Raster Vectorization

This is a revision from a previous version.

Bentley Descartes offers the possibility to detect vector like group of pixels (i.e. : line on a black and white scanned plan) and create a basic geometry element representing that detected group of pixels.

The Vectorize tool box includes a set of enhanced editing tools that support extraction of feature information from images. This tool box can be accessed via the Raster Manager Tools menu or from the Descartes Raster Edit tool box.

Place Element / Continue Element / Insert Element

The first three tools, Place Element, Continue Element and Insert Element, are for placing a new element or continuing or editing an existing one. They are manual and directly controlled by the operator when raster data is not present or not used. They are more or less automatic when raster data is present and used.

Place Element  is used to place a new vector element in the design file and can be used with or without raster data being present.  The resulting element is a complex chain which consist of one or more line string elements and can contain arcs.  Tool settings include:

Erase Raster ‐ If on, the raster in the selected extent will be erased. Entity ‐ Defines where to place the nodes. Mask ‐ Used to select the color mask. Max. Line Width ‐ Used to determine the maximum width of raster lines. Intersections ‐ If this option is on, the line‐following engine will not stop at the intersections. Gap Jumping ‐ Use to set the Maximum Gap Length and the Searching Angle. Confirm Jump ‐ When this option is ON, the vectorize engine stops after it performs a jump and waits for user input to continue. Auto Convert Arcs ‐ If on, this feature tries to recognize arcs along the line strings generated with the convert function. Generalization ‐ If on, when a vector line is generated by automatic line conversion, it is passed through a generalization process. This process removes points from the original result to keep only those considered as necessary to characterize the original pathway of the vector line. Tolerance ‐ Use to enter the tolerance factor that determines the smoothness of curves

Continue Element  allows you to select an existing element and continue placing line strings or arcs from the chosen end. The result is one continuous line string. This tool works exactly like Place Element, except it allows you to select an existing element before continuing. It shares the same tool settings as Place Element.  

Insert Element is a combination of MicroStation's Partial Delete and Insert Vertex. It removes a portion of a line which is defined by two data points. A new line string is then placed between the points. After specifying the portion to be modified, this tool works like Place Element and it has the same settings except Auto Convert which is not present. The resulting element, made of two former portions and the new one, will be a complex chain. You can place the new part of the element with any of the three modes: Point, Stream or Arc.

Productivity Tip:  Use the F1 function key to quickly toggle between the three placement modes:  Points, Stream, and Arcs.

Convert

Automatic line conversion consists of following lines and outlines, or following the boundaries between a mass of pixels belonging to a given color mask and the rest of the image. Bentley Descartes provides a rich collection of tools for automatic document conversion.  These tools are:  Convert Lines, Convert Area, Convert All Connected Lines,  Convert Contour Lines, and Convert Circle.

Productivity Tip:  Ensure that the active colour is different than the pixel colours.  This will help you to keep track of your progress as you vectorize the document.  

Lines

The Convert Lines tool is used to automatically convert one or more raster lines and to place a line string for each raster line which is converted. The behavior of this tools depends upon where it is started.  For example, if it's started along a raster line, then that raster line is converted in both directions until a break point (raster intersection point, raster end point, or node) is hit.  Likewise, if the tool is started at a raster intersection, all branches will be processed until a break point is hit.

Tool settings are the same as Place Element with the addition of Auto Continue.   If  Auto Continue is turned on, then Continue Element will be automatically selected after a part of the line is converted.

Productivity Tip:  Always set the the Max Line Width value slightly larger than what you've measured.

Area

The Convert Area tool is used to convert a predefined area as specified in the tool settings. This tool shares many of the same options as Convert Lines.

Productivity Tip:   Set the line quality of your document in the Convert tab of Descartes preferences. To assess the quality of the lines, display a few samples with a 1:1 resolution. If you see smooth, solid lines, the quality is excellent. And lines with  with holes and repeating gaps would be declared as having poor or bad quality.  

All Connected Lines

Convert All Connected Lines is used to convert all raster branches forming a network of lines connected at intersections and to place a vector string for each raster line which is converted. This tool can start conversion from anywhere on the network of lines to be
converted: along a line, on an intersection, at the end of a line, or even on a node. Convert All Connected Lines shares many of the same tool options as Convert Lines and Convert Area.

Contour Lines

Convert Contour Lines  is used to automatically convert one or more raster contour lines and to place a line string for each raster line which is converted. In addition, it is possible to assign the elevation for the contour being traced and to increment the Z value for subsequent contour lines. This tool is used only with 3D design files and shares many of the same options as Convert Lines.

Circle

Convert Circle is used to automatically convert a raster circle and place a vector circle in the design file. Tool settings specify options to control the behaviour of this tool.  Attention should be made to the Lock option which constrains the conversion to the same diameter or radius setting and ignores the round off setting. Additional settings, that are specific to vectorizing circles, can be found in the Vectorize tab of the Descartes preferences.

Generalize Vector

The Generalize Vector tool removes vertices from the vector linework created by the convert tool.  When processed, it only keeps those points that are considered necessary to characterize the original pathway of the vector line.

Productivity Tip:  To avoid drastic, and perhaps unwanted,  changes to the elements, always start with a small tolerance and increase it until the desired results are obtained. 

Place Nodes / Move Nodes / Delete Nodes

Nodes are features allowing the user to control the vectorize functions by stopping or constraining automatic line conversion. The main property of a node, which is a point surrounded by a control area, is to stop all automatic line conversion functions and to force the generated vector element to snap to it. They are manually placed with the Place Nodes tool, moved with the Move Nodes tool and removed with the Delete Nodes tool. They can also be automatically placed at specific points with the Auto Place Nodes toggle on.

Nodes can be created, displayed, deleted, or saved into files and restored for later use.

Productivity Tip:  The node colour can be changed in the Vectorize tab of the Descartes preferences which is found in the Workspace preferences dialog.  

  Productivity Tip: The size of a node varies with the situation: If it is placed on a line with a raster snap, it is a little larger than the line. If it is placed where there is no line, or with no raster snap, or with an "outline" snap, it is the size of the locate tolerance set in MicroStation.