Talk:Perspective projection distortion

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"An example is a view where one is standing facing north towards a road which runs perfectly east-west. In an artificial perspective projection, every car on the road would be drawn at the same size, even though it is clear in reality that the farther away from the center of the picture that a car is, the farther away from the viewer that car would be."

Agree...I am this original poster of this article .... it has been mangled beyond recognition....I am going to try to repost with improved version all around.....But rather than anyone trying to "correct" or who challenges the technology, please lets talk it out before editing the orginal...thanks Patkelso 14:39, 7 September 2006 (UTC) ..Pat Kelso

This is simply untrue. The more distant cars will be painted on more distant parts of the canvas, and will therefore appear smaller to the viewer, despite being painted at the same size.

There seems to me to be much confusion arising from the notion that perspective in some way emulates the behaviour of the retina. It doesn't: it allows a canvas (screen, etc) to emulate a window on the world, when viewed from a given point (and if you move your viewpoint with respect to the canvas, the correct perspective will be a different linear perspective, not a queer 'spherical' one.

The article currently says

Imagine that on an infinite plane there is an infinitely long and infinitely straight railroad, and that you are standing between the parallel rails. As you peer down the track in one direction the rails appear to intersect (on the horizon). As you peer in the opposite direction they again appear to intersect. ... You similarly test the other rail and discover it too is straight. How can this happen?
...
This conclusively demonstrates that the retina itself is spherical in shape.

I really like this "thought experiment" and "picture sphere" illustrations. However, the conclusion does not follow. I can take photographs with a camera (let's say it's a pinhole camera) in all the locations and directions mentioned in the "thought experiment". After I develop the film, I can take a ruler and put it on the photographs and see that the image of the rail in each image appears straight, and intersects the image of the other rail in both directions. Does this conclusively demonstrate that the film is spherical in shape ? (The retina does happen to be almost perfectly spherical, but that fact is irrelevant to this article).

I agree completely. Simply put, stuff that is farther away from the point where it is viewed from looks smaller since there is more to be seen. Think of the fact that the moon and the sun appear to be the same size, yet the moon is hundreds of times smaller. This is because it is also hundreds of times closer. Likewise, the parallel tracks appear to converge since the distance between them appears smaller at a distance - converging to zero as the distance aproaches infinity. The retina does distort images, but this is taken care of by the visual cortex. Convergence of the tracks is in no way related to the retina since, as you point out, any camera (no matter the lense, projected image shape, etc.) captures this effect. Evilrho 23:48, 23 September 2005 (UTC)

(It is, of course, impossible to assemble all my photographs in a collage on a flat surface, such that the images of one rail are in a straight line from one vanishing point to the other, and also the images of the other rail are in a straight line).

-- DavidCary 19:56, 18 Jun 2004 (UTC)

Proving mathematically that all retinas and perhaps even all cube-map textures containing railway lines are spherical was a bit much for me... The article is probably less readable, but more accurate now. Κσυπ Cyp   2004年11月25日 (木) 20:23 (UTC)

The article mentions "the correct way to project the image", implying that any other method is "incorrect". But I fail to see (no pun intended) any difference. What advantage is there to using a sphere (hemisphere ?) ?

The article specifically mentions:

* when someone stands somewhere *other* than at the "station point" in front of the flat image, there will be distortion.

That's true, but don't we get just as much distortion when someone stands somewhere *other* than at the center of the image sphere ?

I see that the image must somehow "wrap around" the viewer to properly show the intersection directly to his left and directly to his right, which is impossible with a single flat picture. But what's so wrong with using several flat pictures, left-front-right etc. ?

-- DavidCary 19:56, 18 Jun 2004 (UTC)

Nothing. I think the 2nd half of this article is flawed. The spherical shape of the retina is completely unrelated to the phenomenon of the converging tracks. Also, the fact that they seem to converge twice but that you can't seem them converge twice without turning 180 degrees is irrelevant. Actually, if you were in a hot air balloon a hundred feet above the center of the tracks you could. Big deal! See my note about the moon and the sun above. This is a case of over-complicating a simple geometric phenomenon occurring with light as viewed by any singular point. Evilrho 23:48, 23 September 2005 (UTC)


Where are Figures 1, 2 and 3?

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