From aab3c8010c690db86affc2670ed3b72f2b108e88 Mon Sep 17 00:00:00 2001
From: Scott Johnston <scott.johnston@warneranimationgroup.net>
Date: Thu, 15 Jun 2023 12:16:06 -0700
Subject: [PATCH] Correct from full FOV to assimp half-fov in camera. Compute
 from filmWidth and focalLength if FOV not specified.

---
 code/AssetLib/FBX/FBXConverter.cpp | 24 ++++++++++++++++++++----
 code/AssetLib/FBX/FBXDocument.h    |  6 +++++-
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/code/AssetLib/FBX/FBXConverter.cpp b/code/AssetLib/FBX/FBXConverter.cpp
index 51ec79583..cca5da596 100644
--- a/code/AssetLib/FBX/FBXConverter.cpp
+++ b/code/AssetLib/FBX/FBXConverter.cpp
@@ -93,6 +93,8 @@ FBXConverter::FBXConverter(aiScene *out, const Document &doc, bool removeEmptyBo
         mSceneOut(out),
         doc(doc),
         mRemoveEmptyBones(removeEmptyBones) {
+
+
     // animations need to be converted first since this will
     // populate the node_anim_chain_bits map, which is needed
     // to determine which nodes need to be generated.
@@ -427,12 +429,26 @@ void FBXConverter::ConvertCamera(const Camera &cam, const std::string &orig_name
     out_camera->mLookAt = aiVector3D(1.0f, 0.0f, 0.0f);
     out_camera->mUp = aiVector3D(0.0f, 1.0f, 0.0f);
 
-    out_camera->mHorizontalFOV = AI_DEG_TO_RAD(cam.FieldOfView());
+    // NOTE: Some software (maya) does not put FieldOfView in FBX, so we compute
+    // mHorizontalFOV from FocalLength and FilmWidth with unit conversion.
 
-    out_camera->mClipPlaneNear = cam.NearPlane();
-    out_camera->mClipPlaneFar = cam.FarPlane();
+    // TODO: This is not a complete solution for how FBX cameras can be stored.
+    // TODO: Incorporate non-square pixel aspect ratio.
+    // TODO: FBX aperture mode might be storing vertical FOV in need of conversion with aspect ratio.
+
+    float fov_deg = cam.FieldOfView();
+    // If FOV not specified in file, compute using FilmWidth and FocalLength.
+    if (fov_deg == FBX_FOV_UNKNOWN) {
+        float film_width_inches = cam.FilmWidth();
+        float focal_length_mm = cam.FocalLength();
+        ASSIMP_LOG_VERBOSE_DEBUG("FBX FOV unspecified. Computing from FilmWidth (", film_width_inches, "inches) and FocalLength (", focal_length_mm, "mm).");
+        double half_fov_rad = std::atan2(film_width_inches * 25.4 * 0.5, focal_length_mm);
+        out_camera->mHorizontalFOV = half_fov_rad;
+    } else {
+        // FBX fov is full-view degrees. We want half-view radians.
+        out_camera->mHorizontalFOV = AI_DEG_TO_RAD(fov_deg) * 0.5;
+    }
 
-    out_camera->mHorizontalFOV = AI_DEG_TO_RAD(cam.FieldOfView());
     out_camera->mClipPlaneNear = cam.NearPlane();
     out_camera->mClipPlaneFar = cam.FarPlane();
 }
diff --git a/code/AssetLib/FBX/FBXDocument.h b/code/AssetLib/FBX/FBXDocument.h
index e229eef52..503c94833 100644
--- a/code/AssetLib/FBX/FBXDocument.h
+++ b/code/AssetLib/FBX/FBXDocument.h
@@ -55,9 +55,13 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define _AI_CONCAT(a,b)  a ## b
 #define  AI_CONCAT(a,b)  _AI_CONCAT(a,b)
 
+/* Use an 'illegal' default FOV value to detect if the FBX camera has set the FOV. */
+#define FBX_FOV_UNKNOWN -1.0f
+
 namespace Assimp {
 namespace FBX {
 
+
 class Parser;
 class Object;
 struct ImportSettings;
@@ -247,7 +251,7 @@ public:
     fbx_simple_property(FilmAspectRatio, float, 1.0f)
     fbx_simple_property(ApertureMode, int, 0)
 
-    fbx_simple_property(FieldOfView, float, 1.0f)
+    fbx_simple_property(FieldOfView, float, FBX_FOV_UNKNOWN)
     fbx_simple_property(FocalLength, float, 1.0f)
 };