Does anyone know of an iPad/iPhone app that will apply scale to a photograph? If I have know one dimension (say wheel rim diameter) it would be helpful to have an application that would use the same scale to determine the dimensions of another part in the photograph.
Specifically, I know the dimensions of a carburetor, and would like to use those dimensions to determine the size (height and length) of the engine it is mounted on.
If the photo was taken with a relatively long lens opposed to a wide angle lens and if the camera was oriented perpendicular to the dimensions in question, then is would be easy to scale directly on the image. In most cases that is probably not the case but it may still be possible to get a reasonable estimate of the dimensions
Most simple photo editing programs have a scale built in but because of all the variables I doubt that there is a single app.
You might try using the measure feature in GIMP 2:
Showing the Model T Wheelbase of 100" is 427.1 pixels at 11.2 degrees. Using that ratio of (100" / 427.1 pixels) you can get a good estimate of any measurement parallel to the original one.
This image shows the location of the Measure tool:
You will need at least three different ratios:
1. Side to side on the car
2. Front to back on the car
3. Bottom to top on the car
These correspond to the three orthogonal axes X. Y and Z.
Every photo will have different ratios depending on the angle of the camera to the car.
GIMP is available free from The Gnu Image Manipulation Program