How to get the dimensions correct in a 3D Technical Illustration of a car that you have never seen.

One of the biggest problems that face a 3D technical illustrator is to find enough accurate dimensions. Using actual photographs as a reference is probably the best way to get reasonably accurate measurements, but unfortunately, lenses and camera positions tend to distort matters.

If the Item Exists and you have Access to it…

If your client has supplied you with a sample or has allowed you to actually measure and photograph your subject then count yourself very lucky. You want to make sure that you get as much information as possible. No matter how careful you are, you will allways find that just that one dimension that you need is missing. Try to get the following (if possible):

  • The original design drawings (this would be first prize!)
  • An overall blueprint with dimensions.
  • Orthogonal photographs (Front, Sides, Back, Below, Above) using a long focal length lens from as far away as possible. (This reduces parallax errors). Try to get the camera set at an elevation which is exactly half the height of the object.
  • Physical measurements of the distances between identifiable objects on the photos.
  • As many isometric views and detailed close-ups as time permits. Digital camera memory is cheap – take hundreds of pictures – with and without flash. You will never have enough anyway.
  • Photographs of some areas of pure colour so that you have a colour reference when you go back to the computer in the office. Take a colour colourful glossy magazine, or reference colour booklet with you and photograph it next to various critical coloured parts of your subject so that you will be able to match the image on the photo on the monitor to the actual magazine or colour-swatch image and thereby filter out any colour casts. You will then be able to produce a pretty good representation of  the correct colour of your object.

If you don’t have Access to your Subject…

In the case of this 1929 Bentley Blower technical illustration, I have never even seen one in real life. In a case like this where either your subject doesn’t exist or that you have to draw something but can’t get close to it,  then all you can do is to scour the internet (and in a case like the Bentley Blower every coffee-table book you can find) for reference material. I searched the Internet for months for pictures and dimension specifications and everywhere I looked, I found lots of dimensions, but few of them agreed with one another. Eventually I found an orthogonal blueprint image from Google Images which is an illustration and therefore stylised but nevertheless quite well done and usable for my purposes of getting the basic proportions correct:

Blueprint of the 1928 Bently Blower

Blueprint of the 1928 Bentley Blower. (I don't know who originally drew this image but I would be happy to include a credit if the author happens to see this blog post and contacts me.)

Once you have an orthogonal blueprint or some good orthogonal photographs, you need to bring them into AutoCAD as a background image ( IMAGEATTACH ) and scale them up to real size (with the SCALE – REFERENCE command). You need only one dimension, but that one has to be exactly correct because everythng else will depend upon it. The one dimension that I found which was consistent most of the time was the wheelbase. (The wheelbase dimension is the distance between the centre of the front axle to the centre of the back axle). In a perfect world with perfect illustrations and 100% undistorted photographs, every other dimension must then be proportional. Of course, errors start to creep in, but with enough photographs and a generous dollop of common sense, you can use AutoCAD to construct/calculate what you need.

Using AutoCAD you will find that you can often correct the most horrendous parallax and lens distortion errors. Make sure that you roughly dimension the drawing so that you can mathematically check that your drawing makes sense and that you have the correct measurements for the constrained Inventor sketch.

Here is an example of constructing the headlight profile using the master-scaled blueprint in the background:

Headlight drawing on blueprint Image

Headlight drawing on blueprint Image (Click to Enlarge)


My workflow is simple – if I have the dimensions I need and the shape is relatively straightforward, I work directly in Autodesk Inventor to create the parts. If the part is a bit more complicated, I find it easier to construct the 2D profile image of the object I am currently working on and then copy the outline (or part thereof) from AutoCAD into an Autodesk Inventor sketch. From this I start creating the actual 3D part. Once all the parts are constrained into Assemblies and sub-assemblies, the resulting 3D model is transferred into 3DS Max for rendering and animation.

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