A material is distinguished mainly by two characteristics; surface texture and light interaction components...
Surface Texture: Think of texture as being the small-scale form - e.g. bumps (hemispheres), fibres (crossed cylinders) etc. These micro forms dictate the shadow shapes, reflection shapes etc. when the material is viewed from a distance. This will probably be what dictates the the type of brush you use to paint it.
Light Interaction Components: The value of one specific part of a material is a combination of 3 components:
- Diffuse - This is where light gets bounced in lots of different directions at a micro-level, scattering them before they reach your eye. The diffuse component appears the same from different angles.
- Specular - This is the reflective component where light is bounced off the surface and stays relatively unscattered (the surface is smooth at a micro-level). If you move your viewpoint the specular component appears to change or move with you. As a side note, the amount of light reflected in this way increases as the viewing angle decreases - this is why you can see the ground clearly in puddles right below your feet (less reflection) but can see more of the sky in puddles in the distance (more reflection).
- Transparency - This is how much light is allowed to pass through the material. The value of the transparency is therefore dictated by the value of what's behind the material.
The combination of these 3 components for any given material MUST add up to 100%. There are other elements which can come into play within the transparency component; like refraction (when the light is bent while travelling through the material), subsurface scattering (where light is bounced around inside the material/object and passed back out), transmitted light (light travels though the material, taking on colour from it as it emerges on the other side) etc. but I won't get into those here.
Here are a few examples with approximate components..
A puddle of rainwater on the pavement (sidewalk for you American peoples) could be:
- 45% transparent - Some light is let through the water's surface, hits the pavement below which is then made visible to the viewer.
- 10% diffuse - There's all sorts of crap in that puddle making it murky.
- 45% specular - Some light is bounced back in the form of a reflection. N.b. when something is reflected in a puddle or other body of water, especially lightly coloured objects, their reflection usually appears to be darker than the original. This is because the value you're seeing is only this 45% or so of the original mixed with darker values in the diffuse and transparent (i.e. the dark pavement below and dark murky crap floating in it) components.
A mirror shows you what you look like, it shows you that your painting is broken and it lets you know if someone is trying to sneak up on you with a machete. It also has these components:
- 0 % transparent - Pretty much no light gets through.
- 0% diffuse - No scattering of light. It's super smooth at a micro level.
- 100% specular - Almost everything gets reflected back in a cohesive form without getting scattered.
Cotton tshirts provide warmth and have holes for your arms and stuff. They have the following components:
- 5% transparent - A white thin tshirt is more transparent than a thick dark tshirt, but in general they're designed to prevent people seeing what's underneath. Cotton itself isn't transparent as such, but there are thousands of tiny gaps between threads which add up to an over-all slight transparency.
- 90% diffuse - Cotton mostly reflects light in a diffuse way, scattering the light in all directions so we see it as a mostly matt surface.
- 5% specular - There will usually be a slight specular reflection happening - most noticeably strong lights being reflected on the edge of folds.
Ideally a window is designed to let light into a building and let people see outside through it. Therefore it has these components:
- 90% transparent - A good window is almost invisible if it's clean and has no damage. A friend of mine tried to walk through a wall-height window not too long ago because of this.
- 1% diffuse - There's always at least a little bit of muck.
- 9% specular - Depending on the lighting situation and viewing angles, there'll be more or less of a reflection bouncing back like a mirror.
Another thing to mention is gloss (or exponent as it's sometimes referred to) - a sub-component of specularity. This controls how diffuse (not to be confused with the diffuse component mentioned above!) the specular reflections are. For example, something with a high specular value, but low glossiness will have softer reflections which include a wider, fuzzier highlight from a light source. When the gloss value is increased, the reflections get tighter/more accurate and therefore the highlights get smaller. A mirror would have close to 100% gloss and 100% specular - full reflection and full tightness of those reflections. This rendered example from the lovely people at Valve shows this quite well:
There is also another sub-component of specularity which we'll I'll call tint. This is a % value which dictates how much the specular element is coloured by the diffuse element. For example - gold tends to have a high tint value, so reflections appear golden coloured rather than the colour of the original light. If you look at your reflection in a gold bar (because we all have one lying around nearby of course) your face will be a yellowish hue rather than fleshy coloured like it would be in an average mirror. In general, metallic materials tend to have high tint values (except mirror-like metals like chrome) and non-metallic materials like wood have low tint values. It should also be noted that reflections that are tinted tend to look more saturated, since the specular and diffuse elements are coloured in the same way. Tint is somewhat simplified here, but this is probably as far as we 2d artists need to go with it. Exceptions start to arise when the specular colours vary from the diffuse (mother of pearl?) but for the most part they're pretty similar. For an excellent in-depth description of how this is handled in 3d rendering I'd recommend reading this excellent article by Pedro Toledo http://www.manufato.com/?p=902 (thanks to Edward for the link and Pedro for the article).
I hope this post has made some kind of sense and helps in some way. Like I say, I'm still playing with all of this theory and figuring things out every day so any info/corrections would be appreciated. Thanks for reading!
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