# GfsSurface

### From Gerris

A GfsSurface is an oriented surface (in 3D) or an oriented curve (in 2D).

The surface can be defined implicitly using for example:

(x*x + y*y + z*z - 0.1*0.1)

which defines the surface as the set of points of coordinates `(x,y,z)`

such that `x*x + y*y + z*z - 0.1*0.1 = 0`

(i.e. a sphere of radius 0.1 centered on the origin).

The sign of the implicit function defines the surface orientation.

Surfaces can also be defined explicitly using GTS files. For example:

sphere.gts

When using GTS surfaces in two dimensions, the oriented curve is defined as the intersection of the GTS surface with the `z = 0`

plane.

The surface definition can be followed by an optional parameter block with the following syntax:

{ tx = 0.1 ty = -0.2 tz = 0.4 sx = 2. sy = 1.5 sz = -1 rx = 0.5 ry = -0.1 rz = -0.3 angle = -45 flip = 1 twod = 1 }

where `(tx,ty,tz)`

is a translation vector and `(sx,sy,sz)`

is a scaling vector. The vector `(rx,ry,rz)`

defines a rotation axis and `angle`

the associated rotation angle (in degrees).

The `flip`

parameter can be used to flip the surface orientation.

If set to one the `twod`

parameter "flattens" the surface on the `z = 0`

plane (this is used in 3D by the GfsRefineSurface object).

Several simple implicit surfaces are pre-defined:

`ellipse(x,y,a,b)`

- an ellipse centered on
`(x,y)`

and with semimajor axis`a`

and semiminor axis`b`

.

`sphere(x,y,z,r)`

- a sphere centered on
`(x,y,z)`

and of radius`r`

.