Add Color::absorptionAtDistance() to compute absorption for refractio…

…n. (google#1964)

Specifying the transmittance color at a specific distance is more
user-friendly than specifying absorption coefficients directly.

assets/models/prism/prism.mtl

@@ -0,0 +1,10 @@
+# Blender MTL File: 'None'
+# Material Count: 1
+
+newmtl None
+Ns 500
+Ka 0.8 0.8 0.8
+Kd 0.8 0.8 0.8
+Ks 0.8 0.8 0.8
+d 1
+illum 2

assets/models/prism/prism.obj

@@ -0,0 +1,34 @@
+# Blender v2.81 (sub 16) OBJ File: ''
+# www.blender.org
+mtllib prism.mtl
+o Cube_Cube.001
+v 0.500000 0.000000 0.325000
+v 0.500000 1.000000 -0.000000
+v -0.500000 0.000000 0.325000
+v -0.500000 1.000000 0.000000
+v 0.500000 0.000000 -0.325000
+v -0.500000 0.000000 -0.325000
+vt 0.375000 0.000000
+vt 0.625000 0.000000
+vt 0.625000 0.250000
+vt 0.375000 0.250000
+vt 0.375000 0.500000
+vt 0.375000 0.750000
+vt 0.625000 1.000000
+vt 0.375000 1.000000
+vt 0.125000 0.500000
+vt 0.125000 0.750000
+vt 0.875000 0.500000
+vt 0.875000 0.750000
+vn 0.0000 0.3091 0.9510
+vn -1.0000 0.0000 0.0000
+vn 1.0000 0.0000 0.0000
+vn 0.0000 -1.0000 0.0000
+vn -0.0000 0.3091 -0.9510
+usemtl None
+s off
+f 1/1/1 2/2/1 4/3/1 3/4/1
+f 3/4/2 4/3/2 6/5/2
+f 5/6/3 2/7/3 1/8/3
+f 3/9/4 6/5/4 5/6/4 1/10/4
+f 4/11/5 2/12/5 5/6/5 6/5/5

filament/include/filament/Color.h

@@ -125,6 +125,20 @@ class UTILS_PUBLIC Color {
      */
     static LinearColor illuminantD(float K);
 
+    /**
+     * computes the Beer-Lambert absorption coefficients from the specified
+     * transmittance color and distance. The computed absorption will guarantee
+     * the white light will become the specified color at the specified distance.
+     * The output of this function can be used as the absorption parameter of
+     * materials that use refraction.
+     *
+     * @param color the desired linear RGB color in sRGB space
+     * @param distance the distance at which white light should become the specified color
+     *
+     * @return absorption coefficients for the Beer-Lambert law
+     */
+    static math::float3 absorptionAtDistance(LinearColor const& color, float distance);
+
 private:
     static math::float3 sRGBToLinear(math::float3 color) noexcept;
     static math::float3 linearToSRGB(math::float3 color) noexcept;

filament/src/Color.cpp

@@ -20,6 +20,8 @@
 
 #include <math/mat3.h>
 
+#include <cmath>
+
 using namespace filament::math;
 
 namespace filament {
@@ -77,4 +79,8 @@ LinearColor Color::illuminantD(float K) {
     return saturate(linear / std::max(1e-5f, max(linear)));
 }
 
+LinearColor Color::absorptionAtDistance(LinearColor const& color, float distance) {
+    return -log(clamp(color, 1e-5f, 1.0f)) / std::max(1e-5f, distance);
+}
+
 } // namespace filament

libs/math/include/math/TVecHelpers.h

@@ -472,6 +472,13 @@ class TVecFunctions {
         return v;
     }
 
+    friend inline VECTOR<T> MATH_PURE log(VECTOR<T> v) {
+        for (size_t i = 0; i < v.size(); i++) {
+            v[i] = std::log(v[i]);
+        }
+        return v;
+    }
+
     friend inline constexpr VECTOR<T> MATH_PURE saturate(const VECTOR<T>& lv) {
         return clamp(lv, T(0), T(1));
     }

samples/material_sandbox.cpp

@@ -330,7 +330,8 @@ static void gui(filament::Engine* engine, filament::View*) {
                     ImGui::SliderFloat("ior", &params.ior, 1.0f, 3.0f);
                     ImGui::SliderFloat("transmission", &params.transmission, 0.0f, 1.0f);
                     ImGui::SliderFloat("thickness", &params.thickness, 0.0f, 1.0f);
-                    ImGui::ColorEdit3("absorption", &params.absorption.r);
+                    ImGui::ColorEdit3("transmittance", &params.transmittanceColor.r);
+                    ImGui::SliderFloat("distance", &params.distance, 0.0f, 4.0f);
                 }
             }
         }

samples/material_sandbox.h

@@ -71,10 +71,10 @@ struct SandboxParameters {
     float specularAntiAliasingVariance = 0.0f;
     float specularAntiAliasingThreshold = 0.0f;
     float transmission = 1.0f;
-    filament::math::float3 absorption =  0.0f;
+    float distance = 1.0f;
     float ior = 1.5;
-
-    filament::sRGBColor specularColor = {0.0f, 0.0f, 0.0f};
+    filament::sRGBColor transmittanceColor =  { 1.0f };
+    filament::sRGBColor specularColor = {0.0f };
     filament::sRGBColor subsurfaceColor = {0.0f};
     filament::sRGBColor sheenColor = {0.83f, 0.0f, 0.0f};
     int currentMaterialModel = MATERIAL_MODEL_LIT;
@@ -205,7 +205,9 @@ inline filament::MaterialInstance* updateInstances(SandboxParameters& params,
             materialInstance->setParameter("alpha", params.alpha);
         }
         if  (hasRefraction) {
-            materialInstance->setParameter("absorption", params.absorption);
+            math::float3 color = Color::toLinear(params.transmittanceColor);
+            materialInstance->setParameter("absorption",
+                    Color::absorptionAtDistance(color, params.distance));
             materialInstance->setParameter("ior", params.ior);
             materialInstance->setParameter("transmission", params.transmission);
             materialInstance->setParameter("thickness", params.thickness);