Rsl - Texture Channels

RSL
Texture Channels

Introduction

This note shows how 5 texture maps can be "packed" into a five channel image - one map per channel. Figure 1 shows a five channel PhotoShop image. The last two channels have black backgrounds because PhotoShop considers them to be alpha or transparency channels.

Figure 1

When applied by the shader shown in listing 1 the maps can provide the illusion of, say the discoloration of the surface of a fruit, becoming larger over time. The shader simply blends from one map channel into another - figures 1 and 2.

Figure 2
Progressive use of a 5 channel map

Figure 3
The straightstep function uses simple linear interpolation

The five channel PhotoShop image was saved as a tif file and converted to a texture using Pixar's txmake utility.

Listing 1

float straightstep(float begin, end, val)
{
float out;
if(val <= begin)
    out = 0;
else if(val >= end)
    out = 1;
else
        out = (val - begin)/(end - begin);
return 1;
}
//------------------------------------------
surface blotch(float Kd = 1,
                     ime = 0;
            string   map = "";
            color    blotch_color = 0)
{
color    surfcolor = 1;
normal   n = normalize(N);
normal   nf = faceforward(n, I);
float    tex;
  
if(map != "")
    {
    if(time <= 0.20)
        tex = mix(1, texture(map[0]), smoothstep(0,0.20,time));
    else if(time > 0.20 && time <= 0.4)
        tex = mix(texture(map[0]), 
                  texture(map[1]), smoothstep(0.2,0.4,time));
    else if(time > 0.4 && time <= 0.6)
        tex = mix(texture(map[1]), 
                  texture(map[2]), smoothstep(0.4,0.6,time));
    else if(time > 0.6 && time <= 0.8)
        tex = mix(texture(map[2]), 
              1 - texture(map[3]), smoothstep(0.6,0.8,time));
    else if(time > 0.8)
        tex = mix(texture(map[3]), 
              1 - texture(map[4]), smoothstep(0.8,1.0,time));
    // composite the texture map color over the background
    surfcolor = mix(blotch_color, Cs, tex);
    }
Oi = Os;
Ci = Oi * Cs * surfcolor * Kd * diffuse(nf);
}

Slightly different results can be obtained by substituting the RSL function smoothstep() with the custom straightstep() function.

RSL Texture Channels

Introduction

RSL
Texture Channels