View
227
Download
1
Tags:
Embed Size (px)
Citation preview
3D Scanning
How can we get the form?
Projection
Capturing design intents
Generators Fillet radius change Knuckles
Scanning
3D Scanning is the process of getting a set of surface points from a physical object
Touch probe, optical, laser ranging, etc.Usually the points are structured
3D Scanning equipment
Contact type Touch probe scanner CMM
Non-contact type Laser scanner Optical scanner
Touch probe scanning
Co-ordinate measurement
Laser range scanning
Optical scanning
Application range
From very small (tip of a drill bit) to very large (the Lee Ka Shing tower)
Rigid or softColoured, glossy, transparentRestrictions: Smoke, hair, cloud, very fast
moving objects, etc.
Data captured
X Y Z location of pointsFreshly scanned data sets are usually
structured, as a array in XY plane with different Z values
May become un-structured, un-even spaced in all directions after re-sampling
Usually stored as an ASCII text file
Point cloud
The collection of points is called point clouds
A point cloud define the SURFACE of an object
Some scanner (like CT and MRI) can measure physical properties of a object’s entire solid mass, the collection of points is not point cloud, it is called VOXEL
Level of details (LOD)
LOD is determined by point density, which is determined during scanning
LOD is proportional to file sizeScanning itself is not selective, the result
file size of a flat surface is the same as a carved board
Shape consideration
Optical scanner capture a 2D array of points each time
Steep surface will have less points cast on it, thus fewer details
3D Optical scanning
Take ‘pictures’ of the objectOutput a set of 3D points
Shape VS position
Point clouds gives only position information
The shape of the object, if there is any, is not defined in the point cloud data
Human can infer the shape from the clouds easily, but it is very difficult for machines
Scan registration and merge
Scanning and stitching
Scan the object, coat the surface if necessary
If the scan result cannot cover entire surface, scan from other angles
Use point cloud manipulation software to align and merge scan results
Re-sampling
Re-sampling is the process to reduce the number of points or polygons so that the file size become manageable
Adaptive re-sampling reduce more data points on smooth surface and keep more in regions that have abrupt changes
Alignment
Points are only x,y,z values
Positioning and scaling
Data from optical scanner is not necessary correct in size
Positioning of point cloud is not necessary suitable for surface patching
Alignment may need to be adjusted
Tessellation of Point cloud
Triangulation
The process of converting point clouds to polygon models
Triangulation is computation intensiveIf two points are close together, they are
most probably on one surfaceIf there are more that one possible shape,
the shape that form a CONVEX HULL are more likely the correct choice
Multiple solution
Create curve network
Feature curves
Feature curves are curves that define distinctive visual features Sharp edges Fillets Holes
3D scanning cannot capture feature curves directly
Irregularities may be corrected
Repair model
Surface modelling
Generate surface patches using feature curves as guide lines
Lofting is easier but may generate surfaces with unwanted undulation
Sweeping and ruling generate fair surfaces but the feature curves are more difficult to draw
Irregularities may be corrected
NURBS surface creation
Surface recreation
Cut Sections by user defined plane, and then create surface according to these sections
‘Draw’ 3D feature curves directly on the point cloud
Full auto generation
Steps to retrieve CAD from foam modelScanning and stitchingRe-samplingAutomatic surfacing or manual surfacingFor manual surfacing
Positioning and scaling Draw feature curves Draw Surface patches Check deviation
Application
Related applications
Specialised applications Surfacer Rapid Form Raindrop Geomagic
Scanning modules Pro/E, UG, CATIA, ALIAS