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Monday
Feb072011

Shading tutorial – Concepts

This tutorial is an introduction to some of the concepts of shading objects to look three dimensional, and requires no software or hardware.

To start I’d like to make a note on where people go wrong.

Bad people! Naughty People! Stop that!

Step 0 – That You Do That You Should Not Do.

There’s an unfortunate tendency for people to try and automate tasks, resulting in over use of filters, such as lens flare, bevel and emboss and so on. There’s also a trend for shading in specific ways that are poor at best.

Since we’re going to be using a sphere as a tutorial, let’s look at a badly shaded sphere:

A badly shaded sphere

We have a light side, we have a dark side. We have a shadow and a highlight. Yet there’s something a bit… well, crap, about the whole thing. It doesn’t exactly reach out and say “I am a three dimensional object with a set of recognisable properties” – in fact it more or less sits there making a sort of ‘Durrrr’ noise.

The shading is of a flat object with a curved edge, like a plate’s rim, which makes it look flat. The highlight is slapped on as a dot that doesn’t match the lighting, and the shadow doesn’t match the the illumination. It is in fact, sloppy and poor, and doesn't match any observable spherical object.

Step one – Environment.

Nothing sits in a vacuum, not even things that sit in a vacuum. There’s always other stuff around it, influencing it.

To start with we’re going to create the infamous ‘Endless, featureless grey plain’.

This turns out to be easy! Fill a square with grey and add some dark and light gradients.

The dark gradient is a type of Falloff. This is a word we’re going to refer to a lot.

shading-1

Falloff is what happens in the real world when light falls on an object.

It hits, some of it’s energy is absorbed, and then what’s left is reflected and bounces in quite easy to predict ways.

Some of the light gets into your eyes and does all sorts of interesting stuff which eventually causes you to believe you’re seeing something.

Objects further away bounce less light into your eyes for various reasons (Attenuation, diffusion, atmospheric absorption, diffraction…) so it appears darker.

For instance, objects with a high angle of curve bounce less light into your eyes from those curves, making the curve appear darker.

Thus we can start to use this knowledge in our pictures. Let’s assume our featureless grey plain is 1: Evenly lit and 2: There’s air. Air diffuses light.

Working on this assumption we can add some distance by attenuating the light – e.g. making things further away darker. Like the ground in the distance!

We can also assume that the air is refracting and diffusing the light and that it will be brighter at the horizon which is where there’s most air volume between you and… well whatever is on the horizon, which is really just the furthest point way from you that you can see.

Step 2 – Adding a feature to a featureless plain

shading-2

A blue circle appears.

You gain 1 xp, level up, and are assailed by a sense of existential dread as you realise that you cannot determine the object’s distance from you – Or if it is a flat object that happens to be facing you, a sphere, the end of a cylinder or anything else that might appear to be a circle.

You ask yourself “What makes a circle look different from a sphere?” and because you just levelled up and got some extra INT points, realise that there’s no visual cues that would impart this information to you, as there would be in the real world.

Let’s add some of the things we seldom notice, but use.

Step 3 – More falloffshading-3-falloff

You are looking at a sphere. The light is bizarrely coming from the centre of your forehead and shining onto the sphere.

You notice that the sphere appears to be almost perfectly matte (There’s no reflection or highlights) but that as the apparent angle of the surface become ssteeper as the object curves away from you, it appears darker.

The closest point of the sphere to you is the part facing you, in the middle.

It’s fully illuminated because it has a low angle - Perfectly in the centre of the part facing you, the angle is 0° and all the light bounces right back at you.

As you move towards the sides, the angle increases to 90° and thus less light is reflected back to you.

You wonder idly what it’d look like if the light wasn’t emanating directly from you…

Step 4 – Falloff and shadow

shading-4-Shadow

As if by explanatory convenience, the light moves from your forehead, to a position where it’s shining from some unknown distance onto the sphere, at an angle of about 45° from horizontal and vertical, and now you see that part of the sphere is in shadow.

You admire the matte sphere but it’d look better if it weren’t so… dull. Let’s add a little more lighting effect.

Step 5 – Fiat Lux (And all that jazz)

shading-5-lighting

After a little more consideration you realise that the light is not only shining directly onto the sphere in a way that we shall dub ‘Direct Lighting’ but also shining onto the ground, and that the ground doesn’t absorb all the light (If it did, it’d be black!) but reflects some back onto the shadowed part of the sphere.

Also the sky (Which is not black) is evidently bouncing some light back down - And since the sky is all around, it’s also adding a little light into the shadowed areas, which is why the shadow isn’t deep dark black.

Let’s call this ‘Indirect’ lighting. We now have two sorts of light. Direct and Indirect, which are both lighting up the object in slightly different ways – the indirect light is dimmer, for a start and diffuse.

Hmm. Diffuse. There’s that word again. Don’t things usually have some sort of highlight?

Step 5 – What is a highlight?

shading-6-reflection

The sphere mysteriously becomes glossy and reflecting – which is amazingly convenient for me because I need to explain what’s going on.

You notice a bright, slightly fuzzy edged ‘Specular’ highlight, and realise it’s actually a reflection of the light-source that is shining onto the sphere.

Looking up you’re dazzled by a large luminous white ball.

Looking back you realise that a ball or circle of light would look exactly like that highlight if you looked at it in a spherical mirror. Because the reflection is not sharp and in focus, you can guess that the sphere’s texture is also not completely smooth, it must be ‘glossy’ rather than mirror finish.

You level up again. ‘Gratz.

But while you’re looking, you notice that there’s something else – there’s a reflection of the featureless, endless plain but only the light part – because the plain is neutral grey (Being exactly half way between white and black) it’s not really showing up, only the bright horizon is.

You also realise that the reflection is stronger at the edges of the sphere than in the middle.

This bakes your noodle but good, until you guess that it’s like the falloff from before – only in reverse.

The high angles at the edges of the sphere are bouncing more light to you, in the form of a reflection of the environment!

If you were at home you could rush off and start lining up things like ceramic salt pots, coffee mugs, bathroom sinks and so on and check this, and then compare the way they reflect to metal or glass, and come to the conclusion that the blue sphere must be some sort of glazed ceramic, or at least share some reflective properties with it!

Step 6 – The Shadow Knows

shading-7shadow

You still can’t decide if the sphere is really huge, but far away or very small but close.You can’t even tell how close to the floor it is. You really wish there was some sort of cue with which to infer these things.

Whish granted – there’s a shadow now!

Obviously the sphere is sitting right there on the ground, and you also infer there isn’t a vacuum because:

  • Your lungs didn’t turn inside out
  • You can see that on one edge of the shadow, it’s soft and fuzzy, because the air acts to diffuse the light.

Shadows generally become weaker and more blurred the further from the point of origin they are. You can test this with a torch and a wall and your hand.

Also shadows cast from sufficiently wide and/or tall lights will also be fuzzy.

Given our huge glowing sphere of pure white light close by our sphere, the shadow should be very diffuse all around - But let’s pretend you haven’t noticed that. It will be our secret.

What we now know

shading-8-lightsource

We now know that if can see a shadow and highlights we can infer where the light-source is.

If we know where the light source is we can guess where the highlights and shadows will be. As you can see – the specular highlight wasn’t quite in the right place.

We also know:

  • Objects are darker at their edges if they’re curved.
  • You can light something directly.
  • You can light something indirectly.
  • Specular highlights are just reflections.
  • Falloff is damned weird because…
    • Sometimes it makes edges darker
    • Sometimes it makes reflections brighter
  • Air makes light diffuse. The more air, the more diffuse. That’s why far away shadows are blurry.

Then you realise you can’t see your own reflection in the sphere and realise that means you’re not really there! Ooooeeeeeeoooo.

Subscribe to the RSS feed to get notifications when I post the next thrilling instalment – “Lighting and colour”.

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