FEATURE Machine vision


The DLPC350 controller from


Texas Instruments provides input and output trigger signals for


synchronising displayed patterns with a camera. It works with digital micromirror devices,


designed to impart 3D machine vision to industrial, medical, and security equipment


• Pattern recognition is used to identify specific objects. At its simplest, this might mean looking for a specific well-defined mechanical part on a conveyor.


encoding and compression. Advantages over analogue image processing include minimised noise and distortion, as well as the availability of more algorithms. One early image enhancement use was correction of the first close-range images of the lunar surface. This used photogrammetric mapping as well as noise filters and corrections for the geometric distortions arising from the imaging camera’s alignment. Digital image enhancement often involves increasing the image contrast, and may also make geometric corrections for viewing angle and lens distortion. Compression is typically achieved by approximating a complex signal to a combination of cosine functions – a type of Fourier transform known as a discrete cosine transform, or DCT. The JPEG file format is the most popular application of DCT. Image


restoration may also use Fourier transforms to remove noise and blurring. • Photogrammetry uses a form of feature identification to extract measurements from images. These measurements can include 3D information when multiple images of the same scene have been obtained from different positions.


The simplest photogrammetry systems measure the distance between two points in an image employing a scale. Including a known scale reference in the image is normally required for this purpose. • Feature detection allows computers to identify edges and corners or points in an image. This is a required first step for photogrammetry, as well as the identification of objects and motion. Blob detection can identify regions with edges that are too smooth for edge or corner detection.


3D scanners capture 2D images of an object to create a 3D model of it. In some cases, the digital models are then used to 3D print copies


• 3D reconstruction determines the 3D form of objects from 2D images. It can be achieved by photogrammetric methods in which the height of common features (identified in images from different observation points) are determined by triangulation. 3D reconstruction is also possible using a single 2D image; here, software interprets (among other things) the geometric relationships between edges or regions of shading. A human can mentally reconstruct a cube from a simple line-art representation with ease – and a sphere from a shaded circle. Shading gives indication of the surfaces’ slopes. However, the process of such deduction is more complicated than it seems, because shading is a one-dimensional parameter while slope occurs in two dimensions. This can lead to ambiguities – a fact demonstrated by art depicting objects that can’t be physically rendered.


How machine-vision tasks are ordered


Many machine-vision systems progressively combine these techniques, by starting with low-level operations and then advancing one by one to higher- level operations. At the lowest level, all of an image’s pixels are held as high- bandwidth data. Then each operation in the sequence identifies image features and represents information of interest with relatively small amounts of data. The low-level operations of image enhancement and restoration come first, followed by feature detection. Where multiple sensors are used, low-level operations may therefore be carried out by distributed processes dedicated to individual sensors. Once features in individual images are detected, higher- level photogrammetric measurements can occur – as can any object identification or other tasks relying on the combined data from multiple images and sensors.


Direct computations and learning algorithms A direct computation in the context of machine vision is a set of mathematical functions that are manually defined


automationmagazine.co.uk Automation | May 2024 17


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