I haven't mentioned The Polar Express or Beowulf, and for a good reason. They are creepy.

The use of computer-generated imagery (CGI) in films has come a hugely long way since it was first introduced. Obviously, that was a long time ago - the first use (according to Wikipedia) was in 1973's Westworld. Given the general speed of development in the computing industry, a hypothetical alien who has just landed on our planet might assume that by now, CGI has advanced to being completely perfect and realistic. (Whether such an alien exists, and why he'd be particularly interested in technological advances in film and TV, are questions that are somewhat beyond me right now).

Sadly, our alien friend would be disappointed. Although CGI has been enthusiastically taken up by studios throughout the industry, it can be just as unconvincing as it was back in the day. So why is this? To find out, let's look at some examples of good and bad practice in CGI use.

Jurassic Park (1993) - The T-Rex attack

For my money, this is not only the best scene in the movie, it's one of the best scenes in any film that I've ever seen. Notice how the scene isn't about the T-Rex - it's shot almost entirely from the perspective of Alan (Sam Neill) and the children (Ariana Richards and Joseph Mazzello), meaning that they're the characters we focus on. The scene is also completely unscored, letting the rain and the thunder do all the work that music might otherwise ruin. (The sudden crash of thunder right after the goat's leg hits the jeep is far more effective than the orchestra hit you might expect.)

Understatement of this type is the main reason why this scene works so well, and the same thing is true of the CGI. A lot of the work is done with puppets - despite several shots of the T-Rex being completely synthetic, these shots are short and don't require much in the way of movement. The result is a completely convincing scene that still gives me goosebumps 15 years on.

The Abyss (1989) - Trailer

I couldn't find the relevant scene anywhere online, but you get a couple of fleeting glimpses of the watery alien creature in the trailer (drag the slider to 1:40 and 1:54, for example). The Abyss is quite a good movie in its own right, despite a few horrible clichés (notably Ed Harris's ability to bring his wife back from drowning by screaming "FIIIIIGHT!" at her from about 3 inches away), and given that it was made in the '80s the CGI work is incredible. Again, it's not overused - you see one creature, whose defining feature is that it has no defining features, being made entirely of water. By not overreaching themselves, the special effects unit have created something which, although technically not outstanding, looks convincing and just eerie enough to be accepted as alien.

The Lord of the Rings: The Fellowship of the Ring (2001) - Fleeing through Moria

Watching the first 30 seconds of this clip will be enough for our purposes, but feel free to watch the rest if you like a bit of silly yet epic cinema. Now, if you're going to have your characters run across a gigantic underground hall, the likes of which has never been seen in this world, it might be tempting to spend a lot of time on the hall itself. This is, apparently, what the film makers chose to do. Unfortunately, this means that they were not concentrating on less grand things, such as good motion animation of the main characters. And as all eyes at this point are on the main characters, it becomes really rather obvious that they are waddling at high speed with exaggerated arm movements. Maybe this sequence was actually an extended advert for Lord of the Rings Action Figures.

Constantine (2005) - Demon-slaying

The weird thing about this scene (apologies for the awful quality, by the way) is that it would have been superb as a cut-scene in a videogame of Constantine. For all I know, maybe it was. As it is, the use of a CGI Keanu Reeves almost all the way through, even in little shots where they could easily have used bluescreen, produces an oddly shiny and rubbery look to the scene. Especially at the moment (at about 0:25) where "Reeves" spins the magazine on his shotgun, there's something very wrong about his hands - I think it's that they look too solid and meaty. I saw this movie at the cinema, and I think I was laughing all the way through this scene; even in a film where plot consistency, theology and even basic physics go right out the window, you just can't get away with vastly over-egging the CGI pudding and expect the audience not to notice.

Casino Royale (2006) - Final scene

Don't play this clip unless you've already seen the film! It spoils the ending, and whoever stuck it on Youtube has also inexplicably put the Pirates of the Caribbean music on top of it. If you have seen the film and don't mind inexplicable piratey music, read on.

I wouldn't normally say anything bad about Casino Royale, because not only is it a brilliant film, it's also excellent when it comes to CGI use. When Bond drives a JCB through a fence and his enemy has to run up a wall to evade him? They actually did that. When they jump from crane to crane 200 feet up? They did that too, with safety harnesses but no other tricks.

The CGI in this sequence is fairly subtle, and you may not have noticed it. Watch it again if you didn't. For those too bored or lazy to keep guessing, I'll tell you - the house on the lakeshore is entirely built in CGI. Even though it's not obvious unless you're looking for it, as soon as you realise that it's not real, you start to notice the flaws. The motion of the car is too perfect. The camera movement is too smooth. The lighting is too crisp. All of these little things, hardly noticeable in their own right, add up to an effect that just spoils the scene for me. It's entirely possible to do buildings successfully in CGI, and in fact Batman Begins, another film which generally does everything for real, pulls it off extremely successfully. I'm not sure why Casino Royale doesn't manage it, but for me it's an annoyance that detracts from an otherwise brilliant film.

The Lord of the Rings: The Return of the King (2003) - Legolas vs Huge Elephant Thingy

Now, this one is just silly. While it was obviously great fun to script (and would have made a supremely fun addition to Tolkien's original writing, had he been given to writing awesome stunt sequences), it was just too much for the special effects boys to handle. Once again, we're stuck with a sequence that looks like a cut scene from a videogame, complete with unconvincing movement, plastic-like skin tone and a synthetic actor that manages to be even more wooden than Orlando Bloom. I have to admit, it does still look fantastic; there is a definite problem, though, with any sequence that causes you to say "well, for an entirely impossible action it looked good". The whole point of special effects is to make you believe the impossible; this doesn't.

There are several other scenes and films that I could have mentioned here (the unconvincing wolves from The Day After Tomorrow, the unbelievably poorly done Jar Jar Binks from Star Wars Episodes I & II, the excellent motion-captured Gollum from the Lord of the Rings trilogy, and so on), but I think you get the picture already. So what makes a good CGI scene? I think we can list some ideas...

• Understated. You want the audience to notice the story, not the effects. If they come out of the cinema going "the effects were AMAZING!" rather than "I loved the scene where...", you're doing it wrong.
  
• Plausible. The reason we don't believe that Legolas couldn't take down the elephant-type thingy isn't so much to do with the effects - it's more that we had never seen Legolas do anything as ridiculous throughout the rest of the trilogy. If you build up a character slowly (eg. Spider-Man), you can get away with a whole lot more than if you suddenly give him completely stupid powers that no-one has ever done before. Of course, stunning new effects can work (Bullet-Time in The Matrix, for example), but only if they are done really well.
  
• Within the bounds of current technology. Right now, a fully CGI human is just not plausible. The skin, the muscle movement and the way each part interacts with all the others is far too complex to pull off in any kind of sustained fashion - stick to what works.
  
• Willingness not to use it. The rooftop chase scene in The Bourne Ultimatum works as well as it does because every shot was done by a stuntman. Yes, CGI probably could have been used - but it looked a whole lot better done for real.

That's a list that could be extended enormously, but I think I'll leave it up to you to do so. In the meantime, I'll leave you with one of the most shockingly brilliant stunts in the history of cinema - Buster Keaton in Steamboat Bill, Jr.

Copyright notice: All video clips, with the exception of the Steamboat Bill, Jr. clip (which is public domain), are copyright their respective owners. I believe that their use in this context - comment and research - constitutes fair dealing, but in other contexts this might not apply. None of the clips are released under my Creative Commons licence.