From d714b89014d019a1bbead105ad382e002e30649b Mon Sep 17 00:00:00 2001
From: Paul Molodowitch <pmolodowitch@nvidia.com>
Date: Wed, 5 Jun 2024 17:23:34 -0700
Subject: [PATCH] [hdEmbree] ies: add PxrIESFile subclass, which calculates
 power via integration

---
 pxr/imaging/plugin/hdEmbree/CMakeLists.txt    |  2 +
 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp | 78 +++++++++++++++++++
 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h   | 47 +++++++++++
 3 files changed, 127 insertions(+)
 create mode 100644 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
 create mode 100644 pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h

diff --git a/pxr/imaging/plugin/hdEmbree/CMakeLists.txt b/pxr/imaging/plugin/hdEmbree/CMakeLists.txt
index 22ba8bc76b..8e4a5d68b8 100644
--- a/pxr/imaging/plugin/hdEmbree/CMakeLists.txt
+++ b/pxr/imaging/plugin/hdEmbree/CMakeLists.txt
@@ -51,10 +51,12 @@ pxr_plugin(hdEmbree
 
     PRIVATE_HEADERS
         pxrIES/ies.h
+        pxrIES/pxrIES.h
         pxrPbrt/pbrtUtils.h
 
     CPPFILES
         pxrIES/ies.cpp
+        pxrIES/pxrIES.cpp
 
     RESOURCE_FILES
         plugInfo.json
diff --git a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
new file mode 100644
index 0000000000..407ae08e67
--- /dev/null
+++ b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.cpp
@@ -0,0 +1,78 @@
+//
+// Copyright 2024 Pixar
+//
+// Licensed under the terms set forth in the LICENSE.txt file available at
+// https://openusd.org/license.
+//
+#include "pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h"
+
+#include <algorithm>
+
+
+#define _USE_MATH_DEFINES
+#include <cmath>
+
+#if !defined(M_PI)
+#define M_PI 3.14159265358979323846
+#endif
+
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+bool
+PxrIESFile::load(std::string const& ies)  // non-virtual "override"
+{
+    clear();
+    if (!Base::load(ies)) {
+        return false;
+    }
+    pxr_extra_process();
+    return true;
+}
+
+void
+PxrIESFile::clear()  // non-virtual "override"
+{
+    Base::clear();
+    _power = 0;
+}
+
+void
+PxrIESFile::pxr_extra_process()
+{
+    // find max v_angle delta, as a way to estimate whether the distribution
+    // is over a hemisphere or sphere
+    const auto [v_angleMin, v_angleMax] = std::minmax_element(
+        v_angles.cbegin(), v_angles.cend());
+
+    // does the distribution cover the whole sphere?
+    bool is_sphere = false;
+    if ((*v_angleMax - *v_angleMin) > (M_PI / 2 + 0.1 /* fudge factor*/)) {
+        is_sphere = true;
+    }
+
+    _power = 0;
+
+    // integrate the intensity over solid angle to get power
+    for (size_t h = 0; h < h_angles.size() - 1; ++h) {
+        for (size_t v = 0; v < v_angles.size() - 1; ++v) {
+            // approximate dimensions of the patch
+            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 i1 =
+                (intensity[h + 1][v] + intensity[h + 1][v + 1]) / 2;
+            float center_intensity = (i0 + i1) / 2;
+            // solid angle of the patch
+            float dS = dh * dv * sinf(v_angles[v] + dv / 2);
+            _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);
+}
+
+PXR_NAMESPACE_CLOSE_SCOPE
diff --git a/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h
new file mode 100644
index 0000000000..a406b62531
--- /dev/null
+++ b/pxr/imaging/plugin/hdEmbree/pxrIES/pxrIES.h
@@ -0,0 +1,47 @@
+//
+// Copyright 2024 Pixar
+//
+// Licensed under the terms set forth in the LICENSE.txt file available at
+// https://openusd.org/license.
+//
+#ifndef PXR_IMAGING_PLUGIN_HD_EMBREE_PXRIES_PXRIES_H
+#define PXR_IMAGING_PLUGIN_HD_EMBREE_PXRIES_PXRIES_H
+
+#include "pxr/pxr.h"
+#include "pxr/imaging/plugin/hdEmbree/pxrIES/ies.h"
+
+#include <string>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+/// \class PxrIESFile
+///
+/// Extends / overrides some functionality of standard IESFile.
+///
+class PxrIESFile : public pxr_ccl::IESFile {
+private:
+    using Base = pxr_ccl::IESFile;
+
+public:
+
+    bool load(std::string const& ies);  // non-virtual "override"
+    void clear();  // non-virtual "override"
+
+    /// \brief The light's power, as calculated when parsing
+    inline float power() const
+    {
+        return _power;
+    }
+
+protected:
+    // Extra processing we do on-top of the "standard" process() from IESFile
+    void pxr_extra_process();
+
+private:
+
+    float _power = 0;
+};
+
+PXR_NAMESPACE_CLOSE_SCOPE
+
+#endif // PXR_IMAGING_PLUGIN_HD_EMBREE_PXRIES_PXRIES_H