Computes surface normals using four images with light source in different places.

To compute surface normals using four images with light source in different places, we can use the photometric stereo technique. This technique involves capturing multiple images of an object under different lighting conditions and using the variations in brightness to infer the surface normals.

Here are the steps involved in computing surface normals using photometric stereo:

1. Capture four images of the object with the light source in different positions. The images should be taken from the same viewpoint and under the same camera settings.

2. Convert the images to grayscale.

3. Normalize the pixel values in each image so that they have the same mean and standard deviation.

4. For each pixel in the images, compute the surface normal using the following equation:

N = (L1 - L3) x (L2 - L4)

where N is the surface normal, L1, L2, L3, and L4 are the pixel values in the four images, and x denotes the cross product.

5. Normalize the surface normals to have unit length.

6. Generate a surface reconstruction using the computed surface normals.

7. Apply smoothing and filtering to the surface reconstruction to remove noise and artifacts.

By following these steps, we can compute accurate surface normals for an object using photometric stereo. This technique is widely used in computer vision and 3D scanning applications.