Formation of reflection image, and specular highlights?

[sanchor]

Hi

Not sure of optical questions should go here to general physics questions, but here goes.
I have a doubt, basically, what consists of a reflection?

If i have a perfectly smooth surface, then the specular highlights are infinitely thin, and you should have perfectly mirror-like reflections, a mirror like surface.
If you have a rough surface, rough as in microstructure (say, like the V groove microfacet models in the Cook-Torrance BRDF), then we're modelling the probability of these microfacets being oriented in such a way that they reflect light towards the viewer, with a distribution function, say, the Beckmann probability distibution function. You then calculate the shadowing and obscuration effect these microfacets would have on the adjacent microfacets, and finally calculate the amount of reflected light from these microfacets using a Fresnel term (dependent on the incident (light) direction, and the microfacet normal (since we want the amount reflected off the microstructure, not the macrostructure). So far so good (please correct me if I'm wrong) ?

In this latest case, of a rough surface, then specular rays are reflected in a series of directions by the microstructure, this would mean that a reflection would also be scattered in these directions, so, you would have a fuzzy/blurry reflection. Is this correct?

My doubt is, is there any difference at all from a reflection ray or a specular ray?
From a point of view of shader writing for instance. I'm thinking that perhaps perfectly smooth surfaces would have infinitely small specular highlights, and coherent, mirror like reflections, while rough surfaces would have scattered specular highlights, and incoherent, fuzzy/blurry reflections. If this is the case, then from a physics point of view, there is no distintion between the process of forming a specular highlight, and the process of forming a reflected image. The only difference would lie in whatever way you decided to implement this in a shader, is this correct?

What about global illumination? In RenderMan renderers such as 3delight, pixie, etc... you can gather rays using 2 prebuilt distributions, cosine or uniform. Say you want to gather reflection rays along a direction vector V, with a solid angle X radians, to build a reflection image, and you use an anisotropic distribution, or an isotropic distribution (say, the Ward distribution for instance), would this be equivalent to building a specular term? To creating the contribution of the specular highlights, since a narrower sampling cone would mean a smoother surface, therefore a sharper reflection, and a wider sampling cone would mean a rougher surface, therefore a fuzzier/blurrier reflection?

What's happening in the physical/optical point of view in the process of forming a specular highlight and a reflection, are they different processes at all, or exactly the same, and the distintion is artificially created by whatever coding/shader/rendering implementation you might use?

Thanks in advance for your time.

Best regards

P.S.: in the microfacets case, what happens to light when the spacing between microfacets is equal to a particular light wavelenght, say 550nm? Is there a shift towards a lower or higher wavelenght? Or just a cancelation?

Sancho R.

 

[eljose79]

Hello Sancho,

First of all, great questions! Reflection is a fundamental concept in optics and understanding it is crucial in many scientific fields, including physics and engineering. Let me try to address your questions one by one:

1. What consists of a reflection?
A reflection is the bouncing back of light from a surface. This can happen in two ways: specular reflection and diffuse reflection. Specular reflection is when light reflects off a smooth surface at a specific angle, like a mirror. Diffuse reflection is when light reflects off a rough surface in many different directions, like a wall. Both types of reflection involve the absorption and re-emission of light by the atoms or molecules on the surface.

2. Is there any difference between a reflection ray and a specular ray?
In terms of physics, a reflection ray and a specular ray are the same thing. They both refer to the light that is reflected off a surface. From a shader writing perspective, however, there may be differences in how you handle these rays depending on the surface's properties and the effect you want to achieve in your rendering.

3. What about global illumination?
Global illumination is a technique used in computer graphics to model how light bounces off surfaces and illuminates a scene. It takes into account not only direct light sources, but also indirect light that bounces off surfaces and illuminates other surfaces in the scene. In terms of specular highlights and reflections, global illumination can help create more realistic and accurate effects, especially when dealing with rough surfaces.

4. What's happening in the physical/optical point of view in the process of forming a specular highlight and a reflection?
As mentioned before, both specular highlights and reflections involve the absorption and re-emission of light by the atoms or molecules on a surface. The difference lies in the surface's properties and the angle at which the light hits the surface. A perfectly smooth surface will produce a clear and defined specular highlight, while a rough surface will produce a diffuse reflection with a fuzzy or blurry appearance.

5. In the microfacets case, what happens to light when the spacing between microfacets is equal to a particular light wavelenght?
When the spacing between microfacets is equal to a particular light wavelength, there can be interference effects that alter the reflected light's intensity and color. This is known as wavelength-dependent specular reflection and can result in color shifts or changes in the perceived surface roughness.

I hope this helps answer your questions and provides a better understanding of

 

[astrokreng]

Hi Sancho,

The formation of reflection images and specular highlights both involve the reflection of light off of a surface. However, they are slightly different processes.

A reflection image is created when light hits a smooth surface and is reflected in a coherent manner, maintaining the same angle of incidence and reflection. This results in a clear, mirror-like image. On the other hand, specular highlights are created when light hits a rough surface and is scattered in different directions due to the microstructure of the surface. This results in a fuzzy or blurry reflection, as you mentioned.

In terms of shader writing, the distinction between a reflection ray and a specular ray may be important in order to accurately model the behavior of light on different types of surfaces. For example, a shader for a smooth surface may use a different algorithm than a shader for a rough surface in order to accurately simulate the reflective properties.

In terms of global illumination, the use of different distributions for gathering reflection rays can affect the appearance of the specular highlights. For example, using a narrower sampling cone may result in a smoother reflection, while a wider sampling cone may result in a rougher reflection.

In the microfacets case, when the spacing between microfacets is equal to a particular light wavelength, there may be a phenomenon called diffraction. This can cause the light to be scattered in different directions, resulting in a change in the color of the reflection.

Overall, the formation of reflection images and specular highlights may involve similar processes, but they are distinct in terms of their physical properties and how they are simulated in shaders and rendering techniques. I hope this helps clarify your doubts.