%D \module
%D   [       file=mp-tres.mpiv,
%D        version=2017.11.08,
%D          title=\CONTEXT\ \METAPOST\ graphics,
%D       subtitle=Pseudo 3D,
%D         author=Alan Braslau,
%D           date=\currentdate,
%D      copyright={PRAGMA ADE \& \CONTEXT\ Development Team}]
%C
%C This module is part of the \CONTEXT\ macro||package and is
%C therefore copyrighted by \PRAGMA. See licen-en.pdf for
%C details.

%D This module provides simple 3D->2D projections. The code is an adaption of code
%D by Urs Oswald, Dr.sc.math.\ ETH as presented in:
%D
%D \starttyping
%D http://www.ursoswald.ch/metapost/tutorial.html.
%D \stoptyping
%D
%D We need a bit more so it got extended.

if known context_three : endinput ; fi ;

boolean context_three ; context_three := true ;

pair mfun_three_xy ; % this avoids the costly save and allocate
pair mfun_three_yz ;
pair mfun_three_zx ;

transform Txy ;

def setTxy(expr ori, ang) =
    begingroup
        save P, t ;
        pair P[] ;
        transform t ;

        P0 = ori ;                                  % origin in MetaPost coordinates (bp)
        P1 = (left rotated ang) scaled (cosd ang) ; % x axis (in mathematical coordinates)

        % A sort of "cavalier projection".

        % t: maps mathematical 2D coordinates onto MetaPost coordinates.

        t := identity shifted P0 ; % Note: no shift when P0 = origin!

        % Txy: maps the x-y plane of 3D space onto MetaPost coordinates,
        % z is the vertical (MetaPost y).

        % Txy:=identity
        %   reflectedabout((0,0), (1,1))
        %     yscaled ypart P1
        %       slanted (xpart P1/ypart P1)
        %         transformed t ;

        % Alternatively, defined via Pxy:

        % Pxy is the projection of the 3D plane z=0 onto the mathematical 2D coordinates.
        % Pxy is determined by  3  e q u a t i o n s  describing
        % how (1,0), (0,1), and (0,0) are mapped

        save      Pxy ;
        transform Pxy ;
           P1 = (1,0) transformed Pxy ;   % Pxy: (1,0) --> P1
        (1,0) = (0,1) transformed Pxy ;   %      (0,1) --> (1,0)
        (0,0) = (0,0) transformed Pxy ;   %      (0,0) --> (0,0)

        % mathematical 2D coordinates --> MetaPost coordinates

        Txy := Pxy transformed t ;
    endgroup
enddef ;

setTxy(origin,60) ;

%D We already define triplet (as rgbcolor synonym) in in \type {mp-tool.mpiv}:

let Xpart = redpart ;
let Ypart = greenpart ;
let Zpart = bluepart ;

primarydef p Xscaled q =
  (q*Xpart p, Ypart p, Zpart p)
enddef ;

primarydef p Yscaled q =
  (Xpart p, q*Ypart p, Zpart p)
enddef ;

primarydef p Zscaled q =
  (Xpart p, Ypart p, q*Zpart p)
enddef ;

primarydef p XYZscaled q =
  (q*Xpart p,q*Ypart p,q*Zpart p)
enddef ;

vardef projection primary t =
    (if triplet t :
        (Xpart t, Ypart t) transformed Txy shifted (0,Zpart t)
     elseif pair t :
        t transformed Txy
     else :
        origin transformed Txy
     fi)
enddef ;

vardef Abs primary p =
    if triplet p :
        sqrt((Xpart p)**2+(Ypart p)**2+(Zpart p)**2)
    else :
        length p
    fi
enddef ;

vardef Unitvector primary z =
    z/Abs z
enddef ;

primarydef p dotproduct q =
    ((Xpart p)*(Xpart q) + (Ypart p)*(Ypart q) + (Zpart p)*(Zpart q))
enddef ;

primarydef p crossproduct q =
    (
        (Ypart p)*(Zpart q) - (Zpart p)*(Ypart q),
        (Zpart p)*(Xpart q) - (Xpart p)*(Zpart q),
        (Xpart p)*(Ypart q) - (Ypart p)*(Xpart q)
    )
enddef ;

primarydef p rotatedaboutX q =
    hide (
        mfun_three_yz := (Ypart p, Zpart p) ;
        mfun_three_yz := mfun_three_yz rotated q ;
    )
    (Xpart p, xpart mfun_three_yz, ypart mfun_three_yz)
enddef ;

primarydef p rotatedaboutY q =
    hide(
        mfun_three_zx := (Zpart p, Xpart p) ;
        mfun_three_zx := mfun_three_zx rotated q ;
    )
    (ypart mfun_three_zx, Ypart p, xpart mfun_three_zx)
enddef ;

primarydef p rotatedaboutZ q =
    hide (
        mfun_three_xy := (Xpart p, Ypart p) ;
        mfun_three_xy := mfun_three_xy rotated q ;
    )
    (xpart mfun_three_xy, ypart mfun_three_xy, Zpart p)
enddef ;

%D We can use a rotation about an arbitrary direction t ... (easy)

% primarydef p rotatedabout(expr t, a) =
% enddef ;

vardef draw_vector@# (expr v, s) text t =
    if triplet v :
        drawarrow projection(Origin) -- projection(v) t ;
        if string s :
            label@#(s, projection(v)) t ;
        fi
    fi
enddef ;

%D Transform a (2D) path to a 3D plane

def XYpath primary p =
    (for i=0 upto (length p) if cycle p: -1 fi :
        if i>0 : .. fi
        projection (xpart point i of p,
                    ypart point i of p,
                    0)
        ..controls
        projection (xpart postcontrol i of p,
                    ypart postcontrol i of p,
                    0)
        and
        projection (xpart precontrol (i+1) of p,
                    ypart precontrol (i+1) of p,
                    0)
        endfor if cycle p: ..cycle fi)
enddef ;

def XZpath primary p =
    (for i=0 upto (length p) if cycle p: -1 fi :
        if i>0 : .. fi
        projection (xpart point i of p,
                    0,
                    ypart point i of p)
        ..controls
        projection (xpart postcontrol i of p,
                    0,
                    ypart postcontrol i of p)
        and
        projection (xpart precontrol (i+1) of p,
                    0,
                    ypart precontrol (i+1) of p)
        endfor if cycle p: ..cycle fi)
enddef ;

def YZpath primary p =
    (for i=0 upto (length p) if cycle p: -1 fi :
        if i>0 : .. fi
        projection (0,
                    xpart point i of p,
                    ypart point i of p)
        ..controls
        projection (0,
                    xpart postcontrol i of p,
                    ypart postcontrol i of p)
        and
        projection (0,
                    xpart precontrol (i+1) of p,
                    ypart precontrol (i+1) of p)
        endfor if cycle p: ..cycle fi)
enddef ;

%D Some constants...

triplet Origin, Xunitvector, Yunitvector, Zunitvector ;

Origin      := (0,0,0) ;
Xunitvector := (1,0,0) ;
Yunitvector := (0,1,0) ;
Zunitvector := (0,0,1) ;

%D We could do but don't:

% let normalxpart = xpart ;
% let normalypart = ypart ;

% vardef xpart expr p = if triplet p : redpart   else : normalxpart fi p enddef ;
% vardef ypart expr p = if triplet p : greenpart else : normalypart fi p enddef ;
% vardef zpart expr p =                bluepart                        p enddef ;

