Digital Imaging Artifacts: Image aliasing & color moiré

Today we’re gonna start learning about various image problems and draw backs that are most associated with digital imaging systems and the whole revolution of digital sensors. I’m going to expand these problems over a series of posts, starting today with Image Aliasing and Color Moiré.

Image aliasing occurs when attempting to sample a high frequency  signal onto a lower frequency sampling medium. When an exposure is made, the captured scene is sampled on discrete spatial image positions. If the highest frequency in the scene exceeds the sampling frequency of the digital imaging system, aliasing can occur producing evident moiré patterns.

Moiré patterns are undesired artifacts in images produced by various digital imaging and computer graphics techniques when two grids are overlaid at an angle or when they have slightly different mesh size, and are seen as interfering lines or bands of colors covering areas of the image where the level of texture and color detail is very high. Aliasing can also occur when resizing a high resolution image into a much smaller one.

Most digital cameras nowadays use an optical anti-aliasing filter (also known as optical lowpass filter, blur filter, or AA filter) between the image sensor and the lens to limit the range of input frequencies coming through the lens onto the sensor, to a range that is equivalent or lower than the digital image sampling frequency, thus reducing aliasing to an acceptable limit. Anti-aliasing can in turn produce blurring in the reproduced image, something that camera manufacturers try to balance or control in the design of their filters.

If you like this article, stay tuned to the next post in the series where we will be talking about Image Noise…