Schewed walls in Perspective

[Bob Moore]

I was working on a project with a series of walls off set 3/4". after laying out the building and looking at it in 3d modes every thing was as it should appear. After adding site elements (sidewalks and mesh) I reviewed the project again in 3d and the walls appear skewed when shown in perspective but not in isometic, sections or elevations. By the way at one point archicad crashed during the rendering process. I can attach the pln. if that would help.

[Anonymous]

It's hard to tell what you're seeing without being able to zoom in further, whether it's a trick of perspective or pixels / resolution. If the camera is exactly horizontal (camera height and target height identical) there shouldn't be any tapering at all. Baffling.
Changing the view might reveal some clues.

[Anonymous]

Stuart,
If Archicad is drawing a two point perspective then what you are saying
is exactly true. But if Archicad is drawing a true four point perspective
then wouldn't the walls taper closer together upward from the eye level line and taper together downward from the eye level line ?

I was just checking how AC does a perspective of a slab 60 feet by
60 feet and it's top at plus 100 feet and its bottom at minus 100 feet.
The eye level is set at 5 feet and the target is set at 5 feet.

To my surprise, AC drew the vertical edges top to bottom as parallel
straight lines. if the target level is changed to minus 10 feet then
they are drawn still straight but tapering from the top of the slab
inward, and the opposite if the camera level is set at plus 10 feet.

From what I have learned about true perspective and what looks
right to my eye and is to me logical, AC should, in the case of eye level
and target level being equal, draw the edges tapering in both directions
from the eye level.
We have all seen photographs taken with a fisheye lens exaggerate this effect.
Peter Devlin

[Anonymous]

Peter wrote:
Stuart,
If Archicad is drawing a two point perspective then what you are saying is exactly true. But if Archicad is drawing a true four point perspective then wouldn't the walls taper closer together upward from the eye level line and taper together downward from the eye level line ?....
From what I have learned about true perspective and what looks right to my eye and is to me logical, AC should, in the case of eye level and target level being equal, draw the edges tapering in both directions from the eye level.

Peter Devlin

I'm not disagreeing with you, Peter. I understand what you are saying. It should be four point perspective, but it isn't. I don't think the software is that clever. If you look at Bob's perspective the corners of the building are all vertical.

[Anonymous]

Stuart,
When I look at Bob's image I see the corners tapering inward from
the parapet down to the ground as though he had set his target
level lower than eye/camera level.
My eyes may be playing tricks on me so I am going to try to download
Bob's .jpg and paste it into AC and trace those edges to check.

Bob,
I did not mean to divert this thread away from your question.
I would try doing what Stuart suggested
and also set the 3D to hidden line mode so we can see what is
happening more clearly.
Peter Devlin

[Bob Moore]

The walls are not supposed to be tapering at all. When I resized the window (made it smaller) in the 3d window settings it cleared up. Then I made the window larger again an the skew reappeared. Then I change it back to the small window and it was fine. Go figure...

[Anonymous]

I think it's just the resolution playing tricks with you then. Is that just an OpenGL image you've posted? If so how does it render?

[Anonymous]

Stuart,
I downloaded Bob's jpg and pasted as a figure in AC.
I tried to trace as carfully as I could the left and right corners.
The left and right corners are not parallel.
See attachement
Peter Devlin

[Bob Moore]

It is OpenGL. The rendering in Light works is correct, as was the isometric.

[Anonymous]

Peter wrote:
Stuart,
I downloaded Bob's jpg and pasted as a figure in AC.
I tried to trace as carfully as I could the left and right corners.
The left and right corners are not parallel.
See attachement
Peter Devlin

And what was your margin or error, about +/-0.02degrees?
(nearly time for a cold one, TGIF)

[Anonymous]

Stuart,
True and definitely True.
Cheers,
Peter Devlin