INTERFACE PZImage;

(* Tools for image processing *)

IMPORT PZLR3Chain;

TYPE
  GrayLevel = BITS 8 FOR [0..255];
  T = ARRAY OF ARRAY OF GrayLevel;  

PROCEDURE ReadPGM(fName: TEXT): REF T;
  (* 
    Reads a PGM file "fName" and returns the image *)

PROCEDURE WritePGM(fName :TEXT; READONLY M: T);
  (*
    Writes the image "M" as file "fName", in PGM format. *)

TYPE
  Side = {Inner, Interpolated, Outer};

PROCEDURE ExtractBoundary(
    READONLY M: T;
    black: CARDINAL;
    grey: CARDINAL;
    side: Side := Side.Interpolated; 
  ): REF PZLR3Chain.T; 
  (*
    Given a PGM image "M" of a single 4-connected puzzle piece,
    returns the piece's outline as a 2D closed curve. 
    
    Assumes that "M[y,x] <= black" if and only if pixel "(x,y)" is outside the piece.
    The result traces the level curve "M[x,y] = grey" with subpixel accurancy, except
    that it will not deviate more than two pixels from the "M[x,y] = black" curve.
    Also assumes that the piece does not touch the boundary of "M".
    
    The "side" parameter specifies which contour to return. The 
    "Inner" and "outer" contours have integer vertices. *)

END PZImage.