Changes between Version 10 and Version 11 of Ticket #3719, comment 1


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Timestamp:
10/17/17 04:04:55 (7 years ago)
Author:
tiiipponen

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  • Ticket #3719, comment 1

    v10 v11  
    2929I don't know the stroking value perQuad, but if it is default PostGIS value 32, it leads to arc length 0.78 m, which is more than length of actual arc 0.71m. That would create straight line as a stroking result of that arc and that can give the error message "Invalid number of points...".
    3030\\I adjusted example coordinates a little so that arc was longer and error message disappeared.
    31 
    32 If this is the reason, then this problem is not so easy to fix. We can have very short arcs in our data and using same absolute stroking length means troubles.
    33 \\I would think more, if things could be done completely without stroking.
    34 \\At least I would use relative stroking compared to arc length.
    35 \\As a stroking method, I like most the tolerance method. Tolerance means maximum difference between original arc and stroked line. For short arcs the parametric vertices count comes more important.
    36 \\I created following code with Python for you to copy (and test!) freely:
    37 \\User can give parameters accuracy "tolerance" or number of "vertices" in resulting stroked line.
    38 
    39 {{{
    40     # Radius (r), centroid (cx,cy) and angles (a1 and a3) are now
    41     # calculated to arc geometry (p1, p2, p3).
    42     # Angles a1 and a3 are specified properly when compared to sweeping angle direction.
    43     # Then we start stroking the geometry
    44 
    45     angleArc = a3 - a1
    46     toleranceVertices = 0
    47     verticesCount = 0
    48     StrokingTolerance = 0.0
    49    
    50     # If user gave the stroking tolerance, we calculate maximum angle of one stroke sector.
    51     # from equation: cos(angle / 2) = (radius - tolerance) / radius
    52     # We calculate also minimum count of vertices for whole arc that satisfies tolerance.
    53 
    54     if tolerance > 0.0:
    55         StrokingTolerance = tolerance
    56         if StrokingTolerance > 2 * r:
    57             StrokingTolerance = 2 * r
    58         angleSectorFromTolerance = 2 * math.acos((r - StrokingTolerance) / r)
    59         toleranceVertices = int(abs(angleArc / angleSectorFromTolerance))
    60 
    61     # If user gave exact amount of vertices to use,
    62     # we use it or we use vertice count calculated from tolerance,
    63     # which ever is bigger so meaning more accurate.
    64 
    65     if vertices > toleranceVertices:
    66         verticesCount = vertices
    67     else:
    68         verticesCount = toleranceVertices
    69 
    70     # Now calculate exact angle of one stroke sector from count of vertices.
    71 
    72     angleSector = angleArc / (verticesCount + 1)
    73 
    74     # Calculate vertice points and create line from them.
    75 
    76     pointList = [(p1x, p1y)]
    77 
    78     for i in range(1, verticesCount + 1):
    79         currentAngle = a1 + i * angleSector
    80         newPointX = cx + r * math.cos(currentAngle)
    81         newPointY = cy + r * math.sin(currentAngle)
    82         pointList.append((newPointX, newPointY))
    83        
    84     pointList.append((p3x, p3y))
    85 }}}
    86 
    87 Ok, lots of text and actual error is maybe located somewhere else.
    88 \\I wish good luck for solving this, because function ST_Intersects is widely used in QGIS, Openlayers, etc. and this error results missing geometries and error messages in data that holds arcs.