[MUG] Question about texturing using Maple

[MUG] Re: Question about texturing using Maple
Author: Maple User Group    Posted: Wed, 22 May 2002 16:37:40 -0400
>> From: Maple User Group "maple_gr" | >> From: "bguerrieri" | > plot3d({cos(x)*sin(y), 0},x=-Pi..Pi,y=-Pi..Pi,color=cos(x*y); | | which shows a section of the x-y plane as well as its "transform" | under f:=(x,y)->cos(x)*cos(y), both shaded according to the law g := | (x,y)->x*y; | | I seem to run into difficulty when I attempt to "design" my own | coloring function, admittedly an "ugly" one such as | g := proc(x,y) | local a; | if (x>=-evalf(Pi) and x<0) then a := 1: fi: | if (x>=0 and x<=evalf(Pi)) then a := 2: fi: | RETURN(a); | end: | | The call | plot3d(0,u=-Pi..Pi,v=-Pi..Pi,color=g(u,v)); generates the following: | Error, (in g) cannot evaluate boolean: ... -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Fri, 17 May 2002 17:24:27 -0300 (BRT) From: Humberto Jose Bortolossi "hjbortol" To: MUG "maple-list" Subject: Question about texturing using Maple Hi! Try the following: g := proc(x,y) local a; if not(type(x, numeric)) or not(type(y, numeric)) then RETURN('g'(x, y)); fi: if (x>=-evalf(Pi) and x<0) then a := 1: fi: if (x>=0 and x<=evalf(Pi)) then a := 2: fi: RETURN(a/2); end: You have to include the ``if not(type(x, numeric)) ...'' to avoid the evaluation of g(x, y) with the symbolic x since Maple tries to evaluate g(x, y) before x and y assume some numeric value. Best regards, Humberto. "hjbortol" -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Sat, 18 May 2002 23:22:37 -0700 (PDT) From: Robert Israel "israel" To: MUG "maple-list" Subject: Question about texturing using Maple Premature evaluation strikes again! The main problem is that your g(u,v) can't be evaluated when u and v are symbolic variables. Try this: g:= proc(x,y) proc(x,y) local a; if not type([x,y],list(numeric)) then 'procname(x,y)' elif (x >= -evalf(Pi) and x < 0) then 1/2 else 1 fi; end; Note that the colour function is interpreted as a "hue", which should be in the interval 0 to 1; both 0 and 1 are red. Robert Israel "israel" Department of Mathematics http://www.math.ubc.ca/~israel University of British Columbia Vancouver, BC, Canada V6T 1Z2 -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- From: Stanley J Houghton "S.J.Houghton" Date: Mon, 20 May 2002 15:19:08 +0100 To: "bguerrieri" "maple-list" Subject: Question about texturing using Maple It is another case of premature evaluation. I solved it by quoting the color specifier. However, I had to use two pairs of quotes as in color=''g(u,v)'' An alternative is to ensure that your proc is "self-quoting" when its paramteres are symbolic (i.e. not constant values) as follows: > g := proc(x,y) > local a; > if not(type(x,constant)and type(y,constant)) then > RETURN('procname'(u,v)) > fi; > if (x>=-evalf(Pi) and x<0) then a := .5: fi: > if (x>=0 and x<=evalf(Pi)) then a := 1: fi: > RETURN(a); > end: Hope it helps Stan -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- Date: Tue, 21 May 2002 11:28:19 +0200 From: Preben Alsholm "ifakpa" To: "maple-list" "bguerrieri" Subject: Question about texturing using Maple If you make your procedure g return unevaluated when it receives a non-numeric input then everything is OK. E,g. like this: g := proc(x,y) > local a; > if not type(x,numeric) then return 'procname(x,y)' fi; > if (x>=-evalf(Pi) and x<0) then a := 1: fi: > if (x>=0 and x<=evalf(Pi)) then a := 1/2: fi: > a; > end: -- Preben Alsholm Institut for matematik (Department of Mathematics) DTU (Technical University of Denmark) -=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=- From: Alec Mihailovs "amihailo" To: "maple-list" Subject: Question about texturing using Maple Date: Tue, 21 May 2002 20:40:08 -0400 For this particular example, both a:=1 and a:=2 would give the same red color, because the HUE color depends only on the fractional value. Let's change it to something different, say > g:= (x,y)->Heaviside(x)/2; or > g := (x,y)->`if`(x>=0,.5,0); or > g := proc(x,y) if x<0 then 0 else .5 fi end: or the example of g in your letter. Then > plot3d((u,v)->0,-Pi..Pi,-Pi..Pi,color=g); will produce the plot. Best wishes, Alec Mihailovs http://webpages.shepherd.edu/amihailo/

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