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Explain xkcd: It's 'cause you're dumb.
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Color Pattern
♫ When the spacing is tight / And the difference is slight / That's a moiré ♫
Title text: ♫ When the spacing is tight / And the difference is slight / That's a moiré ♫

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Does the TV screen explanation also cover computer screens, which would be more relevant here? Is the title text explanation correct?

The comic references moiré patterns in a parody of the song That's Amore made famous by Dean Martin in 1953. (See trivia for pronunciation).

Cueball complains that the photo he just took of his computer screen is covered in weird rainbow patterns (the color patterns from the title). In photography (or videography), a moiré pattern occurs when the image sensors are aligned in a pattern, while photographing something else that is also aligned in a pattern that's not 100% identically aligned.

In mathematics, physics, and art, a moiré pattern or moiré fringes are large scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar pattern. For the moiré interference pattern to appear, the two patterns must not be completely identical in that they must be displaced, rotated, etc., or have different but similar pitch. A moiré pattern is a kind of aliasing. Moiré patterns appear in many different situations. In printing, the printed pattern of dots can negatively interfere with the image. In television and digital photography, a pattern on an object being photographed can interfere with the shape of the light sensors to generate unwanted artifacts.

Photographs of a TV screen taken with a digital camera often exhibit moiré patterns. 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.

Megan replies with a song that explains that what Cueball sees is a moiré pattern. But she chooses to use the song That's Amore, where "Amore" means "love" in Italian. The pun is that "That's a Moiré" and "That's Amore" are phonetically quite similar. Her explanation that it happens when a grid is misaligned with another behind fits well with the more detailed explanation above.

The song has a second verse given in the title text, again with musical notes indicating that it should be song. More information on when moiré patterns occur is given here, indicating that the space between the grid lines should be small and the two grids should be almost identical, for the maximum moiré effect.

The songs

The entire version of Megan's (Randall's) song is:

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

The two first verse in the original song:

When a moon hits your eye
like a big pizza pie
That's amore
When the world seems to shine
like you've had too much wine
That's amore

A similar song based on the same pun was made by Craig Swanson in 1993 and can be found on his web comic Perspicuity in this comic: That's a Moiré. His song text was:

When new lines hit your eyes
From two screen when they ply
That's a Moire!

Transcript

[Cueball holds up his smartphone in front of his laptop which stands in front of him on a desk. Megan is sitting in an armchair reading, facing away from Cueball. She is singing her reply, as indicated with four double musical notes around her two lines of text.]
Cueball: I took a picture of my computer screen—why is the photo covered in these weird rainbow patterns?
Megan: When a grid's misaligned with another behind
Megan: That's a moiré...

Trivia

  • Pronunciation:
    • Moiré (/ˈmwɑːreɪ/ - /mwɑˈreɪ/ - French: [mwaˈʁe])
    • That's amore /ðæts aˈmɔːrɛ/.
    • That's a moiré /ðæts ə ˈmwɑːreɪ/.


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