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Introduction to Vectorization of Engineering Drawings
Song Jiqiang18/9/2001
Concept
• Vectorization: the conversion from a raster image to its vector-form file.
Engineering drawings
in paper form
Raster images
Vector-form(CAD files)
Scan Vectorization
Why do vectorization?
• A lot of old drawings to be reused, and CAD files are more editable than images.
• Preprocess of automatic drawing understanding systems (information statistic, 3D reconstruction).
• Save the storage space.
History• Begin research on vectorization at late 70’s;
• Begin that for engineering drawings at late
80’s;• Related organizations & publications
– IAPR TC-10, IEEE– IEEE T.PAMI, PR, PRL, CVIU, CVGIP, etc.– ICPR, ICDAR, IEEE CVPR, etc.
State of arts
• K. Tombre (LNCS vol.1389, 1998) : “None of these methods works. … Actually, the m
ethods do work, but none of them is perfect.”
• 《 CADALYST 》 performs the annual evaluation on commercial vectorization systems.
Review of existing methods
• Thinning based • CT(Contour Tracking) based• RLE(Run Length Encoding) based• SPT(Sparse Pixel Tracking) based
Thinning-based methods
CT-based methods
RLE-based methods
SPT-based methods
P3
P0
P1
P2P0 P1 P2 P3 P4
Existing difficulties
• Lines with intersections broken into pieces.
• Texts touch lines misrecognition.
• The interference of recognized objects repetitive detection, false detection.
Our research
• Analysis the vectorization model of existing methods.
• Propose an efficient vectorization model for engineering drawings.
• Propose a group of new graphical object recognition algorithms.
Common model of existing methods ( 2PV - 2 Phase Vectorization )
Raster image
Low level vector form:Short line segments
Graphical objects:Straight lines, circle/arc, curve
Phase 1: Skeletonization or medial-axis approach
Phase 2: Vector-based graphical object recognition
Motivation of 2PV
• Internal memory (RAM)– Used to be high price & limit capacity– High pixel access frequency cause swap
• Pixel tracking algorithm– No guide direction– Repetitive tracking
Object-Oriented Progressive-Simplification based Vectorization Model
• 1 phase model– Imitate the way that humans read drawings
• Recognize a graphical object in its entirety– Object-oriented feature
• Simplify the image data as the recognition goes on– Progressive-simplification feature
Workflow of OOPSV
S-LineRecognition
ArcRecognition
CurveRecognition
SymbolRecognition
TextRecognition
LineSymbolText
ArcCurveSymbolText
CurveSymbolText
SymbolText
Text
S-Line Arc Curve Symbol Text
Graphical object recognition
• Get the intrinsic characteristic of individual type of graphical object.
• Use the characteristic as a guide to track the graphic object in complex environment.
Straight line recognition
Seed segment detection
A
Seed Segment
Irregular run Regular run
Direction guided tracking — based on the Bresenham algorithm
Straight line recognition
O
Vo Vp
Seed segment
Perpendicular runs
Black segment
White segment
Straight line recognition
Dynamic adjustment to tracking direction
PP’
Vp
O
P
P’
Vp
O Pe
Line net recognition
• A line net is a group of intersecting lines.• Take advantage of the intersecting
relationship to accelerate recognition.• Example:
Circle/Arc recognition• Arc segment detection
– get initial arc center, radius, thickness
• Circular tracking– based on the Bresenham algorithm for circle
Circular tracking
P2
P1
O
P
P’
PE
(a) Adjust succeeds (b) Adjust fails
Legend: raster image tracking path medial axis testing path
P2
P1
O P P’
PE
Curve tracking
Tracking result: a sequence of polyline.
P5
P4
P1P2 P3
P8
P9
P7
P6
Pm
P10P11
Image simplification
• Intersection-preserving pixel deletion• Based on the contour detection of the
intersecting branches
Branch at one side
MiddleLine
Branches at both sides
RecognizedLine
Contours of branches
Symbol recognition
• Common symbols– Cartography-based recognition
• Domain-specific symbols– Template-based recognition
Cartography
Symbol template
Text recognition
• Text segmentation– Difficulties: text touches line, similar size
• Character recognition– Stroke-based recognition algorithm
Image before the line recognition
Image after the line recognition
Suspension-Release mechanism
Condition 1: Size( Box(li) ) < Tl Condition 2: p, pl C(p,L) >> C( FP(l,L), L)
n
i=1
Stroke-based character template
• Stroke definition• Black position• White position• Aspect ratio scope• Complexity level
①
②
④
③
⑤
Character recognition
(UMNKLDREFBbhklI1) (BE)
(E) Accept ‘E’, then cut image
Separate connected characters
a. When the rightmost stroke is vertical
b. When the rightmost stroke is not vertical
f(x) x1
•Base on the analysis of the rightmost stroke and the vertical projection.
Experimental result
• Implemented a complete vectorization system running on Windows platform using VC6.0.
• Automatic vectorization of an A0-size drawing (15M) takes about 5 minutes. (PIII500/128M)– Line vectorization takes less than 1 minute (1600 lines),
faster than performing a thinning operation (3.5 mins).
CDI evaluationSize Dp Fp PRI Dv Fv VRI CDI
A0 0.966 0.086 0.940 0.954 0.082 0.936 0.938
A1 0.973 0.077 0.948 0.965 0.084 0.941 0.945
A2 0.975 0.054 0.961 0.963 0.068 0.948 0.955
A3 0.962 0.049 0.957 0.959 0.066 0.947 0.952
This protocol was proposed in Machine Vision Application, (1997)
Manual editing cost evaluationDrawingSize
False_alarms Misses One2many Many2one Total Costs
A0 Ⅰ 124 80 24 37 354Ⅱ 92 62 16 32 263
A1 Ⅰ 63 46 11 18 194Ⅱ 41 33 9 16 142
A2 Ⅰ 30 26 6 8 101Ⅱ 22 18 3 6 67
A3 Ⅰ 16 15 5 7 69Ⅱ 10 13 3 4 44
This protocol was proposed in LNCS V.1389, (1998)
Comparison of editing cost with VPStudio
a. raster image b. our system c. VPStudio
Conclusion: Object-oriented recognition algorithms produce less misrecognition, therefore the editing cost has decreased.
Conclusion
• Progressive simplification decreases both the complexity and workload of vectorization.
• The object-oriented recognition algorithms recognize graphical objects fast and entirely.
Related papers• 7 journal papers & 4 conference papers
• IEEE Trans. PAMI reviewer’s comment: “An efficient model is very important to
recognize engineering drawings …. This paper suggested an object-oriented
progressive-simplification based vectorization system for engineering drawings.
It would bring an impact in this area.”