminated with the same set of light sources.
Orientation Consistency
The key principle behind calibration-based photometric stereo is orientation consistency. It means that if the same surface normal is observed on the object and the calibration sphere, the intensities produced at those points should be identical. This allows for establishing a mapping between the intensity values and surface normals.
The Calibration Process in Detail
The calibration process starts by capturing a set of images of the calibration sphere using the same light sources that will be used for the object of interest. From one of the sphere images, the occluding boundary, or the edge of the sphere, is determined. This boundary provides essential information about the sphere’s shape.
Once the occluding boundary is known, the surface normal at any point on the sphere can be calculated. By knowing the radius of the sphere in pixels, it is possible to map the intensity values measured in the images to surface normals on the sphere, creating a lookup table.
Applying Calibration to the Object of Interest
After the calibration process, the object of interest is illuminated using the same set of light sources as in the calibration phase. A set of images is captured, and for each point on the object, the corresponding intensity values are used to look up the surface orientation from the calibration lookup table. This process generates a surface normal map for the object.
FAQs
Q: How does calibration-based photometric stereo differ from traditional photometric stereo?
A: Calibration-based photometric stereo uses a calibration object to overcome the challenge of accurately modeling the reflectance properties of certain materials, making it possible to apply photometric stereo to objects with unknown shapes.
Q: What is the purpose of the calibration sphere in the calibration process?
A: The calibration sphere is used to establish orientation consistency by comparing the intensities produced at corresponding points on both the calibration object and the sphere.
Q: Can calibration-based photometric stereo be applied to objects made of any material?
A: Calibration-based photometric stereo is particularly useful for objects made of materials like plastics or spray paints, where accurately modeling the bidirectional reflectance distribution function is challenging.
Conclusion
Calibration-based photometric stereo is a data-driven approach that enables the recovery of object shapes when the reflectance properties of the materials are difficult to model. By creating a calibration object with the same material and applying the principles of orientation consistency, a lookup table can be generated. This lookup table then allows for the easy extraction of surface normals from intensity values, resulting in a surface normal map of the object. Calibration-based photometric stereo provides a simple and effective solution for objects with unknown shapes.
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