When is the rendering equation inaccurate?

So if you want to render a photorealistic object, you need to obey this rendering equation? Is this the biggest difference between blinn-phong and the photorealistic render(cause for blinn-phong doesn't obey that rendering equation)?

@Ashley The Blinn-Phong reflection model is an example of a scattering function or BRDF, f_r. Choosing the correct scattering function is important to accurately model how materials reflect light, and having good geometric and reflectance models is one important component when generating photorealistic images. For example, a Blinn-Phong model may not accurately depict how light interacts with skin.

But, to be clear, scattering functions are used in both rasterizers and path tracers. So having a good scattering function is not enough to produce photorealistic images.

@brk @Ashley The rendering equation describes a way to compute the contribution of light bouncing N times through an environment. Each additional bounce is given by an additional recursive call to the function L_i.

Now, this particular form of the rendering equation assumes that light enters and exits the same point p during a reflection. But there are minor modifications that one can make to this expression to handle light scattering through a volume, or to model transient light transport.

The key limitation associated with this equation is that it assumes geometric optics---light that travels in straight lines. It is not appropriate for simulating wave-like phenomena (e.g., Thomas Young's double slit experiment).