                        [Images & Graphics Header]

Neutral File Format

by Eric Haines, 3D/Eye Inc, 2359 North Triphammer Road, Ithaca, NY 14850
email: erich@eye.com, wrath.cs.cornell.edu!eye!erich

Draft #1, 10/3/88
Version 2.7, as of 5/22/90 - added information on hither, light color
Version 3.0, 12/17/90 - minor information changes
Version 3.1, 11/2/92 - more minor information changes

The NFF (Neutral File Format) is designed as a minimal scene description
language. The language was designed in order to test various rendering
algorithms and efficiency schemes. It is meant to describe the geometry and
basic surface characteristics of objects, the placement of lights, and the
viewing frustum for the eye. Some additional information is provided for
esthetic reasons (such as the color of the objects, which is not strictly
necessary for testing the efficiency of rendering algorithms).

Future enhancements could include: circle and torus objects, spline
surfaces with trimming curves, directional lights, characteristics for
positional lights, CSG descriptions, and probably more by the time you read
this. Comments, suggestions, and criticisms are all welcome.

At present the NFF file format is used in conjunction with the SPD
(Standard Procedural Database) software, a package designed to create a
variety of databases for testing rendering schemes. For more information
about SPD see "A Proposal for Standard Graphics Environments," IEEE
Computer Graphics and Applications, vol. 7, no. 11, November 1987, pp. 3-5.
See IEEE CG&A, vol. 8, no. 1, January 1988, p. 18 for the correct image of
the tree database (the only difference is that the sky is blue, not
orange).

The SPD package is available via anonymous FTP from:

        wuarchive.wustl.edu [128.252.135.4]:  /graphics/graphics/objects
        princeton.edu [128.112.128.1]:  /pub/Graphics [note the capital "G"!]

among others. For those without FTP access, write to the netlib automatic
mailer: research!netlib and netlib@ornl.gov are the sites. Send a one line
message "send index" for more information, or "send haines from graphics"
for the latest version of the SPD package.

If you would like to see images of the databases, FTP from:

        ftp.ipl.rpi.edu [128.113.14.50]:  sigma/erich
        nic.funet.fi [128.214.6.100]:  somewhere...
        gondwana.ecr.mu.oz.au [128.250.70.62]:  pub/images/haines

If you're looking for a good scene description language, see Craig Kolb's
Rayshade software (found on weedeater). Rayshade also has an awk program to
convert NFF to Rayshade's format. NFF is meant for testing efficiency
schemes and so has minimal support for lighting and shading.

By providing a minimal interface, NFF is meant to act as a simple format to
allow the programmer to quickly write filters to move from NFF to the local
file format. Presently the following entities are supported:

   * A simple perspective frustum
   * A background color description
   * A positional (vs. directional) light source description
   * A surface properties description
   * Polygon, polygonal patch, cylinder/cone, and sphere descriptions

Files are output as lines of text. For each entity, the first field defines
its type. The rest of the line and possibly other lines contain further
information about the entity. Entities include:

   * "v" - viewing vectors and angles
   * "b" - background color
   * "l" - positional light location
   * "f" - object material properties
   * "c" - cone or cylinder primitive
   * "s" - sphere primitive
   * "p" - polygon primitive
   * "pp" - polygonal patch primitive

These are explained in depth below.
---------------------------------------------------------------------------

Viewpoint location. Description:

    "v"
    "from" Fx Fy Fz
    "at" Ax Ay Az
    "up" Ux Uy Uz
    "angle" angle
    "hither" hither
    "resolution" xres yres

Format:

    v
    from %g %g %g
    at %g %g %g
    up %g %g %g
    angle %g
    hither %g
    resolution %d %d

The parameters are:

    From:  the eye location in XYZ.
    At:    a position to be at the center of the image, in XYZ world
           coordinates.  A.k.a. "lookat".
    Up:    a vector defining which direction is up, as an XYZ vector.
    Angle: in degrees, defined as from the center of top pixel row to
           bottom pixel row and left column to right column.
    Hither: distance of the hither plane (if any) from the eye.  Mostly
           needed for hidden surface algorithms.
    Resolution: in pixels, in x and in y.

Note that no assumptions are made about normalizing the data (e.g. the
from-at distance does not have to be 1). Also, vectors are not required to
be perpendicular to each other.

For all databases some viewing parameters are always the same:

    Yon is "at infinity."
    Aspect ratio is 1.0.

A view entity must be defined before any objects are defined (this
requirement is so that NFF files can be displayed on the fly by hidden
surface machines).
---------------------------------------------------------------------------

Background color. A color is simply RGB with values between 0 and 1:

    "b" R G B

Format:

    b %g %g %g

If no background color is set, assume RGB = {0,0,0}.
---------------------------------------------------------------------------

Positional light. A light is defined by XYZ position. Description:

    "l" X Y Z [R G B]

Format:

    l %g %g %g [%g %g %g]

All light entities must be defined before any objects are defined (this
requirement is so that NFF files can be used by hidden surface machines).
Lights have a non-zero intensity of no particular value, if not specified
(i.e. the program can determine a useful intensity as desired); the
red/green/blue color of the light can optionally be specified.
---------------------------------------------------------------------------

Fill color and shading parameters. Description:

    "f" red green blue Kd Ks Shine T index_of_refraction

Format:

    f %g %g %g %g %g %g %g %g

RGB is in terms of 0.0 to 1.0.

Kd is the diffuse component, Ks the specular, Shine is the Phong cosine
power for highlights, T is transmittance (fraction of contribution of the
transmitting ray). Usually, 0 <= Kd <= 1 and 0 <= Ks <= 1, though it is not
required that Kd + Ks == 1. Note that transmitting objects ( T > 0 ) are
considered to have two sides for algorithms that need these (normally
objects have one side).

The fill color is used to color the objects following it until a new color
is assigned.
---------------------------------------------------------------------------

Objects: all objects are considered one-sided, unless the second side is
needed for transmittance calculations (e.g. you cannot throw out the second
intersection of a transparent sphere in ray tracing).

Cylinder or cone. A cylinder is defined as having a radius and an axis
defined by two points, which also define the top and bottom edge of the
cylinder. A cone is defined similarly, the difference being that the apex
and base radii are different. The apex radius is defined as being smaller
than the base radius. Note that the surface exists without endcaps. The
cone or cylinder description:

    "c"
    base.x base.y base.z base_radius
    apex.x apex.y apex.z apex_radius

Format:

    c
    %g %g %g %g
    %g %g %g %g

A negative value for both radii means that only the inside of the object is
visible (objects are normally considered one sided, with the outside
visible). Note that the base and apex cannot be coincident for a cylinder
or cone. Making them coincident could be used to define endcaps, but none
of the SPD scenes currently make use of this definition.
---------------------------------------------------------------------------

Sphere. A sphere is defined by a radius and center position:

    "s" center.x center.y center.z radius

Format:

    s %g %g %g %g

If the radius is negative, then only the sphere's inside is visible
(objects are normally considered one sided, with the outside visible).
Currently none of the SPD scenes make use of negative radii.
---------------------------------------------------------------------------

Polygon. A polygon is defined by a set of vertices. With these databases, a
polygon is defined to have all points coplanar. A polygon has only one
side, with the order of the vertices being counterclockwise as you face the
polygon (right-handed coordinate system). The first two edges must form a
non-zero convex angle, so that the normal and side visibility can be
determined by using just the first three vertices. Description:

    "p" total_vertices
    vert1.x vert1.y vert1.z
    [etc. for total_vertices vertices]

Format:

    p %d
    [ %g %g %g ] <-- for total_vertices vertices

---------------------------------------------------------------------------

Polygonal patch. A patch is defined by a set of vertices and their normals.
With these databases, a patch is defined to have all points coplanar. A
patch has only one side, with the order of the vertices being
counterclockwise as you face the patch (right-handed coordinate system).
The first two edges must form a non-zero convex angle, so that the normal
and side visibility can be determined. Description:

    "pp" total_vertices
    vert1.x vert1.y vert1.z norm1.x norm1.y norm1.z
    [etc. for total_vertices vertices]

Format:

    pp %d
    [ %g %g %g %g %g %g ] <-- for total_vertices vertices

---------------------------------------------------------------------------

Comment. Description:

    "#" [ string ]

Format:

    # [ string ]

As soon as a "#" character is detected, the rest of the line is considered
a comment.

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