OSL
3D Textures

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Introduction

The features of shading points, such as their position, color and presence, are often determined by their location in the 'st' texture space. For example, listing 1 shows a surface shader that uses the sin() function, based on 's', to generate a cyclic pattern - figure 1.

Listing 1 (simple_sine.osl)

shader
simple_sine(float freq = 10,
            float s = 0 [[int lockgeom = 0]],
            float t = 0 [[int lockgeom = 0]],
        output color resultRGB = 0)
{
float  sinval = sin(s * 2 * M_PI * freq);
  
// Remap from -1 to +1 to 0 to 1
resultRGB = (sinval + 1) * 0.5;
}

Figure 1
Polygonal cylinder with well defined 'st' coordinates.

Of course, if the shader is attached to geometry that has not been assigned texture coordinates the result of using 's' becomes problematic - figure 2.

Figure 2
Polygonal cylinder with undefined texture coordinates.

Even more unpredictable are geometries, such as a Blobby, that cannot be given texture coordinates - figure 3.

Figure 3
Blobby primitives cannot be assigned texture coordinates

The next section demonstrates that shading according to 'xyz' position, rather than texture coordinates, generates a 3D solid texture.

A 3D Sinusoidal Texture

Rather than calculating a sine value based on a texture coordinate, such as 's', the polar coordinate of a shading point (generally a micro-polygon) can be used.

Figure 4

For example, the angular position (theta) of sp shown in figure 4 can be specified by its 2D Polar coordinate. For the purpose of creating a pattern of vertical stripes the height of sp can be ignored. The shader given in listing 2 uses the sine pattern to define an output color as well as a output float that can be used to control "presence".

Listing 2 (sine3d.osl)

shader
sine3d(float    freq = 4,
        float   amp = 0.5,
        float   offset = 0,
        float   blur = 0.05,
        string  spacename = "object",
        output float resultF = 0,
        output color resultRGB = 0)
{
// Convert the shading point to the object coordinate system.
point p = transform(spacename, P);
  
// Get the coordinates of the shading point.
float x = p[0];
float z = p[2];
  
// The arc tangent of the shading point will be in radians 
// (-3.142 to 3.142) ie -180 to 180 degress.
float theta = atan2(x,z);
  
// Generate a sine wave - remapped to 0 to 1.
float wave = (sin(theta * freq) + 1) * 0.5;
  
// Adjust the height and position of the sine wave.
resultF = wave * amp + offset;
  
// Get a filtered value of the wave height.
wave = smoothstep(resultF - blur, resultF + blur, p[1]);
  
// Control the mixing of two colors by the wave.
resultRGB = mix(color(1,1,1), color(0,0,0), wave);
}

Figure 5 shows the sine3d shader controlling the color of a blobby.

Figure 5

    Pattern "PxrOSL" "sine3d4" "string shader" "sine3d"
            "float freq" 34
            "float amp" 2
            "float offset" -1
            "float blur" 0.05
            "string spacename" ["object"]
    Bxdf "PxrDisney" "PxrDisney1"
            "reference color baseColor" ["sine3d4:resultRGB"]
            "float specular" [0.5]

Figure 6 shows the sine3d shader controlling the presence of a blobby.

Figure 6

    Pattern "PxrOSL" "sine3d4" "string shader" "sine3d"
            "float freq" 34
            "float amp" 2
            "float offset" -1
            "float blur" 0.05
            "string spacename" ["object"]
    Bxdf "PxrDisney" "PxrDisney1"
            "float specular" [0.5]
            "reference float presence" ["sine3d4:resultF"]