Boundary conditions in Halide

Posted on September 30, 2014 by Takayuki Muranushi

Zalman Stern has been working on boundary-condition, and the support has been in Halide main branch for a while.

Now is the time for testing! The following code

#include <Halide.h>
#include <stdio.h>

const int NX=4, NY=4;

int main(int argc, char **argv) {
  Halide::Var x, y;
  Halide::Func initial_condition;
  
  initial_condition(x,y) = x+4*y+97;
  

  Halide::Image<int32_t> input = initial_condition.realize(NX,NY);
  Halide::Func extended = Halide::BoundaryConditions::mirror_image(input);
  Halide::Func extended_shifted;
  extended_shifted(x,y) = extended(x-2*NX,y-2*NY);
  Halide::Image<int32_t> output = extended_shifted.realize(5*NX,5*NY);

  for (int j=0;j<5*NY;++j) {
    for (int i=0;i<5*NX;++i) {
      printf("%c ",char(output(i,j)));
    }
    printf("\n");
  }
}

produces the following output.

a b c d d c b a a b c d d c b a a b c d 
e f g h h g f e e f g h h g f e e f g h 
i j k l l k j i i j k l l k j i i j k l 
m n o p p o n m m n o p p o n m m n o p 
m n o p p o n m m n o p p o n m m n o p 
i j k l l k j i i j k l l k j i i j k l 
e f g h h g f e e f g h h g f e e f g h 
a b c d d c b a a b c d d c b a a b c d 
a b c d d c b a a b c d d c b a a b c d 
e f g h h g f e e f g h h g f e e f g h 
i j k l l k j i i j k l l k j i i j k l 
m n o p p o n m m n o p p o n m m n o p 
m n o p p o n m m n o p p o n m m n o p 
i j k l l k j i i j k l l k j i i j k l 
e f g h h g f e e f g h h g f e e f g h 
a b c d d c b a a b c d d c b a a b c d 
a b c d d c b a a b c d d c b a a b c d 
e f g h h g f e e f g h h g f e e f g h 
i j k l l k j i i j k l l k j i i j k l 
m n o p p o n m m n o p p o n m m n o p

According to the documentation on Halide’s boundary condition, Halide supports following boundary conditions at the moment of writing: constant_exterior, repeat_edge, repeat_image, mirror_image, mirror_interior. We can guess what they do from the names.