Real Height versus Perceived Height, of Objects at a Distance.

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In summary, the conversation discusses the concept of perspective and how our perception forms the space around us. The speaker shares their observations of telephone poles and light posts and how their perceived height changes as the distance between them and the observer increases. They also mention using a simple ratio to determine the real height of an object based on its perceived height and distance. The conversation then delves into the idea of perspective in art and how it affects the way we see and create images. The speaker poses questions about the restrictions on space and how our observation may impose them. They also mention the cross-ratio as a conserved quantity in perspective.
  • #1
colinven
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This thought came from staring at telephone poles and light posts along roads and walkways. I noticed that these objects all had the same height more or less and were regularly spaced. The poles and appeared to reduced in height (following some mathematical relationship) as the distance between me and the object increased. When I first thought about this I would hold my fingers up close to my face, and with my pointer finger and thumb encase the height of a telephone pole between my two fingers. The height between my two fingers might have only been a few centimeters (this is the height that the telephone pole seemed to be from my distance to it), but the real height of the telephone pole was much larger. I thought, how could the perceived height and real height be related; is there some way in which I could simply encase any object at some distance and by measuring the height between my fingers know the real height of the object encased?

What I found was a simple ratio: [itex]\frac{h}{l}[/itex]=[itex]\frac{H}{L}[/itex]

where h is the height between my fingers or the perceived height, l is the distance my fingers are from my eye, H is the real height of the object, and L is the distance from my fingers to the object.

Using this formula I only had to know my distance to the object, the height between my two fingers and how far my fingers were from my face in order to know the real height of any object.

The perceived height of telephone poles as the fade into the distance is proportional to [itex]1/L[/itex], or in other words the perceived height is inversely proportional to your distance from each pole. This is true if you are standing directly in line with the poles, however we know from experience that if you are standing directly in line you will not observe this telescoping series. You have to stand some distance from the line of telephone poles in order to observe this phenomenon. To account for this I found the following equation using simple geometry for right triangles: [itex]h[/itex]=[itex]\frac{Hl}{\sqrt{n^{2}d^{2}+L^{2}}}[/itex]

The above equation assumes you are standing parallel to the telephone poles and directly across from the first one in your series. Thus, L is the perpendicular distance to the first telephone pole, d is the pole spacing, and n is the number of telephone pole in the series (the first telephone pole is n=0, the second n=1,...).

The whole denominator in the above equation acts as a way to indirectly measure the distance to each successive telephone pole using the Pythagorean theorem. The above equation has some finite bound, that means as n → 0 h → [itex]\frac{H}{L}[/itex]. Whereas the first equation has no bound, that means as L → 0 h → ∞. Therefore as L → ∞ the [itex]\sqrt{n^{2}d^{2}+L^{2}}[/itex] approximates L and the difference between the two equations becomes negligible.


I offer this as a start to a discussion on how our perception forms the space around us. Is it possible that by our observation we impose restrictions on the allowed heights of an object? Does our observation create a potential curve like [itex]1/L[/itex]? Where does distance come from, us or is it apriori?
 
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  • #2
You have discovered the concept of perspective: http://en.m.wikipedia.org/wiki/Perspective_(graphical )
 
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  • #3
colinven, Welcome to Physics Forums!

Russ is exactly right: If Leonardo Da Vinci was alive today he would congratulate you! Just have a look at his "The Last Supper".
 
  • #4
Thank you Bobbywhy and russ_watters for reading and responding to this post. This subject has been on my mind for a while now, and it feels great to share my ideas.

When we look at telephone poles fade into the distance they do not fade linearly, but instead the relative height of each successive pole reduces proportional to [itex]1/L[/itex]; where L is the distance from the observer to each pole. It would be incorrect, then, to draw a straight line extending from a point in the center of a page to the edge and create my telephone poles under this line. I would have to draw the curve [itex]1/x[/itex] and draw my telephone poles under this function in order to create the correct perspective.

What poses this restriction on space; that an object (when observed) must have a relative height fall under some curve approximating [itex]1/L[/itex]?
 
  • #5
colinven said:
When we look at telephone poles fade into the distance they do not fade linearly, but instead the relative height of each successive pole reduces proportional to [itex]1/L[/itex]; where L is the distance from the observer to each pole. It would be incorrect, then, to draw a straight line extending from a point in the center of a page to the edge and create my telephone poles under this line. I would have to draw the curve [itex]1/x[/itex] and draw my telephone poles under this function in order to create the correct perspective.
No, you're overlooking the fact that they do not appear to be equally spaced either. Projection (perspective) preserves straight lines.
There's an interesting conserved quantity in perspective called the cross-ratio. E.g. if there are four equally spaced points in a straight line (in reality) then in the projection, when you have positioned three of them, you can calculate where the fourth must go.
 
  • #6

1. What is the difference between real height and perceived height of objects at a distance?

The real height of an object is its actual physical height, while the perceived height is how tall the object appears to be from a certain distance. This can be affected by various factors such as angles, distance, and visual cues.

2. How does distance affect the perceived height of an object?

As the distance between an object and the observer increases, the perceived height of the object decreases. This is due to the angle of vision, where the smaller the angle, the smaller the perceived height.

3. Can objects of the same height have different perceived heights?

Yes, objects of the same height can have different perceived heights depending on their distance from the observer and the angle at which they are viewed. For example, a tall building may appear smaller if it is farther away, while a shorter tree may appear taller if it is closer.

4. What are some visual cues that can affect the perceived height of an object?

Visual cues such as relative size, texture, and overlapping can all influence the perceived height of an object. For example, an object with a rough texture may appear taller than an object with a smooth texture, even if they are the same height.

5. How can understanding real height versus perceived height be useful in everyday life?

Understanding the difference between real height and perceived height can be useful for tasks such as estimating distances and sizes, as well as for creating realistic visual representations in art and design. It can also help in understanding optical illusions and how our brains interpret visual information.

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