Games of Daze/Infomagic - Games of Daze (Summer 1995) (Disc 1 of 2).iso/x2ftp/msdos/docs/zed3d050/siggraph.86

Text File | 1995-03-03 | 15KB | 419 lines

Fast Phong Shading. p 103-105
 Gary Bishop
 Room 4E-626
 David M Weimer
 Room 4F-637
 AT&T Bell Laboratories
 Crawfords Corner Road
 Holmdel, NJ 07733

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  SIGGRAPH @1986 - ACM 0-89791-196-2/86/008/0103

Abstract:

Computer image generation systems often represent curved surfaces as a mesh
of polygons that are shaded to restore a smooth appearance.  Phong shading
is a well known algorithm for producing a realistic shading but it has not
been used by real-time systems because of the 3 additions, 1 division and
1 square root required per pixel for its evaluation.  We describe a new
formulation for Phong shading that reduces the amount of computation per
pixel to only 2 additions for simple Lambertian reflection and 5 additions
and 1 memory reference for Phong's complete reflection model. We also show
how to extend our method to compute the specular component with the eye at
a finite distance from the scene rather than at infinity as is usually
assumed. The method can be implemented in hardware for real-time applications
or in software to speed image generation for almost any system.

Introduction:

Most computer image generation (CIG) systems represent curved surfaces as a
mesh of planar polygons because polygons can be transformed and rendered
quickly with well known algorithms.  Since the polygonal representation is an
artifact of the image generation method and is usually of no interest to
viewers (see figure 1), these systems attempt to restore a smooth appearance
to surfaces by varying the intensity across polygons.  The efficiency of this
shading operation is critical to the performance of a CIG system because it
must be performed for one million or more pixels per image.  Although
algorithms for realistic shading of polygons are well known, real-time CIG
systems have not used them because of the large amount of computation they
require per pixel.  This paper describes a new formulation of Phong shading
that drastically reduces the amount of computation compared to previously
known formulations. While the new formulation is well suited to implementation
with special hardware for real-time display, it is also appropriate for
implementation with software for a variety of display systems. Readers who are
unfamiliar with surface representation or shading are referred to one of the
standard graphics texts for more information, [Newman and Sproul, 1979; Foley
and Van Dam, 1983].

Background:

Assume, for simplicity, that the input to the shader consists of triangles,
in screen coordinates, with normals, N, to the original curved surface
specified at each vertex.  The formulations can be extended to polygons with
four or more sides.

The shading algorithms must determine the intensity of each point within a
triangle from the surface normals, and a vector to the light source, L.  The
light source is assumed to be at infinity so that L is independent of the
point on which the light shines.  We will start with diffuse reflection,

                   L * N
    I        = -------------,
     diffuse     |L| * |N|


and later show how to extend the results to include Phong's highlight model
and an extended highlight model.

GOURAUD SHADING: the most commonly used shading method in real-time systems.
Gouraud shading [Gouraud, 1971], computes the intensity at each point by
linear interpolation of the intensities at the vertices. The intensities are
determined using the reflection equation above with normals given at the
vertices. The method is popular for real-time systems because it produces
images of acceptable quality with only 1 addition per pixel, but the shading
has several disturbing characteristics. (1) Moving objects tend to "flatten
out" at certain viewing angles, (2) surfaces appear dull or chalky, and (3)
images show pronounced Mach bands, exaggerations of intensity change at
discontinuities.  Figure 2 is a Gouraud shaded chess piece.

PHONG SHADING: Phong shading, [Phong, 1973], eliminates "flattening out" and
dull surfaces and reduces Mach bands but has not, to my knowledge, been used
in any real-time system because of the large amount of computation required
for its usual formulation.  Phong's method determines the intensity of each
point using an approximate surface normal that is linearly interpolated from
the true surface normals specified at the vertices,

  N(x,y) = Ax + By + C

where A, B and C are chosen to interpolate the normal across the polygon.
Interpolation for successive values of x and y and evaluating the illumination
model requires 7 additions, 6 multiplications, 1 division and 1 square root
per pixel.  Phong proposed a complicated circuit for evaluating this function
but it has not, to my knowledge, been implemented.  Figure 3 is a Phong shaded
chess piece.

Tom Duff has shown, in [Duff, T.,1979, "Smoothly Shaded Renderings of
Polyhedral Objects on Raster Displays", ACM Computer Graphics, vol 13, no 2,
pp 270-275]], that Phong shading can be implemented more efficiently by
combining the interpolation and reflection equations.

                        L        Ax + By + C
       I       (x,y) = --- * -------------------
        diffuse        |L|    |L| |Ax + By + C|


Which can be rewritten as

                          L*Ax + L*By + L*C
       I       (x,y) = ------------------------
        diffuse        |L| |L*Ax + L*By + L*C|

Performing the indicated dot products and expanding the vector magnitude
yields:

                                    ax + by + c
       I       (x,y) = --------------------------------------
        diffuse        SQRT( dx^2 + exy + fy^2 + gx + hy + i)

with
             L * A
         a = ----- ,
              |L|

             L * B
         b = ----- ,
              |L|

             L * C
         c = ----- ,
              |L|

         d = A * A,

         e = 2A * B,

         f = B * B,

         g = 2A * C,

         h = 2B * C, and

         i = C * C.


Using forward differences, this form can be evaluated for successive values
of x and y with 3 additions, 1 division and 1 square root per pixel. This is
a substantial savings over Phong's formulation but the division and square
root are still too expensive for real-time use.

A New Approach:

For display purposes we need not evaluate the reflection equation exactly;
a good approximation will suffice.  The "error" introduced by the
approximation will be of no consuquence since Phong's normal interpolation and
the reflection equation are already approximations.  the one-dimensional form
of Taylor's series is widely used for approximation functions, such as sine
and log, with polynomials.  The less widely used, two-dimensional form will
serve to approximate the reflection equation.

Taylor's series for a function of two variables is:

f(a+h, b+k) = f(a,b) + (h ... + k ...)*f(a,b) + ....... [look it up!]

Shifting the triangle so that (0,0) lies at its center, and expanding in a
Taylor series to the second degree produces


       I       (x,y) = T x^2 + T xy + T y^2 + T x + T y + T
        diffuse         5       4      3       2     1     0

with

        3ig^2 - 4cdi - 4agi
  T  = ---------------------,
   5     8 * i^2 * SQRT(i)


        3cgh - 2cei - 2bgi - 2ahi
  T  = ---------------------------,
   4        4 * i^2 * SQRT(i)


        3ch^2 - 4cfi - 4bhi
  T  = ---------------------,
   3     8 * i^2 * SQRT(i)


           2ai - cg
  T  = -----------------,
   2    2 * i * SQRT(i)


           2bi - ch
  T  = -----------------, and
   1    2 * i * SQRT(i)



           c
  T  = ---------
   0    SQRT(i)


Using this expansion and forward differences, the intensity can be evaluated
with only 2 additions per pixel.

The method can be extended to handle Phong's specular reflection model,
                  n
I        = (N * H) , by using Taylor's series to evaluate the dot product
 specular            and table lookup to do the exponentiation (A table of
                     1K bytes will allow exponentiation to powers up to 20
with less than 1 percent error and a table of 8K bytes allows powers up to
164.)  H is a vector in the direction of maximum highlight which is halfway
between the light direction, L, and the eye direction, E,

                 E * L
            H = -------,
                |E + L|

The eye, like the light source, is assumed for the present to be at infinity.
Phong's reflection equation, I = I       + I       +  I        , can now be
                                  ambient   diffuse    specular
evaluated with only 5 additions and 1 memory access per pixel (the ambient
component can be rolled into the series for the diffuse component and the
series for the specular component can be scaled to allow direct addressing of
the exponentiation table).  It should be simple to configure hardware to do
these operations in less than 100 nanoseconds per pixel.

The additional computation required to determine the Ti for our new method is
offset by the greatly reduced computation per pixel for polygons with 10 or
more pixels.  This estimate is based on the following assumptions: (a) the
algorithms are implemented on sequential processors of conventional design,
(b) with modern hardware, the time for floating point addition,
multiplication, and memory references are all about equal, (c) the
computation of 1 / SQRT(x) requires about 10 operations, and (d) common
subexpressions have been removed from the formulas for Ti. Timings and
analysis of the method as implementd in the raster testbed [Whitted and
Weimer, 1982] also support a 10 pixel break-even point. The break-even point
would be much lower if some simple hardware were added to do the additions
and table lookup for our method in parallel.  Of course, special hardware
could be built for the other methods but it would be much more complicated
and probably not as fast.

The images we have produced with this approximation are indistinguishable
from those produced with Phong's method.  For polygons with more than about
60 degrees of curvature, this new expansion will produce shadings that are
noticeably different from Phong shading but curvatures this large should
never be represented by a single polygon.


Going Further:

Since the form of the polynomial we have to evaluate at each pixel is
independent of the original reflection equation, we can use more elaborate
models. One useful extension is to provide for finite eye distance, thus
requiring that the vector to the eye, E, vary across the scene. The resulting
variation in the specular component produces more natural looking illumination
for scenes rendered with perspective. This is most obvious when there are
planar surfaces in the scene because Phong shading with the eye at infinity,
like flat shading, renders all parallel surfaces with the same intensity.
Figure 4 is a comparision of Phong shading with the eye at infinity and with
the eye at a true perspective distance. The reflection equation for the
specular component with the eye at a finite distance now becomes

                N(x,y) * H(x,y)
  I        = ---------------------,
   specular   |N(x,y)| * |H(x,y)|


where H(x,y) = E(x,y) + L and E(x,y) interpolates the eye vector across the
triangle. This can be expanded just as before

                (Ax + By + C) * (Dx + Ey + F)
  I        = -----------------------------------
   specular   |(Ax + By + C)| * |(Dx + Ey + F)|


After performing the dot products and expanding the vector magnitude as
before

                       ax^2 + bxy + cy^2 + dx + ey + f
I =--------------------------------------------------------------------------
 s  SQRT((gx^2 + hxy + iy^2 + jx + ky + l)*(mx^2 + nxy + oy^2 + px + qy + r))

with

  a = A * D,

  b = A * E + B * D,

  c = B * E,

  d = A * F + C * D,

  e = B * F + C * E,

  f = C * F,

  g = A * A,

  h = 2A * B,

  i = B * B,

  j = 2A * C,

  k = 2B * C,

  l = C * C,

  m = E * E,

  n = D * D,

  o = 2D * E,

  p = 2D * F,

  q = 2E * F, and

  r = F * F.

And expanding in Taylor's series about (0,0).

  I(x,y) = T x^2 + T xy + T y^2 + T x + T y + T
            5       4      3       2     1     0

with

     8al^2r^2 - 4djlr^2 - 4fglr^2 + 3fj^2r^2 - 4dl^2pr + 2fjlpr - 4fl^2mr
T = ----------------------------------------------------------------------
 5                           8l^2r^2 * SQRT(l*r)

          + 3fl^2p^2
    + ---------------------,
       8l^2r^2 * SQRT(l*r)


     4bl^2r^2 - 2dklr^2 - 2ejlr^2 - 2fhlr^2 + 3fjkr^2 - 2dl^2qr
T = ------------------------------------------------------------
 4                       4l^2r^2 * SQRT(l*r)

        - fjlqr - 2el^2pr + fklpr - 2f^2nr + 3fl^2pq
    + ------------------------------------------------,
                      4l^2r^2 * SQRT(l*r)


     8cl^2r^2 - 4eklr^2 - 4filr^2 + 3fk^2r^2 - 4el^2qr + 2fklqr - 4fl^2or
T = ----------------------------------------------------------------------
 3                           8l^2r^2 * SQRT(l*r)

          + 3fl^2q^2
    + ---------------------,
       8l^2r^2 * SQRT(l*r)



      2dlr - fjr - flp
T = -------------------,
 2    2lr * SQRT(l*r)


      2elr - fkr - flq
T = -------------------,
 1    2lr * SQRT(l*r)


         f
T = -----------
 0   SQRT(l*r)




Summary:

We have shown that computer image generation systems can use Phong shading
with only a little more computation per pixel than is required for Gouraud
shading. This result should allow real-time systems to generate more
realistic scenes and conventional systems to produce images more rapidly.

To test the method, we have implemented it as a shader subroutine for the
raster testbed. The cpu time for rendering figure 4 with 14620 polygons
at 2048x2048 resolutions on a DEC microVAX-II are:

Shading method               CPU time     Notes
Gouraud                        406s
Taylor (infinite)              767s       incl 119s for Taylor coeffs
Taylor (finite)                850s       incl 202s for Taylor coeffs
Duff                          2597s       incl 1420 in sqrt
Phong                         3900s       incl 1405 in sqrt


Acknowledgements:


Tom Duff and the reviewers provided many helpful suggestions.




-----------------------------------EOF--------------------------------------
Errata:

 T4 (p 105)

 - f j l q r      =>   + f j l q r
 - 2 f^2 n r      =>   - 2 f l^2 n r