# 16.8 Built-in Module `gl`

This module provides access to the Silicon Graphics Graphics Library. It is available only on Silicon Graphics machines.

Warning: Some illegal calls to the GL library cause the Python interpreter to dump core. In particular, the use of most GL calls is unsafe before the first window is opened.

The module is too large to document here in its entirety, but the following should help you to get started. The parameter conventions for the C functions are translated to Python as follows:

- All (short, long, unsigned) int values are represented by Python integers.
- All float and double values are represented by Python floating point numbers. In most cases, Python integers are also allowed.
- All arrays are represented by one-dimensional Python lists. In most cases, tuples are also allowed.
- All string and character arguments are represented by Python strings, for instance, `winopen('Hi There!')` and `rotate(900, 'z')`.
- All (short, long, unsigned) integer arguments or return values that are only used to specify the length of an array argument are omitted. For example, the C call

      lmdef(deftype, index, np, props)

  is translated to Python as

      lmdef(deftype, index, props)

- Output arguments are omitted from the argument list; they are transmitted as function return values instead. If more than one value must be returned, the return value is a tuple. If the C function has both a regular return value (that is not omitted because of the previous rule) and an output argument, the return value comes first in the tuple. Examples: the C call

      getmcolor(i, &red, &green, &blue)

  is translated to Python as

      red, green, blue = getmcolor(i)

The following functions are non-standard or have special argument conventions:

`varray` (argument)
  Equivalent to but faster than a number of `v3d()` calls. The argument is a list (or tuple) of points. Each point must be a tuple of coordinates `(x, y, z)` or `(x, y)`. The points may be 2- or 3-dimensional but must all have the same dimension. Float and int values may be mixed however. The points are always converted to 3D double precision points by assuming `z = 0.0` if necessary (as indicated in the man page), and for each point `v3d()` is called.

`nvarray` ()
  Equivalent to but faster than a number of `n3f` and `v3f` calls. The argument is an array (list or tuple) of pairs of normals and points. Each pair is a tuple of a point and a normal for that point. Each point or normal must be a tuple of coordinates `(x, y, z)`. Three coordinates must be given. Float and int values may be mixed. For each pair, `n3f()` is called for the normal, and then `v3f()` is called for the point.

`vnarray` ()
  Similar to `nvarray()` but the pairs have the point first and the normal second.

`nurbssurface` (s_k, t_k, ctl, s_ord, t_ord, type)
  Defines a nurbs surface. The dimensions of `ctl[][]` are computed as follows: `[len(s_k) - s_ord]`, `[len(t_k) - t_ord]`.

`nurbscurve` (knots, ctlpoints, order, type)
  Defines a nurbs curve. The length of ctlpoints is `len(knots) - order`.

`pwlcurve` (points, type)
  Defines a piecewise-linear curve. points is a list of points. type must be `N_ST`.

`pick` (n)
  ,selectn The only argument to these functions specifies the desired size of the pick or select buffer.

`endpick` ()
  ,endselect These functions have no arguments. They return a list of integers representing the used part of the pick/select buffer. No method is provided to detect buffer overrun.

Here is a tiny but complete example GL program in Python:

    import gl, GL, time

    def main():
        gl.foreground()
        gl.prefposition(500, 900, 500, 900)
        w = gl.winopen('CrissCross')
        gl.ortho2(0.0, 400.0, 0.0, 400.0)
        gl.color(GL.WHITE)
        gl.clear()
        gl.color(GL.RED)
        gl.bgnline()
        gl.v2f(0.0, 0.0)
        gl.v2f(400.0, 400.0)
        gl.endline()
        gl.bgnline()
        gl.v2f(400.0, 0.0)
        gl.v2f(0.0, 400.0)
        gl.endline()
        time.sleep(5)

    main()