Establishing VPs and horizons in Landscape photos?

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  09 September 2012
Establishing VPs and horizons in Landscape photos?

I understand how to set this up when there are straight edges but how would you do this for the attached exmple image?
  09 September 2012
Well, in the case of the image that you posted, it is quite difficult to get an exact horizon line. You do have a few straight lines in that image but they wont help you much and you can't be sure they aren't on an incline. My suggestion would be to try and figure the horizon line out based on the undulations you can see in the ground and the general lay of the flatter planes in the image. At the end of the day, if you are slightly off, you are unlikely to notice because its so tough to tell the exact perspective anyway. If you have other images of the same location you can use these to give you an accurate horizon, but I can't think of a situation where it would cause issues if your horizon line wasn't 100% in the right place. Can you explain a bit more about what you are trying to do? Or is this just a general question?

  09 September 2012

It was just a general question. I've used 3DS Max's perspective matching tool or Imagemodeler to get my cameras matched in the past, then then other day I learnt how to do this manually as an excercise. I was suprised how easy it was but then that was using a few images with obvious lines of convergence.

I started reading the Matte Painting Handbook as I'd like to learn how to paint, I kinda suck but did my first painting yesterday so can't complain. I want to be able to matte/paint landscapes as backplates for my 3D scenes so figure I need to learn how to set up vanishing points for this type of scene before I can hope to paint one.
  09 September 2012
Yes you are right, if you want to be able to produce matte paintings you should definitely get a grasp of perspective. Learning to paint can be a frustrating experience, probably even more so if you are used to getting good results in your 3d work and have to go back to basics again. Once you get past the first hirdles though and can produce decent paintings without too much difficulty, you will start to find that it will be a powerful tool in your work and will probably help your 3d work too! Learning to paint is a pursuit that never really ends, but is very rewarding.

Maybe post some of your paintings here and we can give you some pointers along the way?

Good luck!
  09 September 2012
Just by looking at the image (if I had to add CG items to it), I would place the horizon line 1/3 of the way up the tree. Both vanishing points for the bridge would be outside the image obviously.

Last edited by ShawnDriscoll : 09 September 2012 at 05:31 AM.
  10 October 2012
to re kindle this thread.

Is it be possible to calculate where vanishing points would be mathmatically by knowing the FOV of this image?

I'm reading a book on perspective that demonstrates setting up some elaborate grids but it doesn't cover this topic. maybe it can't be done or maybe I'm missingthe point somewhere along the way
  10 October 2012
Since the image is over-processed, you can probably get away with simply eyeballing any perspective you need for it manually.
  10 October 2012
The attached is just any old image, I'm thinking for new projects where I may not get the luxury of photogrammetrically calibrating images.
  10 October 2012
You'll need a book on Matchmoving then. That shows how to manually determine the FOV (mm lens used) for an image or some footage so that your imported assets all line up correctly.
  10 October 2012
Yes, if you know the FOV you can draw something technically accurate. Basically when you create your perspective setup you need to make sure that you extend your FOV from the station point at whatever degree you want. If you are trying to base this on a real camera then you will need to get the camera properties from the manufacturers information / specifications and use that to work out the FOV based on a specific focal length and aperture. Then as long as you make sure your vanishing points extend out at 90 degrees from the station point you are all set. Just make sure your picture plane doesn't extend outside your FOV guides and you will be fine.

Couple of things to remember though, to get this to a stage where you are totally accurate, you can't do the usual tricks like ignoring 3rd point convergence if it is only very subtle. Perspective is just a means of getting a 3d object or space onto a 2d plane and can be worked up to a point where it is well beyong the point of a person being able to discern any issues, but you would need to be so perfect to get a measured perspective drawing as accurate as 3d that you could probably learn Maya a couple of times over before you are finished with your drawing. Human error will always introduce slight issues, but if you are careful you can get it to within a negligible margin of error. And once you know how it works is a hell of a lot easier to draw something that can be modelled from fairly easily so its still good to learn!
Also bear in mind your drawing wont have lens distortion so if you are trying to match or extend a photo, delens it first!

I'm not sure whether the normal perspective drawing methods take into account the curvature of the earth either.... Something i need to double check. Again there are easy formulas for determining how far away the horizon would be from the cameras vantage point, but just bear in mind that a flat ground extending out to infinity wont give you an accurate representation of the horizon line as it doesnt take into account the curvature of the earth. If its not earth you are drawing (maybe its Mars?!) i think the formulas can be adapted to suit a planet of a different diameter, but again this is way way further than most people will notice and when working in vfx is usually well outside the margin of acceptable error. Only once have I ever needed to work out the exact distance of the horizon and that is because we were high up and looking straight out to sea. Of course you can go even further as the earth isnt actually 100% spherical (it differs at the poles vs the equator) but if i went into dailies with that, i think the only 100% certainty would be that there would be a lot of blank faces and strange looks
All that is before you consider that there's nowhere on earth that is 100% flat (or following a perfect curve of the earth to be exact) and that sometimes even when you get your perspective totally right it can look and feel a little off, and a supe will simply say 'can you drop that horizon a bit please' and lay waste to all your hard work in an instant!

Anyway i'm rambling now, but yes, you can work it out


Last edited by nickmarshallvfx : 10 October 2012 at 12:30 AM.
  10 October 2012
Cheers for the in depth reply. Some very interesting points. I'm learning this for both 2D matte paintings and for integrating 3D/setting up cameras.

Shawn has a good point with eyeballing it however I'm enjoying learning about it and think these tricks can only help, so I like getting to the nitty gritty.

Nick - if you've got time would you mind adding to this sketch, maybe a couple of notes as I didn't follow all of what you said? I'm assuming that this is the right setup to use?

When you say 'control point', are you refering to the lowest most point in the attached sketch (the book I'm reading calls this the 'eye point')

And when you say 90degrees, what are you refering to? The book I'm reading shows all drawings with 90deg lines going from the Eye point to the horizon to find your VPs.

I tried in CAD adjusting these angles to 110 degrees to see if that was what I needed to change to match my lens. It distorted the grid more but I wasn't shure if this was what I needed to do, nor where I should draw the picture plane.

Thanks again for the replies.

  10 October 2012
I gave this some thought - is this sketch correct for placement of Picture Plane?
  10 October 2012
What you have there is a fine option for a picture plane, although your construction looks a little questionable. I'm in the last 2 weeks of a project at the moment so i'm crazy busy but should have some time at home on Sunday to draw out a quick diagram for you.
I totally agree with you by the way, eyeballing works pretty well in most cases, but being able to draw an object and give a camera FOV and height and a scene scale to him/her and it be a near perfect fit with your perspective is a cool ability to have and will make 3d guys love you
  10 October 2012
Originally Posted by nickmarshallvfx: I totally agree with you by the way, eyeballing works pretty well in most cases, but being able to draw an object and give a camera FOV and height and a scene scale to him/her and it be a near perfect fit with your perspective is a cool ability to have and will make 3d guys love you

You end up with the camera FOV, height, and scale settings after doing the eyeballing. There are books written explaining how to do all this.
  10 October 2012
Originally Posted by ShawnDriscoll: You end up with the camera FOV, height, and scale settings after doing the eyeballing. There are books written explaining how to do all this.

Can you explain what you are referring to in a little more detail? I'm not sure exactly what technique you are talking about here that will allow you to draw freehand and eyeball in your perspective and still end up with something precise enough to be totally accurate right down to specific measurements and camera settings?
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