Rethink SurfaceInteraction regarding C++ mutable members

[wahn]

In C++ the class SurfaceInteraction looks like this:

class SurfaceInteraction : public Interaction {                                                         
  public:                                                                                               
...
    mutable Vector3f dpdx, dpdy;                                                                        
    mutable Float dudx = 0, dvdx = 0, dudy = 0, dvdy = 0;                                               
...                                                                                                        
};                                                                                                      

Most likely we should try to use Rust's interior mutability, e.g. RefCell. See C++ vs Rust and search for mutable keyword.

[wahn]

From the module cell documentation:

Consider using RwLock<T> or Mutex<T> if you need shared mutability in a multi-threaded situation.

[wahn]

I created a branch (issue_114) for this issue. Currently the Rust side looks like this:

pub struct SurfaceInteraction<'p, 's> {
    // Interaction Public Data
    pub p: Point3f,
    pub time: Float,
    pub p_error: Vector3f,
    pub wo: Vector3f,
    pub n: Normal3f,
    pub medium_interface: Option<Arc<MediumInterface>>,
    // SurfaceInteraction Public Data
    pub uv: Point2f,
    pub dpdu: Vector3f,
    pub dpdv: Vector3f,
    pub dndu: Normal3f,
    pub dndv: Normal3f,
    pub dpdx: RwLock<Vector3f>,
    pub dpdy: RwLock<Vector3f>,
    pub dudx: RwLock<Float>,
    pub dvdx: RwLock<Float>,
    pub dudy: RwLock<Float>,
    pub dvdy: RwLock<Float>,
    pub primitive: Option<&'p GeometricPrimitive>,
    pub shading: Shading,
    pub bsdf: Option<Arc<Bsdf>>,
    pub bssrdf: Option<Arc<TabulatedBssrdf>>,
    pub shape: Option<&'s dyn Shape>,
}

I made the remaining files compile, but the mlt and bdpt algorithms had to be removed. I think I have to find a better solution for the pointers involved. On the C++ side they are part of a struct and a class:

struct Interaction {                                                                                    
...
    MediumInterface mediumInterface;                                                                    
};                                                                                                      
...
class SurfaceInteraction : public Interaction {
...
    const Shape *shape = nullptr;                                                                       
...
    const Primitive *primitive = nullptr;                                                               
    BSDF *bsdf = nullptr;                                                                               
    BSSRDF *bssrdf = nullptr;                                                                           
};                                                                                                

[wahn]

After commit 031de340a2e3e5b250fd9e1f0a3d591c9d8e37f3 we got rid of the lifetimes:

pub struct SurfaceInteraction {                                                                         
    // Interaction Public Data                                                                          
    pub p: Point3f,                                                                                     
    pub time: Float,                                                                                    
    pub p_error: Vector3f,                                                                              
    pub wo: Vector3f,                                                                                   
    pub n: Normal3f,                                                                                    
    pub medium_interface: Option<Arc<MediumInterface>>,                                                 
    // SurfaceInteraction Public Data                                                                   
    pub uv: Point2f,                                                                                    
    pub dpdu: Vector3f,                                                                                 
    pub dpdv: Vector3f,                                                                                 
    pub dndu: Normal3f,                                                                                 
    pub dndv: Normal3f,                                                                                 
    pub dpdx: RwLock<Vector3f>,                                                                         
    pub dpdy: RwLock<Vector3f>,                                                                         
    pub dudx: RwLock<Float>,                                                                            
    pub dvdx: RwLock<Float>,                                                                            
    pub dudy: RwLock<Float>,                                                                            
    pub dvdy: RwLock<Float>,                                                                            
    pub primitive: Option<Arc<dyn Primitive + Send + Sync>>,                                            
    pub shading: Shading,                                                                               
    pub bsdf: Option<Arc<Bsdf>>,                                                                        
    pub bssrdf: Option<Arc<TabulatedBssrdf>>,                                                           
    pub shape: Option<Arc<dyn Shape + Send + Sync>>,                                                    
}                                                                                                       

[wahn]

Let's see if we can make the mlt and bdpt algorithms work again ...

[wahn]

After commit e5ca87eff23fefcc4844e913398270299fe0b5a9 bidirectional pathtracing works again:

> ./target/release/rs_pbrt -i assets/scenes/cafe_scene2.pbrt
...
Integrator "bdpt"
  "integer maxdepth" [3]
...
Rendering with 8 thread(s) ...
1024 / 1024 [=======================================================================] 100.00 % 12.19/s  
Writing image "pbrt.png" with bounds Bounds2 { p_min: Point2 { x: 0, y: 0 }, p_max: Point2 { x: 500, y: 500 } }

Now we just need mlt to work again before we can merge the branch back to the master branch.

[wahn]

After commit 84ec3646809fb4f299ccae28609dc8a1f8b97e06 mlt should work again. Everything is merged into the master branch. Let's close the issue then ...