POV-Ray : Newsgroups : povray.advanced-users : Facebook 3D posts : Re: Facebook 3D posts Server Time
8 Jun 2023 07:48:34 EDT (-0400)
  Re: Facebook 3D posts  
From: Bald Eagle
Date: 27 Jun 2021 08:20:00
Message: <web.60d86ce893de9b141f9dae3025979125@news.povray.org>
> I did, but not in POV-Ray. POV-Ray is a rendering tool, it takes as input a
> representation of a 3D scene and output a 2D image. Photogrammetry does the
> opposite, it takes 2D image(s) as input and outputs a representation of the
> corresponding 3D scene. So, I don't see how one could implements photogrammetry
> in POV-Ray, they serve both two opposite purposes.

Yes, but we don't always start with a blank slate and start coding a scene in a
vacuum.  We have all sort of "input data" that we can use for POV-Ray's SDL
language to perform operations upon.  And so sometimes we need to take a 2D
image, reverse the process to obtain the relevant 3D information, and then use
that to render a new 2D image.

Take for example Francois LE COAT's very interesting work:

Forgive me if there's a lot of things I don't understand, properly recall, or
have wrong, but it seems to me that an aerial photograph of a landscape has a
lot of perspective distortion, and part of the photogrammetry process would be
correcting for that.

If I wanted to take a photograph of a piece of writing paper laying on a table
and use the portion of the image with the paper in it as an image_map, I'd
likely have a horribly non-rectangular area to cope with.  Preumably there are
photogrammetry-related tricks to deduce what sort of transformation matrix I
would need to apply in order to mimic reorienting the paper to be perpendicular
to the camera, and have straight edges with 90-degree corners.

Let me preface this one with some information that will help you understand what
I'm talking about:

http://imcs.dvfu.ru/lib.int/docs/Programming/Graphics/Academic Press Graphics
Gems Ii 1995.pdf
(use the wayback machine to get the PDF.   pg 181 (pg 208 of pdf) )

View correlation.
"To combine computer-generated objects into a photographic scene, it is
necessary to render the objects from the same point of view as was used
to make the photo."

Rendering Synthetic Objects into Legacy Photographs
Kevin Karsch Varsha Hedau David Forsyth Derek Hoiem
University of Illinois at Urbana-Champaign




There are plenty of examples of people creating POV-Ray renderings based on real
things - things they often only have photographs of.

But then I was thinking that there are probably actual POV-Ray renderings, but
somehow the source to generate those renderings have been lost due to the code
never being posted, a HDD crash, or just getting - lost.  People might want to
recreate a scene, and having some basic information about the size and placement
of the objects in the scene would greatly speed up the writing of a new scene

I know that I have done some work to recreate some of the documentation images
for things like isosurfaces, and didn't have the code for those images.   I had
to make educated guesses.  I "knew" the probably size of the object, or at least
its relative scale, and then I just needed to place the camera and light source
in the right place to get the image to look the same.   But determining where
the camera and light source are seems to me to be something that could be
calculated using "photogrammetric image cues" and well-established equations.

Let's take a photograph of a room.  It likely has tables and chairs and windows,
and these all have right angles and typical sizes.  It seems to me that there
might be a way to use photogrammetry to compute the 3D points of the corners and
rapidly generate a basic set of vectors for the proper sizing and placement of
everything in a basic rendered version of that same room.

I know that they have cell phone apps that can generate a floor plan of your
house just by snapping a few photos of the rooms from different angles.

Also, the "augmented reality" apps for cell phones.

I think there are a lot of cool things that could be done with POV-Ray in the
"opposite direction" of what people normally do, and the results could be very
interesting, very educational, and give rise to tools that would probably find
every day usage for generated new scenes, and perhaps serve to attract new

It was just my thought that if you do this "every day", that you would have the
level of understanding to quickly implement at least some of the basics, whereas
I'd probably spend the next few months chasing my tail and tying equations up
into Gordian knots before I finally understoof what I was doing wrong.

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