SCN ray-tracing format

1. Introduction

The SCN (SCeNe format) was invented to replace a very simple format called SFF for the Rtrace ray-tracer. It is used to describe environments (geometry, topology, attributes, etc.) that are processed by programs called ray-tracers, although it can be used by other programs.

2. Overview

The RTrace ray-tracer uses a very simplistic format (its only advantage is the simplicity!). It only reads numbers (and strings for file names), but its syntax is very rigid. To avoid this, I decided to create a much more elaborated, easy to write and flexible scene description format, which I called SCN.

To avoid modifying the RTrace input code, a translator from SCN to SFF was implemented as a separate program (Scn2Sff), meant to be used as a preprocessor for RTrace (in UNIX and DOS, these two programs can be used in a pipe like scn2sff | rtrace).

3. Syntax

The SCN format does not have sections like SFF, RTrace's input format, and it supports comments and even other preprocessor constructs (for example, cpp and m4 for UNIX and DOS). Another important difference is that it has default values for some scene parameters, which means that they do not have to be specified at all.

The basic components of SCN are:

comment - they start anywhere on the line with some special character like ; # or % and terminate in the end of line.

integer - any valid integer number, an integer expression inside parenthesis or the result of the int() function. Examples:

	0
	1
	-2
	(1 + 2)     ; 3
	int(10 / 3) ; 3

real - any valid integer or real number, a real expression inside parenthesis or the result of any real-valued function like sin(), cos(), etc. Examples:

	0.0
	1.5
	(1 + 2.1) ; 3.1
	log(2)    ; 0.6931

color - a triplet of RGB reals between 0 and 1. In certain cases, the RGB reals may be bigger than 1 or negative (color_extended). It is also possible to use names that represent the RGB values or the mono keyword followed by just one real. Examples:

	0 0 0    ; black
	1 1 1    ; white
	1 0 0    ; red
	white
	red
	mono 0.5 ; medium gray

point - a triplet of reals representing X Y and Z values or the result of any point-valued function like normvector(), addvector(), etc. Examples:

	0 0 0                     ; world origin
	10 5 1
	1 (2 + 1) (10 * sin(0.1)) ; 1 3 0.0175
	normvector(1.1 2.5 -2.3)  ; 0.3081 0.7001 -0.6441
	addvector(1 2 3, 4 5 6)   ; 5 7 9

vector - similar to a point, but cannot be equal to 0 0 0.

filename - a set of characters (letters, digits and others) with no blanks between. It is recommended to use only file names like those of DOS.

expression - anything inside parenthesis. An expression can be integer or real. The expression operators are + - * / ^ and | . Examples:

	(1 + 2) ; 3
	(1 - 2) ; -1
	(1 * 2) ; 2
	(1 / 2) ; 0.5
	(1 ^ 2) ; 1.0
	(5 | 3) ; 2

function - there are many functions available. The integer/real-valued with integer/real arguments are int(), sin(), cos(), tan(), asin(), acos(), atan(), sqrt(), rtod (), dtor (), exp(), log(), abs(), max() and min(). The dotvector() function has two vector arguments and returns a real number. There are also functions with point/vector arguments that return point/vector quantities like normvector(), addvector(), diffvector(), scalevector() and crossvector(). Examples:

	tan(dtor(45))             ; 1.0
	rtod(atan(1))             ; 45 degrees
	log(exp(2))               ; 2
	abs(-1.5)                 ; 1.5
	max(0, 1)                 ; 1
	dotvector(1 0 0, 0 1 0)   ; 0.0
	scalevector(1 0 0, 2)     ; 2 0 0
	scalevector(2 2 2, 1 2 3) ; 2 4 6
	crossvector(1 0 0, 0 1 0) ; 0 0 1

3.1. Viewing

To specify viewing parameters one can use the following instructions:

eye point - where the camera is (default is 5 0 0); from can also be used.

look point - where the target of the camera is (default 0 0 0); at can also be used.

up vector - the camera vertical direction (default 0 1 0).

fov real [real] - the horizontal/vertical fields of view (default 22.5 degrees); angle can also be used. If there is just one value, then both fields of view are equal.

Examples:

	eye 5 1 1
	look 0 1 0 ; default up is 0 1 0
	fov rtod(atan(320.0 / 200.0 * tan(dtor(15)))) 15 ; 23.21 15

3.2. Ambient and Background

To specify the ambient and background colors, there are the following instructions:

background color - the background color; default light_blue_sky (approx. 0.1 0.5 0.7).

ambient color - the ambient color (diffuse global lighting); default 0.1 0.1 0.1 (dark gray).

Examples:

	background white
	ambient mono 0.2

3.3. Lights

To specify lights attributes, there are several instructions:

light point point [color_extended] - this is a point light that irradiates in any direction. If any of the color components is negative, then there is no distance fall-off. Default color is white.

light directional vector [color] - this is a light like the Sun that irradiates along a direction, with no distance fall-off.

light spot point vector color_extended [angle [factor]] - this light illuminates inside a cone defined by the angle (default 45 degrees) with a transition that can be sharp (factor near 1) or smooth (factor >> 1). The default value for factor is 1.

light extended point color_extended radius samples - a spherical light sampled samples2 rays.

light linear point color_extended vector samples - a thin linear light source defined by the vector and centered at given point. It is sampled by samples rays.

light planar point color_extended vector vector samples samples - a kind of area light source defined by the two vectors (the given point is at the center of the area). It is sampled by the product of samples (8 by 8 is generally good).

Examples:

	light point 2 6 1 yellow
	light directional -1 -1 -1              ; white
	light spot 0 5 0 0 -1 0 red 30 2        ; smooth 30 degrees spot
	light extended -3 5 3 green 0.5 8       ; 0.5 radius light sphere
	light linear 1 8 3 white 0 0 2 32       ; white linear light
	light planar 9 1 8 blue 1 0 0 0 0 1 8 8 ; blue square light

Examples of the six light sources. The basic scene is:

	eye 7 5 0
	at  0 0
	fov 30
	background white
	sphere surface matte mono 0.8 data 0 3 0 1
	cylinder surface matte mono 0.7 data 2 0 2 2 2 2 0.2
	box surface matte white data 0 0 0 5 0.01 5
	light ...

3.4. Surfaces

A surface is a non-geometric attribute that each object must have so that it can be rendered. When a surface is defined in a certain scope, it is considered the current surface and many objects may share it without having to name it explicitly. There is even a default surface (surface white mono 0.9 mono 0.1 3 0). A surface may have its special attributes, which must appear immediately after the surface keyword, like:

emitter - the objects using this surface behave like emitters, ie, they have a constant shading, independent of the light sources.

noshadow - the objects using this surface do not have any shadows.

To specify surfaces, there are the following commands:

surface color [diffusion specularity phong metalness [transparency]] - phong and metalness are reals, the others are colors. Defaults 0.9 0.9 0.9, 0.1 0.1 0.1, 3, 0 and 0.1 0.1 0.1; if only transparency is omitted, then its default is 0 0 0.

surface strauss color smoothness metalness [transparency] - all are colors; default transparency is 0 0 0. This is an alternative to the previous command that uses a more intuitive way of specifying attributes.

surface matte color - 100% diffuse surface.

surface plastic color smoothness phong - plastic surface (high diffusion, small specularity and phong factor).

surface metal color smoothness phong - metallic surface (small diffusion, high specularity and phong factor, maximum metalness).

surface dielectric color transparency refraction - non-opaque surface (no diffusion, small specularity, large phong factor and nometalness). Needs a refraction index.

surface glass color transparency - transparent surface with refraction index near 1.52 and similar to dielectric.

refraction real - this specifies the current refraction index.

Examples:

	surface blue mono 0.7 mono 0.3 15 0.7   ; matte
	surface strauss brown mono 0.9 mono 0.1 ; matte
	surface matte red                       ; 100% matte
	surface plastic beige mono 0.8 0.5      ; plastic
	surface metal white mono 0.9 0.9        ; metallic
	surface dielectric white mono 0.8 1.2   ; translucent
	surface glass white mono 0.95           ; white glass

3.5. Objects

The supported objects in SCN can be of two types. The objects of the first type define a closed volume, while the others do not and must be handled with care inside CSG operations (see the list primitive bellow). Any object may inherit global attributes like surface, textures and transformations or else define its private attributes using the data keyword before its geometric information. It is also possible to group several objects and make them share some attributes using the group ... ungroup keywords. Example:

    transform none
    surface matte white                 ; current surface
    box ...                             ; white
    group
      surface matte red                 ; current surface inside block
      transform rotate y 45 ...         ; current transform inside block
      sphere ...                        ; rotated, red
      cone surface matte green data ... ; rotated, green
      cube bump data ...                ; rotated, red, bump texture
      cylinder ...                      ; rotated, red
    ungroup
    prism ...                           ; white

The closed objects are:

sphere [... data] center radius - a sphere.

box [... data] center sizes - an axis aligned box.

cube [... data] center size - an axis aligned cube.

cone [... data] apex base base_radius - a cone.

cylinder [... data] apex base radius - a cylinder.

cone truncated [... data] apex apex_radius base base_radius - truncated cone.

wedge [... data] point point point depth - a wedge is defined by a triangular face and an extrusion vector whose length is depth; the face is defined counterclockwise and the vector points in the Right Hand Rule direction (this convention is general in SCN).

tetra [... data] point point point point - a general tetrahedron (composed of four triangles).

prism [... data] depth vertices point ... point - a prism defined by a polygonal face and an extrusion vector similar to wedge.

pyramid [... data] depth vertices point ... point - a pyramid.

torus [... data] out_radius in_radius start_angle end_angle [out_samples in_samples] - a torus is centered in 0 0 0 and lies in the XY plane (0 degrees is in the X axis direction and increases counterclockwise). Actually, a torus is composed by a collection of Phong triangles whose density is defined by out_samples in_samples (default 16 8).

text3d file [... data] filename - text primitives stored in a file (high quality 3D letters, symbols, etc).

Examples of closed objects:

The opened objects are:

cone open [... data] apex base base_radius

cylinder open [... data] apex base radius

cone truncated open [... data] apex apex_radius base base_radius

prism open [... data] depth vertices point ... point

pyramid open [... data] depth vertices point ... point

pyramid truncated open [... data] depth apex_scale vertices point ... point - this is an opened pyramid with the apex face scaled by apex_scale in relation to its base face (0 is an opened pyramid and 1 is a opened prism).

disc [... data] center normal radius - a circle.

ring [... data] center normal out_radius in_radius - a circle with a hole.

patch [... data] point ... point (12) - a bicubic patch is defined by its corners and eight exterior points.

patch file [... data] [point [point]] filename - a group of patches stored in a file; first point defines a translation and second a scaling.

polygon [... data] vertices point ... point - polygon without holes.

polygon file [... data] [point [point]] filename - a group of polygons stored in a file.

triangle [... data] point point point

quadrangle [... data] point point point point - four-sided polygon.

triangle normal [... data] point vector point vector point vector - a triangle with normals in its vertices (also called Phong triangles).

triangle normal file [... data] [point [point]] filename - a group of Phong triangles stored in a file.

triangle general [... data] point vector point vector point vector surface surface surface - a triangle with normals and surfaces in its vertices. It is similar to a triangle normal, but allows the interpolation of any (or all) of the surface parameters inside itself.

triangle general file [... data] [point [point]] filename - a group of Phong triangles stored in a file.

torus open [... data] out_radius in_radius start_angle end_angle [out_samples in_samples] - an open torus.

Examples of opened objects:

Finally there are instructions that allow the creation of complex objects:

csg operation [... data] begin - begin of a CSG operation. The type of operation may be union, intersection or subtraction. A CSG (Constructive Solid Geometry) is a binary operation performed on two object operands (which may also be CSG's).

csg next - this instruction separates the definition of the left (first) operand of the CSG from the right (second) operand.

csg end - this instruction terminates the CSG.

Examples of CSG objects:

There is also one instruction that joins several simple objects into a primitive object. This is mandatory for CSG or else errors will appear in the picture:

list [... data] begin - start of a list of objects, at least two.

list end.

This instruction must be used in a CSG context when at least one of the operands is composed of opened objects (please note that each CSG operand must bound a finite volume). Example:

    list surface matte brown begin ; a new object similar to a pencil
      cylinder open  0 -1 0   0 1 0   0.3
      cone open      0 1.3 0  0 1 0   0.3
      sphere         0 -1 0           0.3
    list end

3.6. Transformations

A transformation is an operation that modifies the geometry of an object. Normally, transformations are concatenated to previous transformations, although it is possible to do it in other ways.

transform none - discards all previous transformations.

transform scale factor [factor factor] - scaling transformation.

transform translate point - translation.

transform rotate x angle - rotation about the X axis (angle in degrees).

transform rotate y angle.

transform rotate z angle.

transform rotate vector angle - rotation about an axis passing through origin.

transform general point point point [point] - any transformation matrix (3x3 or 4x4).

3.7. Textures

A texture modifies in some extent the characteristics of a surface. It is possible to apply transformations to textures and even keep them independent from object transformations.

texture none - discards all previous texture transformations.

texture scale factor [factor factor].

texture translate point.

texture rotate x angle.

texture rotate y angle.

texture rotate z angle.

texture rotate vector angle.

texture general point point point [point].

texture local - generate all the necessary texture transformations to access objects without considering their previously defined object transformations. The keyword invariant is also accepted.

Example:

	sphere
	  transform scale 0.2 1 2
	  transform rotate y ANGLE ; ANGLE is defined elsewhere
	  texture local
	  blotch 0.5 surface matte yellow
	  surface matte red
	  data 0 0 0 1

When generating animations, for example, it is important that textures stick to objects independently of their position, scale, etc or else it will produce a undesirable effect. If ANGLE varies from 5 to 30 degrees, the texture still applies correctly to the object and does not float strangely.

The available textures are:

	Name     Parameters
	------------------------------------------------------
	checkers surface [transform]
	blotch   scale surface [filename] [transform]
	bump     scale [transform]
	marble   [filename] [transform]
	fbm      offset scale omega lambda threshold octaves [transform]
	fbmbump  offset scale lambda octaves [transform]
	wood     color [transform]
	round    scale [transform]
	bozo     turbulence [filename] [transform]
	ripples  frequency phase scale [transform]
	waves    frequency phase scale [transform]
	spotted  [filename] [transform]
	dents    scale [transform]
	agate    [filename] [transform]
	wrinkles scale [transform]
	granite  [filename] [transform]
	gradient turbulence direction [filename] [transform]
	imagemap turbulence mode axis filename [transform]
	gloss    scale [transform]
	bump3    scale size [transform]
	------------------------------------------------------

Examples:

	sphere round 0.5 data 0 0 0 0.5
	sphere bump 0.7 scale 20 data 0 1 0 0.5 ; scale of bump is smaller
	sphere bump3 0.5 0.5 scale 1 4 1 data 0 -1 0 0.5 ; strange!