Introduction
A Vista edge set is typically used to represent two-dimensional connected edge points or lines that have
been extracted from images, but it can also be used to represent any sets of vectors of floating point
values.
An edge set in memory is referred to by the C pointer type VistaIOEdges. In data files it's identified by
the type name edges, and in memory, by the VistaIORepnKind code VistaIOEdgesRepn. Since it is a standard
object type with built-in support in the Vista library, edge sets can be read from data files, written to
data files, and manipulated as part of attribute lists by routines such as VistaIOReadFile(3), Vis‐taIOWriteFile(3), and VistaIOGetAttr(3).
A single edge, a member of an edge set, is referred to by the C pointer type VistaIOEdge.
DataStructure
The VistaIOEdges data structure heads a linked list of structures each representing one edge. An edge
consists of an array of connected points along the edge, and each point contains a selected number of
floating point values specifying its location and other properties. The edge itself may also have a se‐
lected number of floating point fields attached to it. The VistaIOEdges data structure also specifies the
size of the display region in which the edges are to be displayed.
By convention, the first two values associated with each point give its x and y location within the dis‐
play region, and the third value can be used to represent a measure of ``strength'' that may map to a
variable display intensity. The coordinate system for display of edges assumes that the origin is at the
lower left corner of the display region. An application is free to assign other meanings to the various
point values (they can be used to represent arbitrary floating point vectors), but then of course they
will not be suitable for viewing with the standard display utilities.
AccessinganEdgeSet
Vista data structures are accessed in an object-oriented style by macros and routines, with no need to
know the details of the underlying C data structures. A macro that is described as ``accessing'' an at‐
tribute can be used either as an rvalue (e.g, on the right hand side of an assignment operator) or as an
lvalue (on the left hand side of an assignment operator).
VistaIOCreateEdges initializes and returns an empty edge set. The nrows and ncolumns arguments give the
size of the display region (usually copied from the size of image from which the edges were derived).
nedge_fields and npoint_fields give the number of floating point values that will be attached to each
edge and point.
VistaIOAddEdge adds a new edge to edges. npoints is the number of points in this edge. The field values
for each of the points is given sequentially in the floating point array points. The values for the
fields attached to this edge are given in edge_fields. closed is TRUE if this edge is closed (i.e., the
last point connects back to the first point). If copy is TRUE, then a new copy is made of the point and
field data so that their storage can be reused.
VistaIOCopyEdges returns a copy of an edge set, made with newly-allocated storage.
VistaIODestroyEdges releases the memory occupied by an edge set, including that occupied by all points
and fields of each the edge.
VistaIONEdgeFields returns the number of floating point fields that are attached to each edge. This num‐
ber is established when the edge set is first created and must not be changed later.
VistaIONPointFields returns the number of floating point fields that are attached to each point. By con‐
vention the first 2 fields are used to represent the x and y coordinates of the point. This number is es‐
tablished when the edge set is first created and must not be changed later.
VistaIOEdgesNRows accesses the number of rows in the image from which the edge set was derived (i.e., it
specifies the height of the display region).
VistaIOEdgesNColumns accesses the number of columns in the image from which the edge set was derived
(i.e., it specifies the width of the display region).
VistaIOEdgesAttrList accesses the attribute list associated with an edge set.
VistaIONEdges returns the number of edges currently in an edge set.
AccessinganIndividualEdgeVistaIOFirstEdge returns the first edge in the linked list of edge records associated with an edge set.
VistaIONextEdge returns the next edge in a set of edges. (The edges are chained in a linked list.)
VistaIOEdgeExists returns FALSE if the edge pointer, edge, does not point to an edge. This is used to de‐
tect the end of the linked list when traversing through an edge set.
VistaIOEdgeClosed returns TRUE if the edge, edge, is closed (i.e., its last point connects back to its
first point).
VistaIOEdgeFields returns the array of floating point values for the fields attached to an edge.
VistaIOEdgeNPoints returns the number of points in an edge.
VistaIOEdgePointArray returns a two-dimensional array, in which the first dimensionthe points and the
second indexes each point fields.
VistaIOEdgesRepresentationinaVistaDataFileattribute-name:edges{nedges:nedgesnpoints:npointsnedge_fields:nedge-fieldsnpoint_fields:npoint-fieldsnrows:nrowsncolumns:ncolumnsdata:data-offsetlength:data-lengthotherattributes}
The attributes listed above are the same as those accessed by the corresponding macros, with the excep‐
tion of npoints which refers to the total number of points in all edges in this edge set.
The binary data is simply a sequence of 32-bit floating point numbers in IEEE standard format. Data is
present for each edge, in sequence. The first value for an edge specifies the number of points, n, in the
edge; it is negated if the edge is closed. This is followed by nedge_fields edge field values, then n
vectors of npoint_fields point field values.
Install Now
PNG Icons
