STL files and Fortran

For me, Stl files came into existence via solidworks. For passing solidworks files from my lab to the machining department, we had to export the solidworks model to stl file formats.

That was it. After that i had no contact with them until this year when I again got some stl models. I was using meshlab to read and view them. Then one boring day decided to see what was inside these files.

That enquiry resulted into a small project and the following program is the result of that project. Binary stl file reader in Fortran. There are many programs available to read stl files, some in matlab, some in c but there wasn’t any available in fortran.

I found one fortran stl file reader but that only worked for ASCII stl files. Thus began this project to read the binary stl files via fortran. After multiple visits to wikipedia, and endless hours of debugging, I have the complete fortran program to read binary stl file. So here’s the result. Fortran STL file reader

program stlread

! Program to Read Binary STL file.
! programmed by Sukhbinder Singh
! stores normals and the triangles in the normals and triangles arrays.
!
! 23rd May 2011

real*4 n(3),x1(3),x2(3),x3(3)
 integer*2 padding
 integer*4 ntri,iunit,irc
 character(len=80) title, filename
 double precision, allocatable :: normals(:,:),triangles(:,:)

 filename="C:\Documents and Settings\sukhbinder\Desktop\projects\stlbinaryread\sample.stl"

iunit=13

 open(unit=iunit,file=filename,status='old',form='unformatted', &
 & access='direct',recl=80)

read(iunit,rec=1)title
 close(iunit,status='keep')

open(unit=iunit,file=filename,status='old',form='unformatted', &
 & access='direct',recl=4)

 read(iunit,rec=21)ntri
 close(iunit,status='keep')

 open(unit=iunit,file=filename,status='old',form='unformatted', &
 & access='direct',recl=2)
 irc=(80+4)/2+1

 allocate(normals(3,ntri))
 allocate(triangles(3,ntri*3))

 k=1
 do i=1,ntri
 call readbin(iunit,n(1),irc)
 call readbin(iunit,n(2),irc)
 call readbin(iunit,n(3),irc)

call readbin(iunit,x1(1),irc)
 call readbin(iunit,x1(2),irc)
 call readbin(iunit,x1(2),irc)

call readbin(iunit,x2(1),irc)
 call readbin(iunit,x2(2),irc)
 call readbin(iunit,x2(3),irc)

call readbin(iunit,x3(1),irc)
 call readbin(iunit,x3(2),irc)
 call readbin(iunit,x3(3),irc)

normals(1,i)=n(1)
 normals(2,i)=n(2)
 normals(3,i)=n(3)

 triangles(1,k)=x1(1)
 triangles(2,k)=x1(2)
 triangles(3,k)=x1(3)

triangles(1,k+1)=x2(1)
 triangles(2,k+1)=x2(2)
 triangles(3,k+1)=x2(3)
 triangles(1,k+2)=x3(1)
 triangles(2,k+2)=x3(2)
 triangles(3,k+2)=x3(3)

k=k+3
 read(iunit,rec=irc)padding
 irc=irc+1
 end do

write(*,*) trim(filename),' has this title ',trim(title),' and has',ntri, ' triangles'
 end program

subroutine readbin(iunit,a,irc)
 real*4 a,b
 integer*2 ib(2)
 equivalence(b,ib)

read(iunit,rec=irc)ib(1)
 irc=irc+1
 read(iunit,rec=irc)ib(2)
 irc=irc+1

 a=b

end