I'd just say that with one eye, you "understand" things in 3D, even though your eye only see 2D.

It's about the way human brain has been trained in interpreting the senses.

Although, in a 3D movie, you don't really have a 3D space in front of you so moving your head around won't help in the slightest.

If i had to make an armchair guess ... part of your brain is dealing with the fact that you always see 2 of everything that you're not directly focusing on even if you're not conscious of it. With one eye shut the other eye sees a ghost of the second image that the shut eye is supposed to be seeing since the glasses are never perfect. The brain may interpret this as two vision streams that it isn't able to combine properly and still feel like it's trying to process stereo.

Something film makers have been using for ages, that our brains have been trained to perceive the world in 3D, and apply that to 2D renderings, for instance an object that is smaller than expected on screen = more distant.
And abused in special effects for just as long :)

Painters and artists create an impression of three dimensional forms using perspective and colour (photographs do it without needing any theory). This is the way a one-eyed person sees the world as having depth. A two-eyed person (and all creatures having two forward facing eyes) have added cues because each eye sees a different picture, the differences providing cues for distance. The greater the differences the stronger the cues, so binocular objectives are spaced further apart than our interocular distance. You can extract further cues by moving your head. The human factors people insist that stereoscopy is limited to 3 metres, and beyond that the differences are too small to be differentiated; this doesn't match my personal observations. I have a friend who has two eyes but who cannot see stereo. Apparently he had to wear an eye patch when he was young and this prevented him learning the skill.

"we use more cues that just stereo vision to judge distance, that could explain your daughters experience." yes, shading/lighting helps define 3D in a 2D pic. Simple examples in Photoshop can be used to demo this to her.

"you don't really have a 3D space in front of you " well this not actual correct but the point that Chienworks is trying to make is; there is no physical object in 3D viewing where position of the viewer makes a critical difference.(that probably no better of an explanation) ;-(

"I have a friend who has two eyes but who cannot see stereo. Apparently he had to wear an eye patch when he was young and this prevented him learning the skill. " Yes, this is very true, it's amazing that vision's depth and motion can't be trained to an adult who has gain vision. Basely, the vision skills learned at a very early age, determines your life's "skills" ;-)

you can loose your 3d vision too. My dad, when I was younger, got a bungee cord in the eye. He had to wear a patch for a while & it messed up his depth perception for quite a while. When he got it off he got some of it back.

Off the subject a little, but pirates used to wear an eye patch over one eye in order to keep their eyes from having to adjust between the outside light and the dark below deck. They would have the patch over their left eye outside and when they went below deck they would switch it to the right eye and the left eye would already be accustomed to the dark and they could see immediately without having to wait for their eyes to adjust. Keep in mind that there were no sunglasses, no lights, and lighting a candle below deck with all that gunpowder around was out of the question.

It makes me sad to think that the poor guys may have lost their depth perception in the process.

Once again another reason to love this forum!!
Thanks to everyone for their great answers.
A special thanks to Serena for reading my mind :-)
I was ready to ask a follow up question about someone who only had one eye since birth and her response took care of that.

Mike

Laurence that was an amazing factoid. All this time I'm thinking pirates had bad luck with BB guns. I wonder why that practice never caught on with other sailors? or did it?

This "ball in the box" demo is a real eye opener.
http://gandalf.psych.umn.edu/users/kersten/kersten-lab/shadows.html

now, with a drop of good old acid, you can see in 4d.

(it's one of those bad head days)

Get some pieces of thick card-stock paper, or even a piece of cardboard, that covers your window entirely. Make sure that your room is as dark as possible and that no light enters (this is much more difficult to achieve than you may think, but perfection is not required). Poke a small hole, about 3 millimeters in diameter, in the paper, so that light from outside your room can enter through the hole (this works best during bright daylight conditions outside). Now get a white towel or sheet, or even a white piece of paper, and hold it right on the hole. Now slowly move the piece of paper/sheet/whatever back from the hole… and be AMAZED! No really, you will be amazed, trust me!

This is from http://www.spiritalchemy.com/blog/221/seeing-upside-down-with-bonus-experiment

The human brain is an awesome accomplishment of liquid engineering

"you can see in 4d." ;-) 3D with time displacement................ ;-) or viewing 3d objects and getting slap on the back of the head!

I am blind in my left eye. A birth defect.

I cannot watch any 3D movies. I either have offset images to deal with or color issues. Usually a headache is involved after about 5 minutes of trying to watch the content.

Technically - i.e. as shown in the Wikipedia article http://en.wikipedia.org/wiki/3-D_film all 3D projecting methods work by sending slightly different pictures to the left eye and to the right eye (through polaroid filters or red/green glasses or etc) and so create the 3D-effect by affecting both eyes.

So, theoretically, the intended 3D effect can not be achieved watching a 3D film with just one eye.

Since, however, the filters are not 100% perfect and because a lot of processing is done between the eye and the "consciousness" it can not be excluded that different people might react differently to seeing a 3D projection with one eye.
Probably ranging from just headache to some kind of 3D awareness.

"It makes me sad to think that the poor guys may have lost their depth perception in the process."

I must confess to being very amused at the extremely non-PC thought, "why should we be distraught at what criminal terrorists did to harm themselves?" ;) Anything they did to make themselves less effective was probably a good thing for everyone else!

Mythbusters on the pirate eyepatch!

(starts at 1:30)

I lost all binocular vision following cranial surgery in 1990 (diagnosed as alternating suppression).
With conventional and computer-aided therapy I became able to read fine print, drive a car again, and work full time by 1995. The prognosis was that I would never regain full 3D vision.

Then it started coming back. In flashes at first, but eventually for longer periods of time. Simple things like taking a walk in a wooded area became a source of amazement. I studied my own rediscovery process every step of the way, and the things I've learned have helped others with visual learning disabilities. I still have trouble with 3D movies and those crazy pictures with hidden 3D objects (they both give me the whirlies). Also staring out over a cliff or laying on my back looking at clouds can make things start to spin. Today my rediscovered 3D vision works most of the time, except when I'm extremely tired or have had too much to drink.

I only see out of one eye at a time due to a condition when I was born.
Therefore the I do not see the 'effects' in 3D movies.

Like others have mentioned, I see '3D' because my mind 'knows' about relationships, so I don't run into things and do drive.

What is being discussed is depth perception. Which must be aquired at a very early age or not at all. One thing it requires is fusion vision. That is, seeing out of both eyes at the same time and overlaping and fuseing the images. While I can look out of both eyes at the same time, I don't have fusion or overlaping of the images so I don't have depth perception. I have been told/read that about 50% of those who think they have depth perception actually don't.

Create a box with absolutely flat lighting. Place two items (sticks or the like) upright inside in such a way you can move them back and forth - perhaps on strings. There must be no shadows in the box or sticks or caused by the sticks in any position. Look in the end of the box and move the sticks so they are exactly next to each other. If you can do that, you have depth perception. (In WWII that was used to test pilots, don't know if is still being done today.)

So, the short answer to the original quesiton is yes and no. <g>

Paul Masters

That would be an extremely stringent test because there would be almost zero difference between the appearance of each stick. I wonder whether actually it would be a test of ranging by eye-convergence ( a factor at near distances). Certainly I would expect such a test to indicate stereopsis is limited to a distance of 3 metres (as stated by human factors people). In the real world there are many more cues, such as the background planes being changed by the foreground objects.

"I must confess to being very amused at the extremely non-PC thought, "why should we be distraught at what criminal terrorists did to harm themselves?" ;) Anything they did to make themselves less effective was probably a good thing for everyone else! "

Whether they were criminals or contractors - it depends on your perspective. What you call pirates were likely privateers - mercenaries (like Blackwater). There were many privateers that worked for the highest bidder. Every major country in the 17th-19th centuries, including the US, employed privateers. Most simply retired rich when their services were no longer needed, but a few went rogue and gave the whole industry a bad rep.