First of all you should prepare the environment needed to take photos.
What should you do?
You should take a great number of photos of the object
while it rotates around a fixed axis. The procedure is as follows:
You should take several dozen of photos (50-100 photos).
- take a photo;
- rotate the object by a fixed angle;
- take another photo;
- rotate the object again by the same fixed angle;
- take another photo;
What do you need? You need:
How can you carry out a fixed angle rotation?
I wrote a simple program for this purpose.
The program lets you use your printer to obtain a graduated disc.
You should stick the graduated disc over the rotating disc, as in figure 1.
- a digital camera (I used a 3.2 megapixel camera: it works just fine!);
- a rotating disc. You can use the disc you put under your TV to easily
rotate it. If you haven't such a tool, you can buy it (usually with less
than 10 euros and maybe you can re-use it later for its original purpose).
Maybe you can build your own rotating disc, but remember: the rotation
should be as accurate as possible: the axis should be fixed and the angle
of rotation should be always the same!
Figure 1a-1b: obtained with: s3d-disc -r173 -n -o disc.
In this figure you can see how I used a piece of polystyrene and a pin
to realize a reference pointer.
I sticked the piece of polystyrene on the table with bi-adhesive tape.
How can you use s3d-disc to obtain a graduated disc?
If your disc has radius = 123 millimeters, you can use:
> s3d-disc -r 123 -n -o disc
The program will create the file 'disc0.eps'.
You should convert it to a PDF file with:
> epstopdf disc0.eps
and print the file 'disc0.pdf' with your printer.
If your disc is too big to be printed on a single sheet (as in figure 1),
s3d-disc will generate several files: 'disc0.eps', 'disc1.eps', ...
Convert them with the command epstopdf, print, clip and glue them together.
If you don't use the A4 format, you can specify the dimension of your paper
with '-p 180x250'. For more details use "s3d-disc --help".
NOTE: all lengths must be given in millimeters.
How should you use your digital camera?
I can give some useful suggestions. You should avoid touching your camera
in any way, during the rotation of the object. If you move the camera,
it won't be possible for Scan3D to find the axis of rotation and this
will lead to a wrong or ugly reconstruction of the surface of the object.
So I give the following suggestions:
How can you reduce prospective-effects?
To reduce prospective-effects you should put your camera far from the object
and use the zoom to avoid too small images.
Scan3D needs the profiles of the object for many different angles.
In future maybe Scan3D will be able to use a photo of the background
to better distinguish the profile of the object. However nowadays Scan3D
uses only a threshold operator to obtain the profiles.
This means that the colours of the object should be different and
distiguishable from the colours of the background.
This also means that Scan3D will have some limitations: you can't scan
a transparent object and you will surely encounters many problems
if you try to scan a black object on a black backround.
One partial solution to this kind of problems is the following:
you should construct a light-emitting screen.
This will be a very simple task. Look at the following photo:
- connect the camera to your PC and use the latter to control the camera.
This should be possible with most digital cameras;
- check the state of charge of the batteries. You won't be able to change
them once you started to take photos;
- be sure that the object is all inside the camera view, when it rotates.
Figure 2a-2b: inside the screen
I put a a lamp inside a carton, and closed the box using paper.
When you turn on the lamp and turn off the light you can see the dark profile
of the object.
Figure 3: the object.
Figure 4: how the screen works: the profile of the object
With this trick, even a white object will seem to be black,
when you turn off the light.
The only problem you could still encounter is that of reflections:
in particular when the surface of the scanned object is reflective
(this happens with varnished surfaces or with metal surfaces).
NOTE: Obviously you should disable the flash-lamp
of your camera, when you take the photos.