The following appendix was taken from Extentions to the graphical interpretation of L-Systems based on turtle geometry by R. Mech and P. Prusinkiewicz.

View file commands

The graphical extensions described in the text include not only new modules with a specific interpretation but also several view file commands setting initial parameters of the graphical interpretations:

contour: allows the user to specify the cross-section of a generalized cylinder as a open or closed Bezier spline (see below for more details);

contour sides: defines the number of polygons drawn around a contour;

twist of cylinders: switches on or off the twist minimizing method applied during the visualization of generalized cylinders;

texture: specifies the texture and the way it is mapped on a surface.

Specifications of contours

Contours are specified in the view file by the command:

contour: id countour_file

The number of polygons around a contour can also be set in the view filw by command:

contour sides: n

The contour file has the following syntax: the first line contains a number of control points, the dimension of the contour (2 or 3), and a identifier of an open or closed contour (the word open or closed). Subsequent lines contain two or three coordinates of control points, one point per line.

A two-dimensional contour is specified by the file:

12 2 closed
0.166482 -1.123751
0.416204 -1.040511
0.582686 -0.332963
1.082131 -0.041620
1.082131 -0.499445
0.499445 0.541065
0.332963 0.915649
-0.374584 1.040511
-0.707547 0.624306
-1.123751 0.166482
-0.874029 -0.749168
-0.416204 -0.665927

A three-dimensional contour is specified by the file:

12 3 closed 
0.166482 -1.123751 1.0
0.416204 -1.040511 0.0
0.582686 -0.332963 0.0
1.082131 -0.041620 0.0
1.082131 -0.499445 0.0
0.499445 0.541065 0.0
0.332963 0.915649 0.0
-0.374584 1.040511 0.0
-0.707547 0.624306 0.0
-1.123751 0.166482 0.0 
-0.874029 -0.749168 0.0
-0.416204 -0.665927 0.0

An open contour is defined by the file:

15 2 open
-0.914286 -0.400000
-0.914286 -0.400000
-0.914286 -0.400000
-0.871429 -0.342857
-0.742857 -0.171429
-0.457143 -0.085714
-0.285714 0.142857
0.000000 0.271429
0.228571 0.114286
0.514286 0.142857
0.657143 -0.028571
0.892857 -0.114286
0.971429 -0.142857
0.971429 -0.142857
0.971429 -0.142857

Note that the first and the last control point is repeated three times to make sure the contour finishes in the given points.

It is possible to create a two-dimensional contour in a drawing program xfig and convert it to the contour text file using a conversion utility fig2con. It is necessary to use either open or closed Bezier spline for the contour coordinates. The created xfig file can be converted using the command:

fig2con contour.xfig contour.spec

Definition of textures

Textures are specified in the view file using a command texture:

Texture: F: image_name H: mag_filter L: min_filter E: env_mode S:

• the parameter image_name specifies the texture image. Currently, it is possible to specify iris rgb, Utah raster toolkit rle, and targa tga images (distinguished by the extension). The image resolution can be arbitrary.

• commands H, L, S, and E are optional.

• command H: mag_filter is used in case texture pixels viewed on the screen are bigger than one window pixel. The flag mag_filter is equal to:
  linear (or l only) - the texture image is smoothened when mapped onto the surface;

  near (or n only) - the texture image is not smoothened, thus texture pixels can appear as big as squares.

  The default is near.

• command L: min_filter is used in case texture pixels viewed on the screen are smaller than one window pixel. The flag min_filter is equal to:
  linear (or l only) - more texture pixels are used to compute the color of a given pixel on the screen;

  near (or n only) - just one texture pixel is used to compute the color of a given pixel on the screen. This mode is faster but can produce some aliasing effects.

  The default is near.

• command E: env_mode controls the way the texture is combined with the surface color. The flag env_modes is equal to:
  modulate (or m only) - the diffuse color of the surface material is multiplied by the color of the texture pixel;

  decal (or d only) - the color of the texture pixel is used as the color of the surface;

  The default is modulate.

• the command S: is recognized only if the texture is used on bicubic surfaces predefined in the view file. If the command is present in the texture specification, the texture image is mapped onto the whole surface. Otherwise, the texture image is mapped onto each surface patch separately.

  As a default, the texture is mapped onto each patch. In this case, texture coordinates are derived from s and t coordinates of the parametric equation of the Bezier patch representing the surface (both s and t goes from 0 to 1). In the case of mapping onto the whole surface, the surface boundaries along x and y axes are found and the texture is mapped into z-plane so that it fits the surface bounding box in x and y coordinates.

Definition of tropisms and twists

Tropisms and twists are specified in the view file using commands tropism or torque:

tropism:T: vector A: angle E: initial_elasticity S: elasticity_increment

torque:T: vector E: initial_elasticity S: elasticity_increment

• parameter vector specifies x, y, z coordinates of the tropism vector. This parameter has to be present.

• parameter angle specifies the diatropism angle. The default value is 0.

• parameter initial elasticity defines the initial value of the elasticity parameter (if not present, the initial value is 0).

• parameter elasticity increment defines the value of elasticity increment used by modules @Ti and @Td.

					Preface Parameters Appendix I Appendix II References
Index	Circles and Spheres	Generalized Cylinders	Surfaces	Textures