This article is so long, it's been split up into 2 parts, you can find part 1 here

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Lighting and Texturing

The light seems to come from everywhere at once
In many 3D packages, under the lighting toolset, there is a button called "Ambient Light". Every time you use this light, a little part of you dies inside. There is no example in the real world of anything that is lit from all sides at once. This is a cheap trick that used to be used back before lighting was accessible and able to be used realistically, and looks something like this:


What you're effectively doing by using ambient light is getting rid of any shadows that may have helped your scene along. If the shadows are too dark, consider looking up things like HDRI and Image Based Lighting, Global Illumination and if you're super curious and love maths, Spherical harmonics..

Don't forget, light is an important aspect of your work, your lighting and composition are visual queues which tell the audience where to look. A good trick once you've finished your lighting, is to desaturate your image and blur it until you can only see broad shapes. If your lighting STILL leads the viewer's eye through the picture, it's working. If not you will end up with a uniform grey mess. Start again and plan it out!

Try the blurring trick on this to see what I mean:


Your shadows are not of this planet
Study shadows in the real world and try to emulate them. Look at how transparent objects cast shadows, how lights closer to an object cast shadows differently to lights far away. If it doesn't look the same in your render as in your real world example, try to fix it, don't just ignore it and hope nobody notices, they will.

Panther in the Jungle at Night Time
Your work is either too dark or too bright. You think it's fine on your monitor, and everyone looking at it should just adjust your settings. This is bad, bad, bad! If nobody can see it, It doesn't matter if your modeling is absolutely amazing.

A lot of the time this problem occurs when you are trying to make something set at night. Setting an image at night doesn't mean the whole thing has to be black, you still need to light it! Night scenes are characteristic, not because they're completely dark, but because they're very saturate (have strong, deep colours) and are very contrasty (very light areas next to very dark areas) and make use of silhouettes. It's not wrong to have dark things in your scene, but you need to contrast them with light things so you can see the dark ones. When in doubt, look at examples of good night lighting. Here's some:



What on earth is it made of?
You've decided your object is going to be "red", and that's as far as you've really gone with texturing. It's important to think about what your models are made of in order to make them look realistic. For example, let's say you have a red table. Is it painted red wood, red metal paint, red paint on plastic, red plastic that's unpainted, maybe it's red-tinted glass with cast-iron legs and supports. Once you know what material it should be, you need to adjust the textures and shaders to make it look right. How shiny is it? Is it reflective? are there flecks in it? Is there dust on it? Is it translucent?

An important thing which is often completely ignored in 3D renders is considering how something's built. If you are making clothes on a character, they should have seams where the material is combined and hemmed. Clothes don't magically weave themselves to the perfect size and shape, and are not made of clay. If you are doing bricks on a house, it's a good idea to physically model in the corners rather than just wrap a brick texture around it. Bricks are a particular size and shape and don't just wrap around corners.

The character's skin looks unnatural, like clay
If you are doing a character, you should be using Subsurface scattering on the skin. If you don't know how to do this in your particular 3D package, look it up. If it doesn't exist, find a way to fake it. Real skin lets light travel through it. When you hold a torch up to your hand you will notice the skin is red when you look at it from the other side, because light travels through it. Even when you don't notice it, it's there. Subsurface Scattering (SSS) is what causes the shadows on a character's face to appear soft rather than black and harsh. A character without even an attempt to fake SSS will appear lifeless and made of clay, no matter how good everything else is.

It takes longer to render than it does to write a thesis
A common sight is people proudly announcing that renders took an hour, two hours, five hours, fifty hours and your render is still grainy and pixelated. It's important to learn what your light and render settings do before cranking them up. If you have nothing transparent that refracts in your scene, don't turn on refraction. If you have a soft light that is looking grainy instead of smooth, don't just crank up the settings as high as they'll go. Adjust and render in small intervals until the problem you're trying to fix is sorted out. Render only a region if you have to, but simple scenes shouldn't take that long. This one took 30 hours and absolutely didn't need to:



Another common mistake is to use seventy billion lights in your scene. The following huge scene from Avatar, uses only two lights. One from the tree and one from the sky:


The more lights you have, the more complicated things will get. In the real world, we are lucky enough to get ambient light for free, It just spills in from outside and bounces around. This doesn't exist in 3D, and the more lights you have, the slower your render will be. A good place to start is to look at Three Point Lighting

It makes my eyes hurt
Your colours clash, your materials are so reflective they bounce everything and make a huge mess, it took an eternity to render, you have 20-odd lights and there's no obvious focus. Now's a good time to start again.

Animation

I will only cover a few things on this one, because there aren't many animators there. Just a few reasons your animations might not look so crash hot.

The movement is jerky and lumpy
You didn't block out your animation and now it's gone horribly wrong. There are a few key principles to animation that you should think about before you start.

Key poses: Block out your main poses. Think about the Line of Action when you are posing a character. Humans respond to curves. If you make the character white and everything else black, you should have a clear silhouette which clearly shows the action.
Arcs: Human movement isn't blocky and doesn't go from point to point. Instead we use Arcs, smooth curves of movement from one point to another. This is really important to make your animation smooth and flow properly.
Tweens: Adding poses between your key ones is important. This is where the term Tweening stems from. A lot of things like feet sliding across the floor and watery animation happen because people forget to add tweens.
Spacing: Spacing is also important and is to do with the timing of your animation. Think about where movement needs to be fast and slow in relation to what you're saying
Secondary animation and Offsetting: This is really important for making your movement look believable. Secondary Animation is animation that is triggered by something else, while Offsetting is moving your keys around so that all movement doesn't happen on the same frame. This leads to more fluid motion.

The whole thing is too long
Never make it longer than it needs to be. This is especially true for show reels. If it's too long it's boring, and people get detracted from what you want them to see and start noticing your animation mistakes instead.

Why on earth is there Metallica playing in the background?
Never animate with inappropriate music. If you are doing a heavy metal rock animation, then fine, use heavy metal in the background (but make sure you have the rights to your music). Otherwise, keep the music in line with the animation. You want people paying attention to your work, not turning it off because they hate your song choices.

Post-Effects and Editing

Finalizing is one of the hardest things to complete when rendering. Once you are certain that you have a good render. You start to go over the final piece and spot errors. This forces you to either post it anyway (who's going to notice?), rip your hair out and scream, then throw away, or use ridiculous heavy filters to try and hide your mistakes.

It has ugly jagged edges
A big tell tale of CG is the jagged edges that you get out of your render. Each rendering package has different options for tackling this. Some 3D packages use interpolation, oversampling, anti-aliasing and other methods to try and avoid these issues. It's a good idea to find out exactly what you should do to fix it. If all else fails, a half-pixel blur in Photoshop can help hide it a bit. It's not enough blur to make it go horrible, but it will hide some CG issues.

Read more about AntiAliasing here.

You've used a Photoshop filter and I can tell exactly which one
If you've never heard the joke about comic sans, look it up. In typography there's an unspoken rule that if you use a fancy font and someone can give you the name of the typeface, then you shouldn't use it. This is why you will never find common fonts like Papyrus, Jokerman, Ravie, Comic Sans and similar in professional printing work. Similarly in computer graphics and 3D, if you've used an aspect of Photoshop and it's very obvious what you've done, you shouldn't do it.

The following filters are easy to spot a mile away: Underpaint, Plastic Wrap, Poster Edges, Paint Daubs. Also shapes like the heart and leaf. Everyone knows what they look like, so don't use them!

You used ugly particle effects for no reason.
People have this misconception that adding real elements to your 3D is somehow cheating. I have to dispel that right here. If you have a 3d candle, do the light in CG but add a real flame to it instead of wasting hours trying to get particles working then more hours rendering them only to find it looks ugly.

You've done nothing at all to it
Some people argue that you shouldn't do any post processing. All the big budget game and film studios do it, so why shouldn't you be allowed to? At the very least you should use levels or curves to make sure your image isn't too dark or too light and make sure you havn't got any jagged edges.

It looks worse than your straight render
When you finish, make sure you go back to your raw render. If your post-processed image looks worse than what came out of your 3D program, scrap it and start post-processing again.

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Hopefully this will help some frustrated people who know there's something wrong, but aren't sure what to fix.

Thank you again to ~Nanaki-Murasaki for his efforts in helping write this!