Make blend shape normals/tangents opt-in. (#40)

The FBX SDK absolutely claims that there is a normal layer to each
FbxShape, with non-trivial data, even when the corresponding FBX file,
upon visual inspection, explicitly contains nothing but zeroes. The only
conclusion I can draw is that the SDK is computing normals from
geometry, without being asked to, which seems kind of sketchy.

These computed normals are often not at all what the artist wanted, they
take up a lot of space -- often pointlessly, since if they're computed,
we could just as well compute them on the client -- and at least in the
case of three.js their inclusion uses up many of the precious 8 morph
target slots in the shader.

So, they are now opt-in, at least until we can solve the mystery of just
what goes on under the hood in the SDK.

README.md

@@ -48,6 +48,10 @@ Usage:
       --pbr-specular-glossiness
                                 (WIP) Experimentally fill in the
                                 KHR_materials_pbrSpecularGlossiness extension.
+      --blend-shape-normals     Include blend shape normals, if reported
+                                present by the FBX SDK.
+      --blend-shape-tangents    Include blend shape tangents, if reported
+                                present by the FBX SDK.
   -k, --keep-attribute arg      Used repeatedly to build a limiting set of
                                 vertex attributes to keep.
   -v, --verbose                 Enable verbose output.
@@ -81,6 +85,13 @@ Some of these switches are not obvious:
   the conversion process. This is a way to trim the size of the resulting glTF
   if you know the FBX contains superfluous attributes. The supported arguments
   are `position`, `normal`, `tangent`, `color`, `uv0`, and `uv1`.
+- When **blend shapes** are present, you may use `--blend-shape-normals` and
+  `--blend-shape-tangents` to include normal and tangent attributes in the glTF
+  morph targets. They are not included by default because they rarely or never
+  seem to be correctly present in the actual FBX source, which means the SDK
+  must be computing them from geometry, unasked? In any case, they are beyond
+  the control of the artist, and can yield strange crinkly behaviour. Since
+  they also take up significant space in the output file, we made them opt-in.
 
 ## Building it on your own
 

src/Raw2Gltf.cpp

@@ -619,10 +619,10 @@ ModelData *Raw2Gltf(
                         auto blendVertex = surfaceModel.GetVertex(jj).blends[channelIx];
                         shapeBounds.AddPoint(blendVertex.position);
                         positions.push_back(blendVertex.position);
-                        if (channel.hasNormals) {
+                        if (options.useBlendShapeTangents && channel.hasNormals) {
                             normals.push_back(blendVertex.normal);
                         }
-                        if (channel.hasTangents) {
+                        if (options.useBlendShapeTangents && channel.hasTangents) {
                             tangents.push_back(blendVertex.tangent);
                         }
                     }
@@ -633,14 +633,14 @@ ModelData *Raw2Gltf(
                     pAcc->max = toStdVec(shapeBounds.max);
 
                     std::shared_ptr<AccessorData> nAcc;
-                    if (channel.hasNormals) {
+                    if (!normals.empty()) {
                         nAcc = gltf->AddAccessorWithView(
                             *gltf->GetAlignedBufferView(buffer, BufferViewData::GL_ARRAY_BUFFER),
                             GLT_VEC3F, normals);
                     }
 
                     std::shared_ptr<AccessorData> tAcc;
-                    if (channel.hasTangents) {
+                    if (!tangents.empty()) {
                         nAcc = gltf->AddAccessorWithView(
                             *gltf->GetAlignedBufferView(buffer, BufferViewData::GL_ARRAY_BUFFER),
                             GLT_VEC4F, tangents);

src/Raw2Gltf.h

@@ -53,6 +53,10 @@ struct GltfOptions
     bool usePBRMetRough;
     /** Whether to use KHR_materials_pbrSpecularGlossiness to extend material definitions. */
     bool usePBRSpecGloss;
+    /** Whether to include blend shape normals, if present according to the SDK. */
+    bool useBlendShapeNormals;
+    /** Whether to include blend shape tangents, if present according to the SDK. */
+    bool useBlendShapeTangents;
 };
 
 struct ComponentType {

src/main.cpp

@@ -49,7 +49,9 @@ int main(int argc, char *argv[])
         false,         // useDraco
         false,         // useKHRMatCom
         false,         // usePBRMetRough
-        false        // usePBRSpecGloss
+        false,         // usePBRSpecGloss
+        false,         // useBlendShapeNormals
+        false,         // useBlendShapeTangents
     };
 
     options.positional_help("[<FBX File>]");
@@ -81,6 +83,12 @@ int main(int argc, char *argv[])
                (
                    "pbr-specular-glossiness", "(WIP) Experimentally fill in the KHR_materials_pbrSpecularGlossiness extension.",
                    cxxopts::value<bool>(gltfOptions.usePBRSpecGloss))
+               (
+                   "blend-shape-normals", "Include blend shape normals, if reported present by the FBX SDK.",
+                   cxxopts::value<bool>(gltfOptions.useBlendShapeNormals))
+               (
+                   "blend-shape-tangents", "Include blend shape tangents, if reported present by the FBX SDK.",
+                   cxxopts::value<bool>(gltfOptions.useBlendShapeTangents))
                (
                    "k,keep-attribute", "Used repeatedly to build a limiting set of vertex attributes to keep.",
                    cxxopts::value<std::vector<std::string>>())