Fixed un-initialized BSDF component bug in MixedBsdf.cpp

[Enigmatisms]

Hi, Benedikt!

I am working on this renderer and I think I found a bug. In line 132-135 of src/core/bsdfs/MixedBsdf.cpp

void MixedBsdf::prepareForRender()
{
    _lobes = BsdfLobes(_bsdf0->lobes(), _bsdf1->lobes());
}

prepareForRender is only called for this class, yet we should also call the prepareForRender method for its components (bsdf0 and bsdf1). This could be serious, since most BSDFs requires prepareForRender to compute the correct inputs.

Here is an example (the actual problem I encountered):

• bsdf0 is RoughPlasticBsdf
• bsdf1 is DiffuseTransmissionBsdf

For RoughPlasticBsdf, since the prepareForRender method is never called: _substrateWeight is 0, so I am having unexpected output for the whole afternoon) and printf is not called, therefore, no terminal output.

_scaledSigmaA = _thickness*_sigmaA;
_avgTransmittance = std::exp(-2.0f*_scaledSigmaA.avg());
_substrateWeight = _albedo->average().avg();
printf("`RoughPlasticBsdf` Prepare for render: %f\n", _substrateWeight);
_diffuseFresnel = Fresnel::computeDiffuseFresnel(_ior, 1000000);

The proposed fix (rather simple):

void MixedBsdf::prepareForRender()
{
    _lobes = BsdfLobes(_bsdf0->lobes(), _bsdf1->lobes());
    // manually call the method:
    _bsdf0->prepareForRender();
    _bsdf1->prepareForRender();
}

Terminal output before fixing:

Loading scene '..\..\data\example-scenes\plant\convert.json'...
`prepareForRender` called for the mixed BSDF.
// Nothing then

After fixing:

Loading scene '..\..\data\example-scenes\plant\convert.json'...
`prepareForRender` called for the mixed BSDF.
`RoughPlasticBsdf` Prepare for render: 0.403922

Rendering result before and after fixing this bug. I guess it is a well-hidden bug, since I won't call the figure 'incorrect' before fixing. However, when I need the diffuse substrate but it is missing, I know there should be something wrong:

before fixing	after fixing

[odil24]

@Enigmatisms Hi. Great solution! Are you have a build for windows? if so can you share with me to test out?

[Enigmatisms]

@odil24 Hi! Do you mean the scene file? I am building on windows, but I think this works perfectly fine on Linux.

[odil24]

@odil24 Hi! Do you mean the scene file? I am building on windows, but I think this works perfectly fine on Linux.

No i mean you said that you have been working on the renderer so can you share with me binary build of your continued renderer for testing?

[Enigmatisms]

Closing this PR due to being ignored for more than 1 year.