I could comment on the skybox itself trying to replicate the parallax effect in motion. However, with regards to motion, I do know that Gordon is running 2-3 times above normal human speed. Games like AA, GRAW, and BF properly reflect human running speeds as nobody can run like a wild dog and keep the same pace straight on. (Im not talking about sprinting, just normal running)
Also, in the case of camera effects, it shakes way too much. Its not with the bobbing or gun fire but more on colission and shock.
On repetitive colours, this could be one reason but the most common colour around is gray. However I can agree to the lighting style as majority of the maps have briefly alternating light and dark regions which, when you are moving at the speed Gordon is, will look like a flashing series of lights. Throw in the camera jiggles and there you have one possible reason for nausea.
I guess this would be a side effect to trying to replicate reality. I do believe that games are far better and more entertaining when they atleast retain some sureal elements. Take World of Warcraft for instance, there are free MMOs out tehre that at the same time are realistic - gory aswell - but one reason why Warcraft is so popular is because the graphics are just so sureal that everyone enjoys looking at them.
Now, I dont think we can blame realism entirely. Games like Doom3 and Farcry tap close into the same quality as Hl2 however they do not, only on a very rare occassion, give nausea to a player. So what exactly makes HL2 different?
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I have another opinion, I think it could be the non-stop no-loading-screen design. Instead of showing a loading screen in between or a cinematic like Doom3, Hl2 just throws you from one map to another. Let us say you are moving, and at the speeds that Gordon runs in, you suddenly stop just to load the next section of the map. In comparison, this would feel very close to riding a car at over 100kph then suddenly in a blink of an eye stopping. Im surprised I nor anyone have thrown themselves into thier screens through simulated inertia that would most likely be caused by the brain as racing games cause the body to imitate centrifugal force.
When exiting cars in HL2, the camera just unaturally moves around - this would also result in similar symptoms as above. CnC Renegade also uses the same animations for exiting and entering vehicles. Not only is this nauseating, its unrealistic as nobody exits a vehicle as if they were on a hand rail. America's Army gets this right by having a complete and proper entry/exit animation.
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In your last point regarding your brother. I think it really would be with the skybox. Since a mojority of the complaints had something to do with the canals and mostly the vehicle maps - I do think the unatural and unrealistic speeds at which you move plus the off the scale paralax is causing the motion sickness.
In reality, paralax would only be constiously visible on nearby objects. A large building in the distance wont appear to move, only the objects close by. This would give you a sense of firm ground. In Hl2, the buildings in the distance to move as if they are right next to you. Add the realistic graphics and you can fool the mind that the building is real - and since the subconscious brain thinks it is real it automatically takes into consideratino that a building of that size in that distance woill not be moving. However it Hl2 is it and it isnt suppose to. This creates a sensation that you are moving over ten times faster than you are. The brain may have been fooled but the body is still firmly in place. Your body knows that it isnt moving but the brain doesnt.
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We could try turning off the skybox entirely and see how this would change the game.
A skybox with background is not a bad thing and I would add great looking detailed skyboxes to my games if I could. However HL2 may have over done it with the paralax animations as the buildinds really do look like tehy have wheels and are on the move.
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I do need the console commands to test them once I recover from the nausea. Doom3 never did this me...
