Why can’t cameras blur each pixel individually to prevent moiré?
Asked 8/10/2015
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I understand that moiré happens when fine scene detail interacts badly with the sensor’s pixel grid, and that some cameras use an optical low-pass (anti-aliasing) filter to blur the image slightly before it reaches the sensor. My question is: why not blur the light separately for each individual pixel so every pixel gets averaged light, but without that blur spilling into neighboring pixels? For example, could each pixel have its own tiny blur element or barrier around it? Would that prevent moiré without reducing overall sharpness, or am I misunderstanding how aliasing works?
Originally by user42962. Source · Licensed CC BY-SA 4.0
user42962
10y ago
2 Answers
5
Moire occurs because the scene is sampled at a discrete resolution which is the nature of digital imaging. There is nothing to do about that but reduce the frequency of the signal reaching the sensor.
In fact, moire occurs in monochrome sensors too which have no sub-pixels. The matrix of pixels form a fixed grid to sample an image formed by incoming light and anytime that image has a higher frequency than the sampling from the sensor, it risks causing aliasing which appears as moire for high-frequency patterns.
You may have noticed that many cameras no longer feature anti-alias filters which allows the possibility of moire but it still occurs very rarely. Most times, one only need to stop down the lens or change the focal-length and moire disappears. There is a demo on my site where you can play with the aperture and see what happens to an image specifically selected to cause moire with two cameras which only differ in the fact that one of them has an anti-alias filter and the other does not. You will notices that moire occurs on both cameras and that with apertures beyond the diffraction limit, neither shows any.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
10y ago
0
Generated from our catalog & community — verify before relying on it.
Moiré is not mainly caused by RGB subpixels being separate; it comes from sampling a scene with a fixed pixel grid. If scene detail is finer than the sensor can properly sample, aliasing occurs, and that can show up as moiré. This also happens with monochrome sensors, so color subpixels are not the root issue.
An anti-aliasing filter works by slightly reducing very fine detail before it reaches the whole sensor, lowering frequencies the sensor cannot sample correctly. If you only blurred light within each individual pixel and prevented any mixing with neighboring pixels, you would still be sampling the image at the same grid spacing, so the aliasing problem would remain.
To reduce moiré, you must reduce high-frequency detail in the image reaching the sensor overall, not just average light inside one pixel well. In practice, many cameras omit AA filters because moiré is relatively rare, and it can often be reduced by changing aperture, focal length, or shooting conditions.
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AI10y ago
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