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Laser Ranging Systems
Laser ranging works on the principle that the surface of the
object reflects laser light back towards a receiver
which then measures the time (or phase difference) between
transmission and reception in order to calculate the depth.
Most laser rangefinders:
- Work at long
distances (greater than )
- Consequently
their depth resolution is inadequate for detailed
vision tasks.
- Shorter range systems exist but still have an
inadequate depth resolution (1cm at best) for most practical
industrial vision purposes.
Structured Light Methods
Basic idea:
- Project patterns of light (grids, stripes,
elliptical patterns etc.) onto an object.
- Surface shapes are
then deduced from the distortions of the patterns that are
produced on Object's Surface.
- Knowing relevant camera and
projector geometry, depth can be inferred by triangulation.
- Many methods have been developed using this
approach.
- Major advantage -- simple to use.
- Low spatial resolution -- patterns become sparser with
distance.
- Some close
range (4cm) sensors exist with good
depth resolution (around 0.05mm) but have very
narrow field of view and close range of operation.
Moire Fringe Methods
The essence of the method is
that a grating is projected onto an object and an image is
formed in the plane of some reference grating as shown in
Fig. 6.
The image then interferes
with the reference grating to form Moire fringe contour
patterns which appear as dark and light stripes, as demonstrated
by Fig. 7. Analysis of the patterns then
gives accurate descriptions of changes in depth and hence shape.
Fig. 6 A moire projection system
Fig. 7 Moire fringe patterns
NOTE: Ambiguities arise in interrogating the fringe
patterns.
- It is not possible to determine
whether adjacent contours are higher or lower in depth.
- Resolve by moving one of the gratings and taking multiple
Moire images.
- Reference grating can also be omitted and its effect can be
simulated in software.
Moire fringe methods are capable of producing very accurate
depth data (resolution to within about 10 microns) but the methods
have certain drawbacks.
- Methods are relatively computationally expensive.
- Surfaces at a large angle are
sometimes unmeasurable -- fringe density becomes too
dense.
Shape from Shading Methods
Methods based on shape from shading employ photometric stereo
techniques to produce depth
measurements.
Using a single camera, two or
more images are taken of an object in a fixed position but under
different lighting conditions.
By studying the changes in
brightness over a surface and employing constraints in the
orientation of surfaces, certain depth
information may be calculated.
Methods based on these techniques
are not suited for general three-dimensional depth data
acquisition:
- Methods are sensitively dependent on the illumination
and surface reflectance properties of objects present in the
scene.
- Methods only work well on objects with
uniform surface texture.
- It is difficult to infer absolute depth, and only
surface orientation is easily inferred.
- Methods are mostly used when it is desired to extract
surface shape information.
Passive Stereoscopic Methods
Stereoscopy as a technique for
measuring range by triangulation to selected locations in a
scene imaged by two cameras already -- further details on
general stereo configurations in Books.
The
primary computational problem of stereoscopy is to find the
correspondence of various points in the two
images.
This requires:
- Reliable
extraction of certain features (such as edges or points) from
both images
- Matching of corresponding features between images.
- Both of these tasks are non-trivial and
computationally complex.
- Passive stereo may not produce depth maps
within a reasonable time.
- the depth data produced is typically sparse since high
level features, such as edges, are used rather than points.
NOTE:
- Problems in finding and accurately locating
features in each image can be hard.
- Care needed not to introduce errors.
- Depth measurements accurate to a few millimetres.
- One such passive stereo vision system is TINA
developed at Sheffield University.
Active Stereoscopic Methods
The problems of passive stereoscopic techniques may be
overcome by
- Illuminating the scene with a strong source of
light (in the form of a point or line of light) which can
be observed by both cameras.
- Known corresponding
points provided in each image.
- Depth maps can then be produced by
sweeping the light source across the whole scene.
- Laser light source typically employed.
- Active stereo can only be applied in controlled
environments -- industrial applications.
Next: Our Active Stereo Vision
Up: 3D imaging
Previous: Introduction to Stereo Imaging
David Marshall 1994-1997