deruvian |
02-22-2005 12:45 PM |
1 Attachment(s)
Well, this is an interesting thread. I think that I may be able to explain this phenomena, and the processes of your vision.
When you stare directly at an object (like a CRT), the "refresh rate" of your vision is approximately 72Hz. When you move your vision to the side of an object, but keep it in your peripheral vision, the "refresh rate" is approximately 80 - 85Hz. This is why a high quality CRT monitor, with a refresh rate of 80Hz or more, is very expensive. Lower quality CRT monitors, that stay around 72 - 75Hz are adequate when you look directly at them. But if you move the monitor to the very edge of your vision, the refresh rate becomes quite apparent. Anything below 70Hz will be visible when looking right at it, and will likely give you headaches and damage your vision.
This variance in rates is likely because your direct vision sees in color, while the very edge of your vision sees in black and white. More on this later, though.
I could most certainly be wrong, but here is my theory on why wheels/spokes appears to spin backwards at high speeds. It will greatly help if you try to still imagine a car driving at freeway speeds, but slow down your visualization of the spokes into a frame-by-frame sequence. I think that they seem to spin backwards because the spokes are spinning a rate slightly faster than the "refresh rate" of your vision. In "frame 1," your brain simply sees the wheel for the first time. Each subsequent frame is compared to the first. So, when the spokes are in a location just before their location in the first frame, it makes it seem as though the wheel is spinning backwards. This is also why they seem to spin backwards very slowly, despite the fact that the car is moving very quickly.
This is the same thing as when you videotape a CRT monitor. With the right camera, and the right monitor, the refresh lines seem to scan upwards. This is not correct, because CRTs scan downwards. This makes me believe that it is not an optical illusion created by your brain, just a difference in the refresh rates. A camera does not have a brain, or at least one capable of fabricating optical illusions.
I quickly made and attached a JPEG of this process. One part of the wheel in the picture is filled in with gray, to show the direction of rotation of the wheel. The other part is colored in red, to illustrate my statements in the previous paragraph (the location of the spokes being just prior to the previous thread, despite the wheel moving forward). Hope this helps.
About the color and black and white stuff... It seems tough to believe, and when I first learned this, I didn't believe it either. You look at the stack of neon green post-it notes on your desk, and then look away from them, and think, "Those are green. I can see that they are green from the edge of my vision."
No, you can't. Your mind remembers that they are green. Try this: Get an assortment of color markers (five or six colors, I prefer Sharpies), and look at them all. Try to remember all the colors. Now, have someone very slowly move the edge of a random marker into your field of vision. Tell them to stop just as you can see the very edge of the marker. Now, tell them what color it is. :D
Believe me now?
Anyway, on with the shouting and yelling. I always seem to bring it to every thread I post in. :D
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