[GWM] The Atlantis Cafe was a greasy spoon Cafe in Bristol. Professor Richard Gregory published a paper based on the illusion created by the tiles on the front of the cafe [http://www.richardgregory.org/papers/cafe_wall/cafe-wall.doc].

[http://mini.net/files/atlantis.gif]

See also [Bulging Line Illusion] [Arrow Illusion].

This application allows you to control some of the parameters of the tiling; since the Cafe has changed hands and name [[I thought the tiles had been removed but: ( [PT] 28-Oct-2004 - Just to point out that the tiles still exist on the wall of this building. It's no longer a cafe though. Well - unless they went in the last few weeks that is. [PT] 01-Nov-2004: I drove past here last night - the tiles are definitely still in place. Maybe next time I'll take a camera :) )]]. 

The relative luminance of the mortar should be midway between the brightness of the light & dark tiles for the illusion to occur most strongly; this toyol (half way between a toy and a tool) could allow you to experiment with the degree of illusion.

{Copy this code into Wish console and run it. 2 scale controls control the offset of the tiles and the lightness of the grouting.}

[GWM] 26.10.04 Minor change to remove some unused variables.

 # Atlantis Cafe effect - first reported by Prof Richard Gregory
 # see http://www.richardgregory.org/papers/cafe_wall/cafe-wall.doc
 # then at Bristol University, UK.  The Atlantis Cafe was at the bottom of St Michael's Hill
 # but has now changed hands and lost the tiles. So, here they are again!
 # The luminance of the mortar between tiles affects the degree of illusion, as does
 # placing the tiles at the bottom of the screen and looking at the top of screen (peripheral vision).
 # The degree of overlap of the tiles also affects the degree of illusion.

 proc makerow {rowdata length height repeat colmort coldark collight} { ;# make an image data set
 # of bricks/tiles length by height, repeating every repeat along length.
 # colmort etc are 3 colours for mortar, dark tile light tile
	set tilelen [expr $repeat/2]
	for {set j 0} {$j<$height} { incr j} { ;# rows of picture
		set pix [list]
		for {set i 0} {$i<$length} { incr i } {
			if {$j<2} { lappend pix $colmort
			} else {
				if {[expr $i%$repeat<$tilelen-1]} { lappend pix $coldark
				} elseif {[expr $i%$repeat>($tilelen+1) && $i%$repeat<$repeat-2]} {
					lappend pix $collight
				} else { lappend pix $colmort
				}
			}
		}
		$rowdata put [list $pix] -to 0  $j
	}	
 }
 proc mortar {ht size lumfrac} { ;# set mortar colour
	set colmo [format %02x [expr int($lumfrac*2.55)] ]
	makerow tilerow $size $ht 60 \#$colmo$colmo$colmo \#000000 \#ffffff
 }
 proc movetiles {ht frac} { ;# slide rows of tiles across the canvas
     	catch [destroy .frm] {} ;# delete it. Same as clear.
	set frm [canvas .frm -width [expr 8*$ht] -height [expr 8*$ht] ]
 
	for {set j 0} {$j<8} { incr j} { ;# rows of tiles
		if {[expr $j%2]} {set dx $frac
		} else { set dx 0}
		label $frm.row$j -image tilerow -bd 0 -relief raised
		place  $frm.row$j -x $dx -y [expr $j*$ht]
	}
	pack .frm
 }
 proc setup {size} {
     	catch [destroy .dt] {} ;# delete it. Same as clear.
     	catch [destroy .dlum] {} ;# delete it. Same as clear.
 
	wm title . "Atlantis Cafe"
	set ht [expr $size/16]
	scale .dt  -variable frac -orient horizontal -label "fraction" -command "movetiles $ht"
	scale .dlum -variable lumf -orient horizontal -label "mortar luminance" -command "mortar $ht $size"

	image create photo tilerow -height $ht -width $size	
	pack .dt .dlum -side top
 }
 global frac lumf
 set frac 10
 set lumf 50
 setup 450

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[Category Graphics] | [Category Application]