Café wall illusion
Café wall illusion
Image source
Café wall illusion
The original Café in Bristol
Image source

Café wall illusion

The café wall illusion is an optical illusion, first described by Richard Gregory. When offset dark and light tiles are alternated, they can create the illusion of tapering horizontal lines. The effect depends on the presence of a visible line of gray mortar between the tiles. It is known as the "café wall illusion" since being observed on the outside wall of a café.

This illusion is created when offset rows of alternating dark and light tiles are surrounded by a visible line of mortar. Ideally, the mortar is a shade somewhere between the two tile colors. When the tiles are offset by half a tile width, the horizontal lines appear to slant diagonally, creating the appearance of wedges. The illusionary effect is affected by both the position of the tiles as well as the thickness and color of the grout in between them. If grout lines are removed, there is no longer any illusion of diagonal lines. (New World Encyclopedia)

Café wall illusion: The horizontal lines are parallel & straight, even if they seem otherwise.


 

The original Café in Bristol

The original Illusory Café in Bristol
According to Gregory, this unusual visual effect was observed by a member of his laboratory in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol.

A close-up view of the original Café

A close-up view of the original Illusory Café in Bristol
Professor Richard Gregory visited the Cafe Wall in February 2010.  Closer view.

Rotating Cafe wall illusion

Rotating Cafe wall illusion
A Simple Illusion that Completely Screws Up Your Sense of Space

Optical illusion building in Melbourne

Optical illusion building in Melbourne
Architecture inspired by the café wall illusion.



 

Proof that the horizontal lines are parallel and straight by turning all or alternating rows of dark "bricks' into light "bricks"

Turning all rows of dark "bricks' into light "bricks"


Café wall illusion         Café wall illusion: the proof

Turning alternating rows of dark "bricks' into light "bricks"


Café wall illusion         Café wall illusion: the proof


 

An animated "proof" that the horizontal lines are parallel and straight can be viewed at www.yoism.org

 

Additional images associated with similar optical illusion

Hering illusion

Optical illusion associated with Café wall illusion

The horizontal lines are parallel & straight.

Note: This image is similar to a type of Perspective illusion: Hering illusion

Elongated cube drawn with parallel sides

Leaning tower illusion: Elongated cube drawn with parallel side

The illusion found with side-by-side replicas of an image of a receding object applies also to a single object, as in this figure. The figure is drawn with parallel sides, yet the impression is of an object that becomes fatter with distance.

Note: This image is associated with a type of Leaning tower illusion