Sijun Forums :: View topic - In-depth Question about Linear Perspective & Optics

downloads157 · junior member Member # Joined: 10 Nov 2006 Posts: 2

Hi,

i've read a few books about Perspective and i would like to seek help in understanding in-depth the concept of :

1. Vanishing points: Using optics(light rays) to explain, what does a Vanishing Point equate to? eg What?why? is there a Vanishing Point for human vision?

2. Horizon line: Also using optics(light rays) What? Why? is the horizon line/plane?

3. What does points that lay ON the horizon line represent in human vision? and what does Vp that does not lie on Horizone line interprete to in human vision?
Very Happy

4. Regarding our cone of vision, objects outside this Circle of view would appear distorted: If I want to draw in the distorted objects in my painting, how do i measure how much distortion there will be as in moves further from the 60 degree undistorted circle? eg vertical/horizontal distortion & by how much? any ratio relationship?

Any references or in depth explanation would be appreciated. i would welcome technical answers in terms of optics/geometry too.

Thanks!

downloads157 · junior member Member # Joined: 10 Nov 2006 Posts: 2

Ian Jones · Posted: Mon Nov 13, 2006 7:22 am

well I think I have a few answers for your first 3 questions:

1. Vanishing points are conceptual representations, therefore infinitely small points. There is not really a strict vanishing point for human vision, its just a concept. I suppose you could say a vanishing point is at the extreme distance of human vision (this is pretty much impossible to gauge anyway, so close enough is good enough). It is the point at which two parralel lines 'appear to converge'.

2. Horizon line is also a conceptual representation. I'm not sure an explanation is really needed, as isn't a complicated concept. In terms of classical perspective, the horizon line is always at 'eye height' (height of view frustrum, center of vision). Assuming we have a perfectly flat ground plane, a object taller than the 'eye height' or center of vision will protrude above the horizon line, as I said because it is related to the height of the viewer 'eye-height'.

3. Answer 2 sort of covers this. A vanishing point is fairly arbitrary, as it doesn;t need to be on the horizon line. The rule is simple, parallel lines eventually converge with human vision because of foreshortening, eg objects further away appear smaller. So, a box whose major parallel lines point off into the sky somewhere above the horizon line follow this rule. They appear to converge in an imaginary 'vanishing point'. A vanishing point on the horizon therefore represents a object whose paralell lines point towards the horizon. or if you like to think of it as being parallel to that perfectly flat imaginary ground plane.

Hope that helps a little.

Affected · Posted: Tue Nov 14, 2006 6:43 am

I'm not entirely sure I understand all of your questions, but there are a few threads on perspective here already.
http://forums.sijun.com/viewtopic.php?t=38954
http://forums.sijun.com/viewtopic.php?t=36275

Read through them, I think you might find some interesting info. I haven't actually checked the second thread fully, only found it just now when searching for the first one, but it seems pretty informative as well.

Capt. Fred · Posted: Thu Nov 30, 2006 2:26 pm

"What? Why? is the horizon line/plane?"

The horizon line is taught as rule of perspective because we are a species that live on a big flat surface, and we are used to having it around.

Mikko K · member Member # Joined: 29 Apr 2003 Posts: 639

What, why, how, when?

So earth is flat after all! The vikings were right!

B0b · Posted: Fri Dec 01, 2006 5:25 am

no its just the circumference of the earth is so big that we don't notice it form on the ground Wink

Jabo · Posted: Fri Dec 01, 2006 6:21 am

Burn him, he's a witch!

Affected · Posted: Wed Dec 13, 2006 8:46 am

The horizon line doesn't have anything to do with the earth. It's an abstraction, just a visual aid to help keep all relevant lines going to the same level.

octavian · Posted: Wed Dec 13, 2006 2:43 pm

Capt. Fred · Posted: Tue Dec 19, 2006 9:42 am

Affected · Posted: Wed Dec 20, 2006 2:41 am

Capt. Fred: lines will only converge "on the ground" if they are parallel with the ground. In any case, in my opinion it's a case of the lines on the ground converging on the horizon as much as any other parallel lines. If you want a sloped surface, you'll either need to have the vanishing point on a different horizon line or construct the slope by drawing inside a box or whatever.

Capt. Fred · Posted: Wed Dec 20, 2006 10:32 am

"If you want a sloped surface, you'll either need to have the vanishing point on a different horizon line or construct the slope by drawing inside a box or whatever."

The most common way of doing it in my experience is the second one: draw a box that IS on the ground and draw a slope inside it - verging on ground-mania.

sweetums · member Member # Joined: 10 Aug 2004 Posts: 236

I wanna know what these "Couple of books on perspective" were, that you don't seem to have a clue as to what you read. Not once, evidently, but twice.

You additionally seem to have no clue as to what an Internet Search Engine is, as I have done a lot of online research into perspective, and every question you have asked has been answered on the Web, with some VERY interesting discussions on the physiological responses to visual stimuli that create our perception of perspective.

I can't believe that you READ not one, but TWO BOOKS, and neither one explained these concepts to you.

PLEASE give us the names of these worthless books, so that they are NEVER recommended to anyone...
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