Volume Grid Isosurface mesh issue

src/volume_grid_scalar_quantity.cpp から始め、MarchingCube の呼び出しと座標変換が示されている createIsosurfaceProgram() と registerIsosurfaceAsMesh() を調べます。両方のパスにおけるグリッド次元の順序を、ボリュームバッファのレイアウトと比較します。volume-grid の等値面が、両方の用途で正しい向きとセル順序でレンダリングされれば完了です。

Hi,

I've run into an issue with the isosurface generation for volume grid scalar quantities. The problem appears to be that the change made to fix the frame transformation for #253 wasn't propagated into the grid dimensions passed to the MarchingCube library, resulting in the grid cells being interpreted in the wrong order.

The following shows a volume grid of the Stanford Bunny and its isosurface visualization with the 2.2.1 release:

By swizzling the grid dimensions passed into MarchingCube we get the following (correct) result instead:

The patch for the above fix is:

diff --git a/src/volume_grid_scalar_quantity.cpp b/src/volume_grid_scalar_quantity.cpp                                 
index cde441a..603841f 100644                                                                                          
--- a/src/volume_grid_scalar_quantity.cpp                                                                              
+++ b/src/volume_grid_scalar_quantity.cpp                                                                              
@@ -165,8 +165,8 @@ void VolumeGridNodeScalarQuantity::createIsosurfaceProgram() {                                     
                                                                                                                       
   // Extract the isosurface from the level set of the scalar field                                                    
   MC::mcMesh isosurfaceMesh;                                                                                          
-  MC::marching_cube(&values.data.front(), isosurfaceLevel.get(), parent.getGridNodeDim().x, parent.getGridNodeDim().y,
-                    parent.getGridNodeDim().z, isosurfaceMesh);                                                       
+  MC::marching_cube(&values.data.front(), isosurfaceLevel.get(), parent.getGridNodeDim().z, parent.getGridNodeDim().y,
+                    parent.getGridNodeDim().x, isosurfaceMesh);                                                       
                                                                                                                       
   // Transform the result to be aligned with our volume's spatial layout                                              
   glm::vec3 scale = parent.gridSpacing();                                                                             
@@ -215,8 +215,8 @@ SurfaceMesh* VolumeGridNodeScalarQuantity::registerIsosurfaceAsMesh(std::string                    
                                                                                                                       
   // extract the mesh                                                                                                 
   MC::mcMesh isosurfaceMesh;                                                                                          
-  MC::marching_cube(&values.data.front(), isosurfaceLevel.get(), parent.getGridNodeDim().x, parent.getGridNodeDim().y,
-                    parent.getGridNodeDim().z, isosurfaceMesh);                                                       
+  MC::marching_cube(&values.data.front(), isosurfaceLevel.get(), parent.getGridNodeDim().z, parent.getGridNodeDim().y,
+                    parent.getGridNodeDim().x, isosurfaceMesh);                                                       
   glm::vec3 scale = parent.gridSpacing();                                                                             
   for (auto& p : isosurfaceMesh.vertices) {                                                                           
     // swizzle to account for change of coordinate/buffer ordering in the MC lib                                      

I'm not sure how you prefer to manage the repo, but if you'd like me to open a PR for the fix I'm happy to. Thanks!