The Materials and Geometry Format

Version 1.0, May 1995

Greg Ward, Lawrence Berkeley Laboratory, GJWard@Lbl.Gov


What makes MGF special?

What does MGF look like?

MGF's place in the world of standards

MGF Basics

Entities and Contexts

Hierarchical Contexts and Transformations

Detailed MGF Example

MGF Entity Reference

MGF Translators

MGF Parser Library

Application Notes

Relation to Standard Practices in Computer Graphics

For those coming from a computer graphics background, some of the choices in the material model may seem strange or even capricious. Why not simply stick with RGB colors and a Phong specular component like everyone else? What is the point in choosing the number of sides to a material?

In the real world, a surface can have only one side, defining the interface between one volume and another. Many object-space rendering packages (e.g. z-buffer algorithms) take advantage of this fact by culling back-facing polygons and thus saving as much as 50% of the preprocessing time. However, many models rely on an approximation whereby a single surface is used to represent a very thin volume, such as a pane of glass, and this also can provide significant calculational savings in an image-space algorithm (such as ray-tracing). Also, many models are created in such a way that the front vs. back information is lost or confused, so that the back side of one or more surfaces may have to serve as the front side during rendering. (AutoCAD is one easily identified culprit in this department.) Since both types of surface models are useful and any rendering algorithm may ultimately be applied, MGF provides a way to specify sidedness rather than picking one interpretation or the other.

The problem with RGB is that there is no accepted standard, and even if we were to set one it would either be impossible to realize (i.e. impossible to create phosphors with the chosen colors) or it would have a gamut that excludes many saturated colors. The CIE color system was very carefully conceived and developed, and is the standard to which all photometric measurements adhere. It is therefore the logical choice in any standard format, though it has been too often ignored by the computer graphics community.

Regarding Phong shading, this was never a physical model and making it behave basic laws of reciprocity and energy balance is difficult. More to the point, specular power has almost nothing to do with surface microstructure, and is difficult to set properly even if every physical characteristic of a material has been carefully measured. This is the ultimate indictment of any physical model -- that it is incapable of reproducing any measurement whatsoever.

Admittedly, the compliment of diffuse and specular component plus surface roughness and index of refraction used in MGF is less than a perfect model, but it is serviceable for most materials and relatively simple to incorporate into a rendering algorithm. In the long term, MGF shall probably include full spectral scattering functions, though the sheer quantity of data involved makes this burdensome from both the measurement side and the simulation side.

Converting between Phong Specular Power and Gaussian Roughness

So-called specular reflection and transmission are modeled using a Gaussian distribution of surface facets. The roughness parameters to the rs and ts entities specify the root-mean-squared (RMS) surface facet slope, which varies from 0 for a perfectly smooth surface to around .2 for a fairly rough one. The effect this will have on the reflected component distribution is well-defined, but predicting the behavior of the transmitted component requires further assumptions. We assume that the surface scatters light passing through it just as much as it scatters reflected light. This assumption is approximately correct for a two-sided transparent material with an index of refraction of 1.5 (like glass) and both sides having the given RMS facet slope.

Oftentimes, one is translating from a Phong exponent on the cosine of the half-vector-to-normal angle to the more physical but less familiar Gaussian model of MGF. The hardest part is translating the specular power to a roughness value. For this, we recommend the following approximation:

	roughness = sqrt(2/specular_power)
It is not a perfect correlation, but it is about as close as one can get.

Converting between RGB and CIE Colors

Unlike most graphics languages, MGF does not use an RGB color model, simply because there is no recognized definition for this model. It is based on computer monitor phosphors, which vary from one CRT to the next. (There is an RGB standard defined in the TV industry, but this has a rather poor correlation to most computer monitors and it is impossible to express many real-world colors within its limited gamut.)

MGF uses two alternative, well-defined standards, spectral power distributions and the 1931 CIE 2 degree standard observer. With the CIE standard, any viewable color may be exactly represented as an (x,y) chromaticity value. Unfortunately, the interaction between colors (i.e. colored light sources and interreflections) cannot be specified exactly with any finite coordinate set, including CIE chromaticities. So, MGF offers the ability to give reflectance, transmittance or emittance as a function of wavelength over the visible spectrum. This function is still discretized, but at a user-selectable resolution. Furthermore, spectral colors may be mixed, providing (nearly) arbitrary basis functions, which can produce more accurate results in some cases and are merely a convenience for translation in others.

Conversion back and forth between CIE chromaticity coordinates and spectral samples is provided within the MGF parser. Unfortunately, conversion to and from RGB values depends on a particular RGB definition, and as we have said, there is no recognized standard. We therefore recommend that you decide yourself what chromaticity values to use for each RGB primary, and adopt the following code to convert between CIE and RGB coordinates.

#ifdef  NTSC
#define  CIE_x_r		0.670		/* standard NTSC primaries */
#define  CIE_y_r		0.330
#define  CIE_x_g		0.210
#define  CIE_y_g		0.710
#define  CIE_x_b		0.140
#define  CIE_y_b		0.080
#define  CIE_x_w		0.3333		/* monitor white point */
#define  CIE_y_w		0.3333
#define  CIE_x_r		0.640		/* nominal CRT primaries */
#define  CIE_y_r		0.330
#define  CIE_x_g		0.290
#define  CIE_y_g		0.600
#define  CIE_x_b		0.150
#define  CIE_y_b		0.060
#define  CIE_x_w		0.3333		/* monitor white point */
#define  CIE_y_w		0.3333

#define CIE_D		(	CIE_x_r*(CIE_y_g - CIE_y_b) + \
				CIE_x_g*(CIE_y_b - CIE_y_r) + \
				CIE_x_b*(CIE_y_r - CIE_y_g)	)
#define CIE_C_rD	( (1./CIE_y_w) * \
				( CIE_x_w*(CIE_y_g - CIE_y_b) - \
				  CIE_y_w*(CIE_x_g - CIE_x_b) + \
				  CIE_x_g*CIE_y_b - CIE_x_b*CIE_y_g	) )
#define CIE_C_gD	( (1./CIE_y_w) * \
				( CIE_x_w*(CIE_y_b - CIE_y_r) - \
				  CIE_y_w*(CIE_x_b - CIE_x_r) - \
				  CIE_x_r*CIE_y_b + CIE_x_b*CIE_y_r	) )
#define CIE_C_bD	( (1./CIE_y_w) * \
				( CIE_x_w*(CIE_y_r - CIE_y_g) - \
				  CIE_y_w*(CIE_x_r - CIE_x_g) + \
				  CIE_x_r*CIE_y_g - CIE_x_g*CIE_y_r	) )

#define CIE_rf		(CIE_y_r*CIE_C_rD/CIE_D)
#define CIE_gf		(CIE_y_g*CIE_C_gD/CIE_D)
#define CIE_bf		(CIE_y_b*CIE_C_bD/CIE_D)

float  xyz2rgbmat[3][3] = {	/* XYZ to RGB */
	{(CIE_y_g - CIE_y_b - CIE_x_b*CIE_y_g + CIE_y_b*CIE_x_g)/CIE_C_rD,
	 (CIE_x_b - CIE_x_g - CIE_x_b*CIE_y_g + CIE_x_g*CIE_y_b)/CIE_C_rD,
	 (CIE_x_g*CIE_y_b - CIE_x_b*CIE_y_g)/CIE_C_rD},
	{(CIE_y_b - CIE_y_r - CIE_y_b*CIE_x_r + CIE_y_r*CIE_x_b)/CIE_C_gD,
	 (CIE_x_r - CIE_x_b - CIE_x_r*CIE_y_b + CIE_x_b*CIE_y_r)/CIE_C_gD,
	 (CIE_x_b*CIE_y_r - CIE_x_r*CIE_y_b)/CIE_C_gD},
	{(CIE_y_r - CIE_y_g - CIE_y_r*CIE_x_g + CIE_y_g*CIE_x_r)/CIE_C_bD,
	 (CIE_x_g - CIE_x_r - CIE_x_g*CIE_y_r + CIE_x_r*CIE_y_g)/CIE_C_bD,
	 (CIE_x_r*CIE_y_g - CIE_x_g*CIE_y_r)/CIE_C_bD}

float  rgb2xyzmat[3][3] = {	/* RGB to XYZ */

cie_rgb(rgbcolor, ciecolor)		/* convert CIE to RGB */
register float  *rgbcolor, *ciecolor;
	register int  i;

	for (i = 0; i < 3; i++) {
		rgbcolor[i] =	xyz2rgbmat[i][0]*ciecolor[0] +
				xyz2rgbmat[i][1]*ciecolor[1] +
				xyz2rgbmat[i][2]*ciecolor[2] ;
		if (rgbcolor[i] < 0.0)		/* watch for negative values */
			rgbcolor[i] = 0.0;

rgb_cie(ciecolor, rgbcolor)		/* convert RGB to CIE */
register float  *ciecolor, *rgbcolor;
	register int  i;

	for (i = 0; i < 3; i++)
		ciecolor[i] =	rgb2xyzmat[i][0]*rgbcolor[0] +
				rgb2xyzmat[i][1]*rgbcolor[1] +
				rgb2xyzmat[i][2]*rgbcolor[2] ;
An alternative to adopting the above code is to use the MGF "cmix" entity to convert from RGB directly by naming the three primaries in terms of their chromaticities, e.g:

c R =
	cxy 0.640 0.330
c G =
	cxy 0.290 0.600
c B =
	cxy 0.150 0.060

Then, converting from RGB to MGF colors is as simple as multiplying each component by its relative luminance in a cmix statement, for instance:

c white =
	cmix 0.265 R 0.670 G 0.065 B

For the chosen RGB standard, the above specification would result a pure white. The reason the coefficients are not all 1 as you might expect is that cmix uses relative luminance as the standard for its weights. Since blue is less luminous for the same energy than red, which is in turn less luminous than green, the weights cannot be the same to achieve an even spectral balance. Unfortunately, computing these relative weights is not straightforward, though it is given in the above macros as CIE_rf, CIE_gf and CIE_bf. (The common factors in these macros may of course be removed since cmix weights are all relative.) Alternatively, one could measure the actual full scale luminance of the phosphors with a luminance probe and get the same relative values.

Relation to IESNA LM-63 and Luminaire Catalogs