,PIET
,PIET IS A PROGRAMMING LANGUAGE WHOSE
SOURCE CODE IS NOT TEXT BUT A PICTURE 
SPECIFICALLY1 AN ABSTRACT IMAGE OF
COLORED BLOCKS THAT LOOKS1 BY DESIGN1
LIKE A ,PIET ,MONDRIAN CANVAS4 ,DAVID
,MORGAN-,MAR CREATED IT AND NAMED IT
AFTER THE ,DUTCH GEOMETRIC
ABSTRACTIONIST1 PRODUCING THE ONLY
LANGUAGE IN THIS CATALOGUE WHOSE
8,HELLO1 ,WORLD60 YOU COULD PLAUSIBLY
HANG IN A GALLERY4.<CITE3 .<,PIET1
ESOTERIC-LANGUAGES WIKI  ,MORGAN-,MAR1
THE ,MONDRIAN NAMESAKE1 AND THE
COLOR-BLOCK EXECUTION MODEL.>.>
,A ,PIET PROGRAM IS AN IMAGE MADE OF
CODELS  THE LANGUAGE'S EQUIVALENT OF
PIXELS1 WHICH MAY BE SCALED TO ARBITRARY
SIZE FOR DISPLAY4 ,CONTIGUOUS
SAME-COLORED CODELS FORM COLOR BLOCKS1
AND EXECUTION PROCEEDS WHEN A DIRECTION
POINTER "<,,DP"> WALKS FROM BLOCK TO
BLOCK ACROSS THE CANVAS4 ,THE COMMAND
EXECUTED AT EACH STEP IS DETERMINED NOT
BY THE COLOR ENTERED BUT BY THE CHANGE
IN HUE AND LIGHTNESS BETWEEN THE BLOCK
LEFT AND THE BLOCK ENTERED  THE PROGRAM
IS ENCODED IN THE TRANSITIONS1 WHICH IS
WHY A ,PIET IMAGE HAS NO DISCERNIBLE
SYNTAX AT ALL3 ONLY
RELATIONSHIPS4.<CITE3 .<,PIET EXECUTION
MODEL  CODELS1 COLOR BLOCKS1 THE
DIRECTION POINTER1 AND THE CODEL
CHOOSER.>.>
,A PALETTE WITH A PROGRAM INSIDE
,PIET DEFINES EXACTLY #BJ COLORS3 SIX
HUES "<RED1 YELLOW1 GREEN1 CYAN1 BLUE1
MAGENTA"> AT THREE LIGHTNESS LEVELS  #AH
CYCLICALLY-ORDERED COLORS  PLUS WHITE1
WHICH THE POINTER PASSES STRAIGHT
THROUGH LIKE BLANK SPACE1 AND BLACK1
WHICH IS IMPASSABLE4 ,COMMANDS COME FROM
THE TWO CYCLES3
- ,HUE CHANGE "<#J#E STEPS FORWARD
AROUND THE SIX-HUE RING"> SELECTS THE
ROW OF THE COMMAND TABLE4 - ,LIGHTNESS
CHANGE "<NONE1 ONE1 OR TWO STEPS ALONG
THE LIGHTDARK CYCLE"> SELECTS THE
COLUMN4 - ,CROSSING FROM MAGENTA TO RED
INCREASES HUE BY ONE STEP  THE CYCLE
WRAPS  WHILE GOING THE OTHER WAY IS A
FIVE-STEP MOVE4
,THE RESULT IS A #AG-COMMAND MACHINE3
THE NO-HUE-CHANGE ROW PUSHES AND POPS2
THE ONE-STEP ROW ADDS1 SUBTRACTS1
MULTIPLIES2 TWO STEPS DIVIDE1 MODULO1
,,NOT2 THREE STEPS COMPARE1 STEER THE
POINTER1 SWITCH THE CODEL CHOOSER2 FOUR
STEPS DUPLICATE1 ROLL1 INPUT NUMBERS2
FIVE STEPS DO CHARACTER ,I_/,O4 ,PUSH
HAS A BEAUTIFUL QUIRK  IT PUSHES THE
SIZE OF THE COLOR BLOCK JUST LEFT1 SO
THE PHYSICAL AREA OF A REGION OF PAINT
IS A NUMERIC LITERAL4.<CITE3 .<,PIET
COMMAND TABLE  THE FULL #AG-COMMAND
HUE_/LIGHTNESS MATRIX AND THE BLOCK-SIZE
PUSH.>.>
,FLOW CONTROL FOR PAINTERS
,THE DIRECTION POINTER STARTS AT THE
UPPER-LEFT CODEL MOVING RIGHT1
ACCOMPANIED BY A CODEL CHOOSER "<,,CC">
THAT BREAKS TIES WHEN SEVERAL BLOCKS
QUALIFY AS 8NEXT40 ,WHEN THE ,,DP HITS A
BLACK WALL IT CANNOT CROSS1 THE CHOOSER
FLIPS1 THEN THE POINTER ROTATES
CLOCKWISE  AND IF EVERY ORIENTATION IS
WALLED IN1 THE PROGRAM ENDS4 ,THE ONLY
WAY TO TERMINATE A ,PIET PROGRAM IS TO
TRAP THE POINTER IN BLACK PAINT4 ,LOOPS
ARE PATHS AROUND THE CANVAS2 BRANCHES
ARE CORRIDORS2 THE POINTER AND SWITCH
COMMANDS LET DATA STEER EXECUTION
MID-FLIGHT4 ,THE WHOLE THING IS A MAZE
THAT COMPUTES4.<CITE3 .<,PIET EXECUTION
MODEL  ,,DP_/,,CC STEERING1 BLACK-WALL
BLOCKING1 AND THE ONLY TERMINATION
PATH.>.>
,AND IT IS NOT A TOY IN THE FORMAL
SENSE3 ,PIET IS ,TURING-COMPLETE WHEN
THE STACK IS UNBOUNDED  A RESULT
DEMONSTRATED CONCRETELY BY THE EXISTENCE
OF A ,BRAINFUCK INTERPRETER WRITTEN IN
,PIET1 AN ABSTRACT PAINTING THAT
EXECUTES ANOTHER LANGUAGE'S CODE4
,BRAINFUCK1 THE LANGUAGE DESIGNED TO BE
AS SMALL AS POSSIBLE1 WAS IMPLEMENTED IN
THE LANGUAGE DESIGNED TO BE AS PRETTY AS
POSSIBLE4.<CITE3 .<,PIET COMPUTATIONAL
CLASS  THE UNBOUNDED-STACK CAVEAT AND
THE ,PIET-HOSTED ,BRAINFUCK
INTERPRETER.>.>
,BECAUSE THE PROGRAM IS THE TRANSITIONS1
"9"9ANY ,PIET PROGRAM CAN BE
RECOLORED"9"9  SHIFT THE ENTIRE PALETTE
ONE HUE STEP AND IT STILL RUNS
IDENTICALLY4 ,PIET IS THE ONLY LANGUAGE
WITH TRUE 8SKINS40
,WHERE IT SITS IN THE WING
,THE ESOLANG CANON BROKE ONE ASSUMPTION
EACH3 ,,INTERCAL BROKE SERIOUSNESS1
,BRAINFUCK SIZE1 ,MALBOLGE THE
PROGRAMMER1 ,WHITESPACE VISIBILITY1
,BEFUNGE DIMENSIONALITY1 ,UNLAMBDA
VARIABLES1 ,,LOLCODE VOCABULARY1 AND
,SHAKESPEARE THE IDENTITY OF CODE AS
CODE4 ,PIET BROKE THE LAST ASSUMPTION
STANDING  THAT A PROGRAM IS TEXT4 ,ITS
ANSWER IS A PROOF BY PAINTING3
EVERYTHING A ,TURING MACHINE NEEDS CAN
BE EXPRESSED IN AREAS OF FLAT COLOR1 AND
THE RESULT IS1 GENUINELY1 THE ONLY
SOURCE CODE IN THIS WIKI THAT IMPROVES A
ROOM4
,CHECK YOUR UNDERSTANDING
- .< .> ,WHAT DETERMINES WHICH COMMAND A
,PIET COLOR BLOCK EXECUTES  ITS COLOR1
OR SOMETHING ELSE8 - .< .> ,WHAT DO
BLACK AND WHITE BLOCKS DO TO THE
DIRECTION POINTER8 - .< .> ,WHAT DOES
PUSH ACTUALLY PUSH ONTO THE STACK8 - .<
.> ,HOW DOES A ,PIET PROGRAM TERMINATE8
#A  NOT ITS COLOR  THE
"9"9TRANSITION"9"9 IN HUE AND LIGHTNESS
BETWEEN THE PREVIOUS BLOCK AND THE
ENTERED BLOCK SELECTS THE COMMAND4 #B 
"9"9WHITE"9"9 IS PASS-THROUGH "<THE
POINTER SLIDES ACROSS1 EXECUTING
NOTHING">2 "9"9BLACK"9"9 IS IMPASSABLE
AND FORCES POINTER_/CHOOSER RETRIES4 #C 
THE "9"9NUMBER OF CODELS IN THE BLOCK
JUST LEFT"9"9  THE LITERAL IS THE
PAINT'S PHYSICAL AREA4 #D  ONLY WHEN THE
POINTER IS COMPLETELY TRAPPED BY BLACK
IN EVERY DIRECTION AND CHOOSER STATE 
THERE IS NO HALT INSTRUCTION4