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| Scanning & Conversion Concepts: |
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The Hardcopy Dilemma
In companies around the world, hardcopy
drawings are squashed in flat files, mashed in rolls, and scattered in
aperture card bins. Paper, vellum, and mylar drawings that have
accumulated over decades are easily damaged in handling and frequently
misfiled. Projects are delayed due to time consuming searches. Even
companies that make extensive use of Computer Aided Design (CAD) live in a
world of paper drawings. Over 80% of the world's engineering drawings
still exist on paper. The challenge is to provide an effective, low-cost
method to put these drawings on computer and to integrate them with the
growing world of CAD designs. More and more organizations feel the need to
manage these drawings electronically both inside and outside the CAD
environment.
Raster Files
Over the last few years, advances in
large document scanning have made the process fast, easy, and cost
effective. A scanner operates in the same way as a FAX machine, converting
a drawing into an electronic image (photocopy) known as a raster file.
Raster files are generally found in two basic forms. In binary or
monochrome images, the file consists of black and white pixels only. In
grayscale or color images, the pixels can vary in shades of gray or color,
similar to black and white or color photographs. Line drawings are scanned
to produce binary raster files, while scanned aerial photographs and
satellite imagery are scanned to produce gray-scale or color raster files.
A raster image is not directly compatible with vector based graphics found
in CAD systems. Raster images contain a series of dots or pixels while
vector files are defined by specific points in a coordinate system. A
circle in CAD is defined by a center point and a radius. No such
information exists for a circle in a raster file.
Sources Of Raster Data
Raster data can be captured from a
variety of sources:
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Large document scanners (most take
blueprints, sepia, vellum, & mylar media) |
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Desktop scanners |
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Aperture card scanners |
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Micro fiche scanners
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Document scanners come in a
full range of document sizes, resolutions, functionality, and cost.
File Size
File size is determined by scan
resolution- which is measured in dots per inch (dpi)-file format, and
complexity of drawing. Scan resolutions vary between applications, but are
usually between 200-400 dpi. The optimal resolution depends on the quality
and complexity of the original, and the application being used.
Raster File Formats
Some common file formats for binary
raster images are:
Uncompressed - Raster data is
stored as a matrix of picture elements called pixels. Each pixel is
displayed as black or white. This uncompressed format results in files
that are extremely large when scanning an engineering size document. A 300
dpi E-size (A0-size) bitmap is approximately 1 6MB. TIFF (uncompressed)
and BMP are typical uncompressed raster tiles.
CCITT Group 3 - Raster data is
compressed in one direction by combining groups of pixels of the same
color into alternating runs of black and white. Usually compression is 8:1
over an uncompressed file. One common type of a CCITT Group 3 file is RLC
or "Run Length Coded.. A 300 dpi E-size (A0-size) RLC file is
approximately 3.2MB.
CCITT Group 4 - A two
dimensional compression format that reduces file size about 5:1 over RLC
and 40:1 over an uncompressed file. CALS and TIFF files can also use Group
4 compression techniques. A 300 dpi E-size (A0-size) Group 4 file is
approximately 580KB.
The following tables show the relation
between resolution, drawing sizes, and file
compression for some typical drawings: |
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| DRAWING
SIZE |
RAW 200 dpi |
RLC
200 dpi |
G4
200 dpi |
RAW
300 dpi |
RLC
300 dpi |
G4
300 dpi |
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A (A4)
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500k
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100K
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18K
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1MB
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200K
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40K
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B (A3)
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1MB
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200K
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35K
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2MB
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400K
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75K
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C (A2)
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2MB
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400K
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70K
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4MB
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820K
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150K
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D (A1)
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4MB
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750K
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140K
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8MB
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1.6MB
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300K
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E (A0)
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7MB
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1.4MB
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270K
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16MB
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3.2MB
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580K
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Raster to Vector (CAD Conversion)
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Conversion Methods
Anyone who has redrawn CAD files from
paper drawings or by using large format digitizers can tell you of the
long hours and high cost of such conversion methods. Various software
conversion solutions have made the job easier. There are three distinct
methods for converting a raster image into a CAD file. The choice of
method depends on the nature of the drawing and how it will be used. With
some of the conversion applications, these methods can be used in
combination to produce the desired results.
On-Screen Digitizing - the most
popular method of converting drawings to CAD, combines raster and vector
technologies. A raster image is used as a backdrop over which the drawing
can be constructed using the drawing commands of the CAD system. This
method provides total control of layers, line types, entity color, text,
and symbology found in traditional CAD files. When an intelligent CAD file
is required, on-screen digitizing has proved to be the most efficient and
cost effective method.
Semi-Automatic, Interactive fine Following -
A combination of interactive and automatic techniques allow the user
to define a set of parameters, select the raster element to convert, and
direct the program through the conversion process. The software traces one
line at a time to an intersection or ambiguous area, then stops and
prompts the user for direction. This method is best for contour, parcel
and other types of maps .
Automatic Vectorization - The user
selects all or part of the drawing to convert, and applies a set of
parameters to fine tune the conversion for the characteristics of each
drawing type. The software uses statistical analysis to recognize patterns
of dots in the raster file into geometric elements such as lines, arcs,
circles and polylines. Some programs attempt to recognize text, but with
hand lettered drawings the quality of recognition is very low. Files
produced with automatic conversion software have an inconsistent entity
structure. To resolve the converted geometry into a traditional layering
scheme with text, symbols and line types, a considerable amount of clean
up may be required. Although this method is fast and easy, it should be
limited to drawings that are in good condition and without fine detail, in
order to minimize the editing and clean up time. |
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Alternative Methods: |
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While conversion of paper drawings to
CAD may seem necessary, many organizations have abandoned the idea of
converting an entire archive of drawings because of the time and expense
involved. With the appropriate software, the user can revise only those
parts of the drawing that need to be changed, leaving the drawing in
raster or raster/vector hybrid format. The resulting file can be plotted
to a raster output device for immediate use. The ability to scan a paper
drawing, make revisions in CAD, and plot a new drawing quickly has
increased productivity for users from one-room shops to Fortune 500
companies.
Raster Drawing Revision - Some of the
available raster editing programs, have the ability to select raster
objects, much in the same way that CAD software selects vector objects.
The selected raster objects can then be erased, moved, rotated, copied, or
scaled depending on the task. Raster drawing commands add to the
software's functionality as well as the ability to cut and create raster
symbols which can be stored in a library. The resulting file can then be
plotted to a raster output device.
Hybrid Or Raster/Vector Drawing Revision -
A hybrid drawing refers to the combination of a raster and a vector
file. Hybrid files can be created, edited, and plotted with the
appropriate software that maintains the scale and correlation of the
raster and vector data. For example, a vector drawing of a water line
might be combined with a raster drawing of a street map. This approach can
be used as a form of incremental conversion for a large archive of paper
drawings.
We hope you found our Scanning 101 page
interesting and informative. Please return for future updates. Please tour
our VisualBASE
EDM page for additional information concerning the software
tools described above.
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