Could the image you are working from already have some distortion or have been tweaked by a camera? May even have had the format changed at some time.

There is also the problem with draft angle on what looks like a cast bracket so your 90deg corners may well be 93 or 87

That does look the best for two planes, front to back is not so easy to find a long straight pair, probably top and bottom of the motor mount plate as they seem to be the only two on the same plane assuming the top edge is parallel to the bottom?

I've tried something not entirely dissimilar when trying to find dimensions for railway vehicles and buildings that are no longer with us.

My first attempts involved measuring the photo and trying to use trig to determine the unknown dimensions from the one that was least uncertain. I thought it should be simple but I could not make it simple or get it to produce sensible results.

My approach now is to take advantage of the fact that I need to make a drawing anyway and then create a 3d model in whatever tool, export it to STL and then import that to Blender. With Blender I can then guess the relative position of the camera and the subject, guess the focal length of the lens and attempt to reproduce the perspective in the photo. The photo can be imported as a background and I can move the opacity slider to compare the two. The same model can be used to overlay multiple photos from different angles if you have them.

Blender takes time to learn but overlaying a photo onto an existing STL is using a tiny part of its features so you only need to learn those parts.

See below for an example of a building. It's the Steam Packet Inn in Hayle and was demolished in the 1960s.

I have not tried to use isometric projection as an interim step TBH.

Well yes and no

Light travels in straight lines by bouncing and dissipating

Never forgot about some chap in the far east who invented a camera which could reconstruct deconstructed light

so looking down a corridor became like looking down a corridor covered in mirrors, so you could see around corners

clever stuff

Another tricky problem from Michael!

This shot from Michael's first link suggests why the answer is hard:

The type of camera that may have been used to photograph the Jason is adjustable in all three planes, and is positioned and tilted by the operator relative to the target to control perspective and lens distortion. He adjusts images by eye so they look good, rather than them being geometrically correct. Getting back to isometric means first undoing the unknown camera corrections, and then removing perspective and lens distortions; the lens is also unknown.

Problem is still difficult even if an ordinary camera was used. The camera sees a perspective image because the object isn't flat. Big cow far away and model cow close up are the same size, making it possible to take photographs like this, which I think are impossible to lay on an isometric grid and recover the dimensions.

In the Pisa example, in translating a 3D image of the tower and girl into 2 dimensions the camera has lost the information that tower and girl are on entirely different scales. Although the Pisa example is extreme, the effect also applies to smaller objects like the Jason. It's easier to work with flat photos and stereoscopic pairs than a single photo of a deeply 3D object.

Martin's on the right track, and as he says 'some experiment is required to get the best overall result.'

The camera never lies? Oh yes it does…

Dave

Here is the effect of rotating to even out the angles and skewing to bring the verticals back to vertical. Then stretching the width to get shallower angles. The quality of the picture just decreases with every geometric adjustment.

Martin C

I've used a similar approach to get a 'side on' view of a locomotive photographed obliquely.

There is a deal of distortion, but it helps with working out dimensions.

I've also found that things made in imperial dimensions tend to be sized conveniently, often in obvious 'steps'.

Neil