In an increasingly digital world, it’s small wonder that we’re constantly searching for ever-more-sophisticated ways to interact with photographs and images: designers scan a 3D prototype and import its dimensions into a computer; medical programs image an internal organ and delineate the tumor to be removed; robots avoid drop-offs by recognizing the shapes that stairs make on their detectors; teenagers transform their selfies into what appears to be a pencil sketch.
Although these tasks range widely in scope, they all rely on a computer being able to accurately judge the boundaries between different parts of an image, like the line that separates a check from its background when you use a mobile deposit app. Currently the technology exists to allow for each of these applications, but professionals say that it leaves a lot to be desired.
To begin with, most methods of edge detection use complicated computer algorithms that require large amounts of processing power, making it difficult to use the results in real time. Even devices that use physical rather than computational means to enhance the contrast between different objects, such as the Nomarski interference prism or metamaterials, rely on highly specialized (not to mention expensive) equipment that puts them out of reach for many individuals. “These methods are either bulky and complex or challenging for precise fabrication,” says Zhejiang University researcher Zhichao Ruan.
That’s why Ruan—and a network of collaborators spanning four Chinese universities—decided to take a step back from all the fancy technologies and focus instead on basic equipment found in every optics lab: mirrors, lenses and polarizers.
Although everyone's familiar with mirrors and lenses, polarizers—which selectively cut out certain types of light—are also common in daily life. To understand how they work, think about light in its wavelike form. Much like a rope that can be shaken up and down, side to side, or any combination of the two, light can also take any number of orientations. A polarizer acts something like a vertical or horizontal fence that only allows certain orientations of light to pass through. This is how polarizing sunglasses cut down on road glare without dimming everything else too much—horizontal surfaces mostly reflect horizontally-polarized light.
Well, almost. That’s certainly the case when the polarizers are stacked, as in the image above, but the research team found that if they placed a mirror between the polarizers, shown below, that wasn’t the whole story. “Despite the common knowledge in textbooks that light cannot pass through two orthogonal polarizers,” Ruan says, “there is light passing through and it corresponds to optical computation of spatial differentiation.” In other words, not only does light pass through the second polarizer, but it results in a remarkably clean edge detection!
spin Hall effect of light forces the orientation of light to rotate slightly in a direction that depends on the specific quantum properties of each photon. This transforms what was previously a uniform polarized light source into a superposition of precisely oriented “bunches” of light that can recombine and interfere with each other as they arrive at the second polarizer.
To fully explain the outline effect, the researchers had to delve far into the mathematics of wave optics. Ultimately, they showed that for a setup like theirs, the intensity of the output light is proportional to the change in intensity at that region for the input image. In other words, if a single pixel is roughly as bright as the pixels on either side of it, the output for that pixel will be quite dim. If, on the other hand, the pixel is right on the border of a shape—meaning its leftmost neighbor is much dimmer than its rightmost—it will shine brightly on the output image.
There is a catch though: this differentiation only works in a single direction. “The edge detection can only be performed along the direction perpendicular to the incident plane,” Ruan explains. This makes it impossible to detect purely vertical and purely horizontal lines simultaneously. Nevertheless, they managed to produce some stunning results, published last week in the American Physical Society journal Physical Review Applied.
Although they aren’t the first to achieve edge detection by a long shot, it is incredible to think that the results of their minimalistic optics setup can rival the most sophisticated of algorithms. Given all that we’ve done with relatively clunky technology so far, who knows what could be on the horizon?
—Eleanor Hook
Although these tasks range widely in scope, they all rely on a computer being able to accurately judge the boundaries between different parts of an image, like the line that separates a check from its background when you use a mobile deposit app. Currently the technology exists to allow for each of these applications, but professionals say that it leaves a lot to be desired.
To begin with, most methods of edge detection use complicated computer algorithms that require large amounts of processing power, making it difficult to use the results in real time. Even devices that use physical rather than computational means to enhance the contrast between different objects, such as the Nomarski interference prism or metamaterials, rely on highly specialized (not to mention expensive) equipment that puts them out of reach for many individuals. “These methods are either bulky and complex or challenging for precise fabrication,” says Zhejiang University researcher Zhichao Ruan.
That’s why Ruan—and a network of collaborators spanning four Chinese universities—decided to take a step back from all the fancy technologies and focus instead on basic equipment found in every optics lab: mirrors, lenses and polarizers.
Although everyone's familiar with mirrors and lenses, polarizers—which selectively cut out certain types of light—are also common in daily life. To understand how they work, think about light in its wavelike form. Much like a rope that can be shaken up and down, side to side, or any combination of the two, light can also take any number of orientations. A polarizer acts something like a vertical or horizontal fence that only allows certain orientations of light to pass through. This is how polarizing sunglasses cut down on road glare without dimming everything else too much—horizontal surfaces mostly reflect horizontally-polarized light.
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Two polarizers lined up perpendicularly typically block all light that tries to pass through them. Image Credit: NielsB via Wikimedia Commons. |
spin Hall effect of light forces the orientation of light to rotate slightly in a direction that depends on the specific quantum properties of each photon. This transforms what was previously a uniform polarized light source into a superposition of precisely oriented “bunches” of light that can recombine and interfere with each other as they arrive at the second polarizer.
To fully explain the outline effect, the researchers had to delve far into the mathematics of wave optics. Ultimately, they showed that for a setup like theirs, the intensity of the output light is proportional to the change in intensity at that region for the input image. In other words, if a single pixel is roughly as bright as the pixels on either side of it, the output for that pixel will be quite dim. If, on the other hand, the pixel is right on the border of a shape—meaning its leftmost neighbor is much dimmer than its rightmost—it will shine brightly on the output image.
There is a catch though: this differentiation only works in a single direction. “The edge detection can only be performed along the direction perpendicular to the incident plane,” Ruan explains. This makes it impossible to detect purely vertical and purely horizontal lines simultaneously. Nevertheless, they managed to produce some stunning results, published last week in the American Physical Society journal Physical Review Applied.
Although they aren’t the first to achieve edge detection by a long shot, it is incredible to think that the results of their minimalistic optics setup can rival the most sophisticated of algorithms. Given all that we’ve done with relatively clunky technology so far, who knows what could be on the horizon?
—Eleanor Hook
I think you linked to the wrong arXiv article--should it be this one https://arxiv.org/abs/1804.06965?
ReplyDeleteAhp—thanks for that! Fixed.
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