From 7eb43eb21bbb0b8f29aa1422d8fc92b524ba1bcb Mon Sep 17 00:00:00 2001
From: Anders Langlands <alanglands@nvidia.com>
Date: Tue, 21 May 2024 12:22:04 -0700
Subject: [PATCH] [hdEmbree] PxrIESFile: add eval() and valid() methods

---
 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp | 104 ++++++++++++++++--
 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h   |  10 ++
 2 files changed, 107 insertions(+), 7 deletions(-)

diff --git a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
index 407ae08e67..b05dbe78af 100644
--- a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
+++ b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
@@ -6,6 +6,8 @@
 //
 #include "pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h"
 
+#include "pxr/base/gf/math.h"
+
 #include <algorithm>
 
 
@@ -16,11 +18,43 @@
 #define M_PI 3.14159265358979323846
 #endif
 
+namespace {
+
+// -------------------------------------------------------------------------
+// Constants
+// -------------------------------------------------------------------------
+
+template <typename T>
+constexpr T _pi = static_cast<T>(M_PI);
+
+constexpr float _hemisphereFudgeFactor = 0.1f;
+
+// -------------------------------------------------------------------------
+// Utility functions
+// -------------------------------------------------------------------------
+
+
+float
+_linearstep(float x, float a, float b)
+{
+    if (x <= a) {
+        return 0.0f;
+    }
+
+    if (x >= b) {
+        return 1.0f;
+    }
+
+    return (x - a) / (b - a);
+}
+
+}  // anonymous namespace
+
 
 PXR_NAMESPACE_OPEN_SCOPE
 
 bool
-PxrIESFile::load(std::string const& ies)  // non-virtual "override"
+PxrIESFile::load(const std::string &ies)  // non-virtual "override"
 {
     clear();
     if (!Base::load(ies)) {
@@ -47,7 +81,8 @@ PxrIESFile::pxr_extra_process()
 
     // does the distribution cover the whole sphere?
     bool is_sphere = false;
-    if ((*v_angleMax - *v_angleMin) > (M_PI / 2 + 0.1 /* fudge factor*/)) {
+    if ((*v_angleMax - *v_angleMin)
+            > (_pi<float> / 2.0f + _hemisphereFudgeFactor)) {
         is_sphere = true;
     }
 
@@ -60,19 +95,74 @@ PxrIESFile::pxr_extra_process()
             float dh = h_angles[h + 1] - h_angles[h];
             float dv = v_angles[v + 1] - v_angles[v];
             // bilinearly interpolate intensity at the patch center
-            float i0 = (intensity[h][v] + intensity[h][v + 1]) / 2;
+            float i0 = (intensity[h][v] + intensity[h][v + 1]) / 2.0f;
             float i1 =
-                (intensity[h + 1][v] + intensity[h + 1][v + 1]) / 2;
-            float center_intensity = (i0 + i1) / 2;
+                (intensity[h + 1][v] + intensity[h + 1][v + 1]) / 2.0f;
+            float center_intensity = (i0 + i1) / 2.0f;
             // solid angle of the patch
-            float dS = dh * dv * sinf(v_angles[v] + dv / 2);
+            float dS = dh * dv * sinf(v_angles[v] + dv / 2.0f);
             _power += dS * center_intensity;
         }
     }
 
     // ...and divide by surface area of a unit sphere (or hemisphere)
     // (this result matches Karma & RIS)
-    _power /= M_PI * (is_sphere ? 4 : 2);
+    _power /= _pi<float> * (is_sphere ? 4.0f : 2.0f);
+}
+
+float
+PxrIESFile::eval(float theta, float phi, float angleScale) const
+{
+    int hi = -1;
+    int vi = -1;
+    float dh = 0.0f;
+    float dv = 0.0f;
+
+    phi = GfMod(phi, 2.0f * _pi<float>);
+    for (size_t i = 0; i < h_angles.size() - 1; ++i) {
+        if (phi >= h_angles[i] && phi < h_angles[i + 1]) {
+            hi = i;
+            dh = _linearstep(phi, h_angles[i], h_angles[i + 1]);
+            break;
+        }
+    }
+
+    // This formula matches Renderman's behavior
+
+    // Scale with origin at "top" (ie, 180 degress / pi), by a factor
+    // of 1 / (1 + angleScale), offset so that angleScale = 0 yields the
+    // identity function.
+    const float profileScale = 1.0f + angleScale;
+    theta = (theta - _pi<float>) / profileScale + _pi<float>;
+    theta = GfClamp(theta, 0.0f, _pi<float>);
+
+    if (theta < 0) {
+        // vi = 0;
+        // dv = 0;
+        return 0.0f;
+    } else if (theta >= _pi<float>) {
+        vi = v_angles.size() - 2;
+        dv = 1;
+    } else {
+        for (size_t i = 0; i < v_angles.size() - 1; ++i) {
+            if (theta >= v_angles[i] && theta < v_angles[i + 1]) {
+                vi = i;
+                dv = _linearstep(theta, v_angles[i], v_angles[i + 1]);
+                break;
+            }
+        }
+    }
+
+    if (hi == -1 || vi == -1) {
+        // XXX: need to indicate error somehow here
+        return 0.0f;
+    }
+
+    // XXX: This should be a cubic interpolation
+    float i0 = GfLerp(dv, intensity[hi][vi], intensity[hi][vi + 1]);
+    float i1 = GfLerp(dv, intensity[hi + 1][vi], intensity[hi + 1][vi + 1]);
+
+    return GfLerp(dh, i0, i1);
 }
 
 PXR_NAMESPACE_CLOSE_SCOPE
diff --git a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h
index a406b62531..3378146df2 100644
--- a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h
+++ b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h
@@ -33,6 +33,16 @@ class PxrIESFile : public pxr_ccl::IESFile {
         return _power;
     }
 
+    // returns true if the IES files was successfully loaded and processed and
+    // is ready to evaluate
+    bool valid() const
+    {
+        return !intensity.empty();
+    }
+
+    // evaluate the IES file for the given spherical coordinates
+    float eval(float theta, float phi, float angleScale) const;
+
 protected:
     // Extra processing we do on-top of the "standard" process() from IESFile
     void pxr_extra_process();