3D imaging technology has come a long way from its roots in academic research labs, and thanks to innovations in sensors, decreasing cost of components, and the emergence of 3D functions in software libraries, 3D vision is now appearing in a variety of machine automation applications. From vision-guided robotic bin-picking to high-precision metrology, the latest generation of processors can now handle the immense data sets and sophisticated algorithms required to extract depth information and quickly make decisions.
There are several ways to calculate depth information using 2D camera sensors or other optical sensing technologies, each of which has its own set of tradeoffs and benefits, and the choice of technique often depends on the application itself. Common techniques for 3D vision include stereovision, laser triangulation, time of flight, and projected light. This article will focus on stereovision, which uses the disparity between images from multiple cameras to extract depth information.
Binocular stereovision algorithms can be used to calculate depth information from multiple cameras. By using calibration information between two cameras, the new algorithms can generate depth images, providing richer data to identify objects, detect defects, and guide robotic arms on how to move and respond.
A binocular stereovision system uses exactly two cameras. Ideally, the two cameras are separated by a short distance, and are mounted almost parallel to one another. In the example shown in Figure 1, a box of spherical chocolates is used to demonstrate the benefits of 3D imaging for automated inspection. After calibrating the two cameras to know the 3D spatial relationship, such as separation and tilt, two different images are acquired to locate potential defects in the chocolate. Using 3D stereovision algorithms, the two images can be combined to calculate depth information and visualize a depth image.
While difficult to characterize using traditional two-dimensional functions, the 3D depth image shows that two of the chocolates are not spherical enough to pass the high quality standards. The image in Figure 2 shows a white box around the defects that have been identified.
One important consideration when using stereovision is that the computation of the disparity is based on locating a feature from a line of the left image, and the same line of the right image. To be able to locate and differentiate the features, the images need to have sufficient detail, and the objects sufficient texture or non-uniformity. To obtain better results, one may need to add this needed detail by illuminating the scene with structured lighting.
Finally, binocular stereovision can be used to calculate the 3D coordinates (X,Y,Z) of points on the surface of an object being inspected. These points are often referred to as point clouds or cloud of points. Point clouds are very useful in visualizing the 3D shape of objects and can also be used by other 3D analysis software.
How Stereovision Works
To better illustrate how binocular stereovision works, Figure 3 shows the diagram of a simplified stereovision setup, where both cameras are mounted perfectly parallel to each other, and have the exact same focal length.
The variables in Figure 3 are: b is the baseline, or distance between the two cameras.
f is the focal length of a camera.
XA is the X-axis of a camera.
ZA is the optical axis of a camera.
P is a real-world point defined by the coordinates X, Y, and Z.
uL is the projection of the real-world point P in an image acquired by the left camera.
uR is the projection of the real-world point P in an image acquired by the right camera.
Since the two cameras are separated by distance “b,” both cameras view the same real-world point P in a different location on the two-dimensional images acquired. The X-coordinates of points uL and uR are given by:
uL = f * X/Z
uR = f * (X-b)/Z