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I went to the National Gallery of Art in Washington D.C. and saw an artwork by Jack Whitten entitled Ascension I. This interesting piece was made by applying a tool to generate wavy lines in a background of acrylic paint to create the diagonal interlace pattern that you see in some baskets. Apart from the art itself, one of the things that caught my attention is that when I moved, the pattern of the art piece seemed to pulsate, displaying changes in color and giving off a glare. I was able to capture this with my phone camera in the video below. These optical effects are an example of what is called spatial aliasing. The most well-known example of spatial aliasing is when two grates or meshes are placed on top of each other and one of them is moved. A series of banding patterns appear, which are called interference patterns or Moiré patterns. Spatial aliasing occurs when a signal is not sampled often enough along an axis in space. Our eye and the camera do not monitor reality in a continuous fashion, instead they take samples and then put them together much in the same way that the sensation of movement is generated in a film by playing individual frames one after the other. Spatial aliasing is a problem in any branch of technology that involves waves, such as when playing or recording sounds or generating or producing light. Many industries, ranging from computer graphics to recording studios, implement anti-aliasing techniques to improve the quality of the images or the sound. Another modality of aliasing, called temporal aliasing, is produced when the sampling rate for the signal is insufficient over time. A classic example is the apparent change in rotation speed and direction of the spokes of a wheel. I have previously made a video of the phenomenon of temporal aliasing I observed when shooting a video of the railroad tracks from a moving train. The photos belong to the author and can only be used with permission.
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I filmed the video below from a train on my way to the town of Kutna Hora in the Czech Republic. The way the tracks seem to “travel” next to each other and then merge is really cool. This is due to the fact that the wooden blocks of the train tracks (called sleepers or ties) seem to be moving in the same direction that the train is moving even though in reality they are receding as the train moves forward. This effect is called "temporal aliasing", and it is the same effect that you see in the spokes of the rotating wheel of a wagon. In the case of the camera, this happens because it does not record reality in a continuous fashion. Rather the camera samples reality a number of times per second, and the samples are put together to generate the video. This is similar to how the old celluloid movies would generate the perception of motion from different successive frames each displaying different stages of a movement. If the sampling rate is not appropriate to record a moving object, the object will appear to move in a direction that is illusory, such as is the case of the wooden blocks in my video. Whether the human eye/brain system takes samples of reality like a camera is controversial, and there is evidence both in favor and against this hypothesis. The interesting thing about my video is that I could not see this forward motion of the wooden blocks with my own eyes, because the blocks became a blur when the train was moving fast. I could only see it with the camera! |
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