RSL
Layered Shading

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Introduction

It is useful to think about a shading effect, say making a colored pattern, as a process of sequentially applying layers of colored foil to a surface. As each layer of foil is applied it is cut out by a digital "cookie-cutter".

Figure 1

Figure 1 shows a "cookie-cutter" and foreground and background colored "foils". The cookie-cutter can also be thought of as a mask that allows one layer of color to be composited onto another layer of color. The cookie-cutter ie. mask, defines areas of transparency and opacity. Opaque areas allow parts of a foreground foil to overlay a background foil.

Defining a Cookie-Cutter

Listing 1 provides the code for a function that returns 0 or 1 depending on whether an location (x, y) is inside or outside a rectangle defined by its top, left, bottom and right edges. The function should be saved in a file called "lib.h".

Listing 1

float rectangle(float top, left, bottom, right, x, y)
{
float result = 0;
if(x >= left && x <= right && y >= top && y <= bottom)
    result = 1;
return result;
}

The value retured from the function is used by mix() in listing 2 to overlay or composite the final color.

Using a Cookie-Cutter

The source for a simple layered shader is shown below in listing 2.

Listing 2

#include "lib.h"
  
surface
layering1(float Kfb = 1,
                top = 0.4,
                left = 0.4,
                bottom = 0.6,
                right = 0.6;
        color   foreground = color(1,0.239,0))
{
color    surfcolor;
float    mask;
  
/* STEP 1  Set the apparent surface opacity.      */
Oi = Os;
  
/* STEP 2  Composite a foreground color over a    */
/*         background color - in this instance    */
/*         Cs is the background color.            */
mask = rectangle(top, left, bottom, right, s, t);
surfcolor = mix(Cs, foreground, mask);
    
/* STEP 3  No need to use Cs again because it was */
/*         used it in step 2.                     */
Ci = Oi * surfcolor * Kfb;
}

If you are using Cutter to compile the RSL source code make sure that both the "lib.h" and "layering1.sl" are in the shader source code directory specified in Cutter's Preferences - figure 2. If you are not using Cutter the full path to the "lib.h" file must be specfied by the #include statement.

Figure 2

Multiple Layers

By repeating step 2 several times, for example,

    mask = rectangle(0.1, 0.3, 0.8, 0.7, s, t);
    surfcolor = mix(Cs, color(0,0,1), mask);

    mask = circle(0.3, 0.5, 0.25, s, t);
    surfcolor = mix(surfcolor, color(1,0.239,0), mask);

a shader can give the illusion the surface to which it is assigned has multiple colored layers - figure 3.

Figure 3

A function called circle() was implemented in the "lib.h" file as follows,

Listing 3

float circle(float locx, locy, radius, x, y)
{
float result = 0;
float d = distance(point(locx,locy,0), point(x,y,0));
if(d <= radius)
    result = 1;
return result;
}