Question about rendering normals with BSDF normalmap awareness

[saeedhd96]

Hi,

I am trying to render a normal map, similar to the aov integrator sh_normal, but I need the normal map to be perturbed on a certain object according to the BSDF normalmap on that shape.

I wrote a custom integrator inspired by normalmap.cpp this line. The issue is the final result looks a little weird to me. I was wondering this is right way to do this?

class NormalIntegrator(mi.SamplingIntegrator):
    def sample(self: mi.SamplingIntegrator, scene: mi.Scene, sampler, ray: mi.RayDifferential3f, medium: mi.Medium = None, active: bool = True):
        si:mi.SurfaceInteraction3f = scene.ray_intersect(ray, mi.RayFlags.All, True, active)
        valid_ray = active & si.is_valid()
        result = mi.Spectrum(0)

        #taken from normalmap.cpp
        def frame(si, active):    
            n = self.normal_map.eval_3(si, active) * 2 -1

            result = mi.Frame3f()
            result.n = dr.normalize(n)
            result.s = dr.normalize(result.n * dr.dot(result.n, si.dp_du) - si.dp_du)
            result.t = dr.cross(result.n, result.s)
            return result

        f = frame(si, si.is_valid())
        sh_normal = si.sh_frame.n       #shading normal
        perturbed_n = f.to_local(sh_normal)   #perturb the original normal
        mask = dr.eq(si.shape, self.shape_of_interest)
        result = dr.select(mask, perturbed_n , sh_normal)    #only for the object of interest, show the perturbed, otherwise show the shading normal

        return result, valid_ray, []

normal_integrator = NormalIntegrator(mi.Properties())



# ... find the shape of interest using scene.shapes()[index]


normal_map = mi.load_dict({'type': 'bitmap',
                    'raw': True,
                    'filename': r'..\data\scenes_mistuba\kitchen\textures\metal\Metal009_4K-PNG_NormalGL.png'
                })

normal_integrator.normal_map = normal_map
normal_integrator.shape_of_interest = shape_of_interest

# my integrator result
normal_rendering = mi.render(scene, integrator=normal_integrator, sensor=sensor)/2 + 0.5
mi.Bitmap(normal_rendering).convert(mi.Bitmap.PixelFormat.RGB, mi.Struct.Type.UInt8, True).write('perturbed.png')

AOV 'sh_normal' result:

My integrator result:

The normal map:

[saeedhd96]

Thanks to @tizian, the correct answer is:

class NormalIntegrator(mi.SamplingIntegrator):
    def sample(self: mi.SamplingIntegrator, scene: mi.Scene, sampler, ray: mi.RayDifferential3f, medium: mi.Medium = None, active: bool = True):
        si:mi.SurfaceInteraction3f = scene.ray_intersect(ray, mi.RayFlags.All, True, active)
        valid_ray = active & si.is_valid()
        result = mi.Spectrum(0)
        def frame(si, active):
            n = self.normal_map.eval_3(si, active) * 2 -1

            result = mi.Frame3f()
            result.n = dr.normalize(n)
            result.s = dr.normalize(result.n * dr.dot(result.n, si.dp_du) - si.dp_du)
            result.t = dr.cross(result.n, result.s)
            return result

        f = frame(si, si.is_valid())
        perturbed_n = si.sh_frame.to_world(f.n)        
        sh_normal = si.sh_frame.n
        mask = dr.eq(si.shape, self.shape_of_interest)
        result = dr.select(mask, perturbed_n, sh_normal)
        return result, valid_ray, []