Procedure for using the PhotometricStereo step

The photometric stereo source images must be of exactly the same scene; otherwise, you must align the images before registration. The images must be taken under different direct lighting orientations, and are combined in the PhotometricStereo step to create a single enhanced image.

You can use 4 or more lights in the photometric stereo setup, whereby each input image must be taken with only one light source. Note that when setting up for a Local Shape operation, your lights should be set up in opposite pairs. That is, for each light in the setup, there should be another light positioned 180 degrees opposite.

See Chapter 28: Acquisition for more information about grabbing or loading an image. See the Photometric stereo registration of a moving object section later in this chapter on how to align images if the object is moving.

Checkmark one or more required Output Images. This sets which photometric stereo operation(s) to perform. For information on each photometric stereo operation, see the Output image types section later in this chapter.

When multiple Output Images are selected, you can right-click on the image display and select from the Display Image context menu item which one to display, or choose from the dropdown list in the Project toolbar. For information on displaying multiple images at once, see the Displaying multiple images subsection of the Quick Evaluate pane section in the Panes and editors reference chapter.

Output images based on shape (Local Shape, Gaussian Curvature, Mean Curvature) are calculated internally using normal vectors whose magnitudes can vary widely depending on image content. Internal calculations are done in floating-point. However, to generate the output image with the same depth as the source images, the calculated values must be mapped or scaled to integer intensity values. By default, the Remap Factor input is set to Automatic mode, which remaps values using the full range of values in the current image. The remap factor is reported (to the left of the Set button). However, with some objects and lighting, it is possible for some (but not all) source images to have peak values that are much bigger than the others in the set (for example, a random, very sharp reflection), which causes that image to use a very different remap factor, possibly diminishing important details in key parts of the image. To avoid the effect of occasional sharp transitions, you can set a constant factor to use for the remapping.

Generally, using a constant Remap Factor produces reliable results. It is recommended to find the reported Automatic mode factor for some typical images and then lock in a value (Set) that is slightly higher. You can try a value that is 3 to 4 times the reported Automatic mode factor. This increases image contrast and helps to bring out smaller details in the photometric stereo output image.

For each source image, you must specify the direction of its light source using a vector.

Set the coordinate system in which you are expressing the source images' light vectors.

• The Spherical coordinate system defines each light vector with 3 values: the zenith (polar) angle (angle between a light and the camera), the azimuth angle (angle of the light vector projected onto the XY-plane, measured from the positive X-axis), and the relative light intensity value.

• The Cartesian coordinate system defines each light vector with 3 coordinates: X,Y, and Z, with the axes oriented as shown in the image below.

Note that the origin is located at the center of the plane upon which the object is placed for the photometric stereo registration operation, directly below the camera (assuming the camera is perpendicular to the scene).

A lighting element specifies the directional light used to illuminate one photometric stereo source image. There should be one lighting element per photometric stereo source image.

Modify the settings of the 4 default Lighting Elements if the defaults do not match your setup. Add more Lighting Elements if required.

Ensure that each lighting element is linked to a different source image.

When working in the Spherical coordinate system, you can modify the relative intensity of each lighting element. The default value is 1.0. If the relative light intensity is set to 1.0 for all lighting elements, each light provides the same intensity to the setup, which is the ideal arrangement. If equal light intensities are not possible, set the strongest light to 1.0 and set the other light intensity values relative to this.

To adjust relative light intensities within the Cartesian coordinate system, note that the light intensity correlates to the norm (magnitude) of the vector formed by X, Y, and Z. Calculate the norm using the following formula: . Once you know the light intensity for the strongest light, adjust the X-, Y-, and Z-coordinates of the other lights accordingly.

If a source image's lighting is significantly uneven across the surface of the scene, enable Non-Uniformity Correction to see if it improves photometric stereo results.

The default setting for Non-Uniformity Correction is Disable. If set to Auto, source images are corrected for non-uniform illumination.

In certain cases, the PhotometricStereo step might take longer than is acceptable for your application. To account for your time constraints, you can set the Timeout property to a maximum time for the operation. The default Timeout limit is 2000 msec. If the limit is exceeded, processing is stopped, and no results are available.

Shape smoothness

Normalization

Object size