xkcd’s comic today, titled “Color Pattern,” is about the famous moiré pattern in photography, the interference patterns that appear when certain patterns are overlaid on each other.
Here’s the short song in the strip (including what appears when you hover your mouse over it on the original page):
When a grid’s misaligned
with another behind
That’s a moiré…
When the spacing is tight
And the difference is slight
That’s a moiré
You may recognize that as a parody of the famous song “That’s Amore,” sung by Dean Martin:
“Photographs of a TV screen taken with a digital camera often exhibit moiré patterns,” writes explain xkcd. “Since both the TV screen and the digital camera use a scanning technique to produce or to capture pictures with horizontal scan lines, the conflicting sets of lines cause the moiré patterns. To avoid the effect, the digital camera can be aimed at an angle of 30 degrees to the TV screen.”
Moiré patterns are a type of aliasing, and traditionally anti-aliasing filters in cameras help combat moiré. In recent times, however, more and more digital cameras are omitting the filter for sharper images at the expense of more moiré.