Making scale drawings from photos

[Hauk]

Those of you that have read my ramblings about buses will know that I am facing the challenge of making a set of scale drawings based on photos with a large degree of perspective. When you want to model pretty obscure stuff, this is a challenge that raises its confusing head quite often.


But I have never been able to figure out the exact procedure for removing the perspective from the photos.
In theory, it should be possible to make a scale drawing by constructing a  two-point perspective drawing "in reverse".

So I tried a little experiment. I wanted to see if I could calculate the correct height (Z) and width (Y) of this wooden box using only the picture and the length (X). I have made it a bit simpler for myself by taking the picture straight on without tilting the camera, so Z is unsharpened. So the challenge is to remove the shortening on X and Y:



Using standard 2-point perspective drawing theory I established the horizon and two vanishing points:



Then I constructed the View distance, station point and measuring points:



Next step was making a measurement bar and finding the unshortened length of the X and Y sides:



Now it should be just a matter of scaling everything so that the width becomes correct based on the known X dimension of 153mm. This should in theory give the correct Y and Z dimensions.  And measuring the height on the drawing gives indeed the correct height within a reasonable margin. But the Y came out  too short, only around 80% of what it should be. I did the same experiment with a diffent box and  got the almost exactly  the same error. So I am doing something wrong. Does anybody have a suggestion for where I went wrong?
I would also love to hear from others that have tried to make scale drawings the same way, and maybe with methods that actually work...

Any input is very welcome !

[mabloodhound]

SketchUp does that for you when you import a photograph into it and scale the photo.  It does require that you have a fairly straight on photo (not tilted) as you have done.  SketchUp has many good features for photo work to drawing.

[finescalerr]

I would guess that part of your problem is lens geometry. You may need some kind of unique equation and knowledge of lens distortion (different for every lens). I am helpless trying to understand anything with numbers and, if you add letters to those numbers I simply shut down, but I do know lens and sensor pairings use a lot of tricky math. -- Russ

[Hauk]

I would guess that part of your problem is lens geometry. You may need some kind of unique equation and knowledge of lens distortion (different for every lens). I am helpless trying to understand anything with numbers and, if you add letters to those numbers I simply shut down, but I do know lens and sensor pairings use a lot of tricky math. -- Russ

You might be on to something. But you should be able to do the exercise without the aid of a image made by a camera at all.
A perspective drawing of a box made using the exact same perspective drawing theory should be possible to "reverse engineer" into a scale drawing without any foreshortening.

Hmm, but if that works, I would actually have proven your theory that it is  the lens that creates the error! So that will be tomorrows exercise.

[5thwheel]

This is a method I taught in a model building class.

[5thwheel]

Instructions

To add second plane add base line E-F crossing A-B at7 out at an angle to the right to match the base of the object such as a box. Extend reference line to the right to establish all verticals up from the baseline as was done in the first plane.

[Chuck Doan]

Great info Bill!

[finescalerr]

Are you a genius or something? I couldn't have figured that out in six lifetimes! -- Russ

[nalmeida]

I use a tool called perpective correction which can be found in most graphic editing programs. I use it in Paint Shop Pro. It's not exact but when you consider all the variables it's quite difficult being exact when using photos. With that said I found out that even using manufacturers plans it's also a challenge, it's interesting how changes were made on the fly ignoring original plans and adapting to the manufacture.

[Hauk]

As usual, any question on this forum results in a lot of interesting replies.

But none of them so far adresses the main problem:

How to remove the foreshortening in the planes without knowing *any* of the dimensions.

I still think this should be possible using perspective drawing techniques in reverse.

Finding a good way to remove the foreshortening would be especially useful on pictures of structures where you have no circular features to aid you in estimating the foreshortening.

So keep the discussions going!

[Bill Gill]

Hauk, I'm confused. If you don't have any dimensions, how would you know there was distortion in the first place?

[eTraxx]

I brought one of the buses into Sketchup via Match Photo and made an attempt to align the perspective bars to it. Then made a 'box' around the bus so I could project the photo on to it.



I projected the image onto the box and switched to Parallel Projection and viewed from right.



this was done 'on the quick' .. spending a little more time and you might get a better view. In Paint Shop Pro you can adjust/correct distortions by the camera lens .. Barrel, Fisheye and Pincushion. Someone who knows a little about the way a camera lens distorts photo can use that prior to using the Sketchup Match Photo and get better results. Here I applied a Fisheye Correction on the first photo with the box around it as it shows the correction made.

[Hauk]

Hauk, I'm confused. If you don't have any dimensions, how would you know there was distortion in the first place?

The dimensions are not important. But the presumption you have to accept in my example is that the object in the photo is box with sides at 90 deg to each other, and that the planes opposite each other are parallel.

This is usually a safe bet with basic structures, boxcars etc.

So when you have picture where you are certain that these criteria is met, you should be able to convert the perspective photo into a  drawing with the perspective removed, and with correct proportions.

But without a dimension from the object (measured or estimated) you have no way of knowing the scale of your drawing. You have no way of knowing if the box in my picture is a giant replica hundred times the size of the original archive box or a scale model.

[peterh]

I know of a way of making a plan from a photo by geometry. You find the station point as you have done in your first post, but then you proceed differently, starting by drawing radial lines up from the station point through corners and edges on the photo.

I could post a sketch, but haven't figured out how to post an image yet. Better still, I could email you a PDF of the magazine article that told me how. If you don't want to put your email on this forum, why not email Marc R (email at the top of the main forum page) and ask if I can send it to him and he will forward it to you.

[Bill Gill]

Hello, Peter. Please share the perspective technique you found with the forum. I'd find it helpful for several projects and others probably will be interested as well. It took me a couple tries to figure out how to post images too. I emailed Marc R for assistance. He said to reduce the image to 72 dpi and that only one image can be posted at a time.

When you are logged on and you want to add an image to a reply or new topic, first click "-Additional Options" just below and to the left of the white text box. Then click "Choose file" that will open a window where you can search for the image you want, click on it and click "open". If you have another image you can click "(more attachments)" and repeat the search, open steps, but you can only post a max of 4 images at one time, and they cannot be larger than 230 KB, for more images you have to add another reply to the post and repeat the steps. Finally click "Post" to post it all.
One quirk I cannot figure out is how to see a "Preview". When I click on it the reply vanishes and I have to start all over. Good Luck, hope to see your success here later.