bool QgsDecorationNorthArrow::calculateNorthDirection()
{
QgsMapCanvas* mapCanvas = QgisApp::instance()->mapCanvas();
bool goodDirn = false;
// Get the shown extent...
QgsRectangle canvasExtent = mapCanvas->extent();
// ... and all layers extent, ...
QgsRectangle fullExtent = mapCanvas->fullExtent();
// ... and combine
QgsRectangle extent = canvasExtent.intersect( & fullExtent );
// If no layers are added or shown, we can't get any direction
if ( mapCanvas->layerCount() > 0 && ! extent.isEmpty() )
{
QgsCoordinateReferenceSystem outputCRS = mapCanvas->mapRenderer()->destinationCrs();
if ( outputCRS.isValid() && !outputCRS.geographicFlag() )
{
// Use a geographic CRS to get lat/long to work out direction
QgsCoordinateReferenceSystem ourCRS;
ourCRS.createFromOgcWmsCrs( GEO_EPSG_CRS_AUTHID );
assert( ourCRS.isValid() );
QgsCoordinateTransform transform( outputCRS, ourCRS );
QgsPoint p1( extent.center() );
// A point a bit above p1. XXX assumes that y increases up!!
// May need to involve the maptopixel transform if this proves
// to be a problem.
QgsPoint p2( p1.x(), p1.y() + extent.height() * 0.25 );
// project p1 and p2 to geographic coords
try
{
p1 = transform.transform( p1 );
p2 = transform.transform( p2 );
}
catch ( QgsCsException &e )
{
Q_UNUSED( e );
// just give up
QgsDebugMsg( "North Arrow: Transformation error, quitting" );
return false;
}
// Work out the value of the initial heading one takes to go
// from point p1 to point p2. The north direction is then that
// many degrees anti-clockwise or vertical.
// Take some care to not divide by zero, etc, and ensure that we
// get sensible results for all possible values for p1 and p2.
goodDirn = true;
double angle = 0.0;
// convert to radians for the equations below
p1.multiply( PI / 180.0 );
p2.multiply( PI / 180.0 );
double y = sin( p2.x() - p1.x() ) * cos( p2.y() );
double x = cos( p1.y() ) * sin( p2.y() ) -
sin( p1.y() ) * cos( p2.y() ) * cos( p2.x() - p1.x() );
// Use TOL to decide if the quotient is big enough.
// Both x and y can be very small, if heavily zoomed
// For small y/x, we set directly angle 0. Not sure
// if this is needed.
if ( y > 0.0 )
{
if ( x > 0.0 && ( y / x ) > TOL )
angle = atan( y / x );
else if ( x < 0.0 && ( y / x ) < -TOL )
angle = PI - atan( -y / x );
else
angle = 0.5 * PI;
}
else if ( y < 0.0 )
{
if ( x > 0.0 && ( y / x ) < -TOL )
angle = -atan( -y / x );
else if ( x < 0.0 && ( y / x ) > TOL )
angle = atan( y / x ) - PI;
else
angle = 1.5 * PI;
}
else
{
if ( x > TOL )
angle = 0.0;
else if ( x < -TOL )
angle = PI;
else
{
angle = 0.0; // p1 = p2
goodDirn = false;
}
}
// And set the angle of the north arrow. Perhaps do something
// different if goodDirn = false.
mRotationInt = qRound( fmod( 360.0 - angle * 180.0 / PI, 360.0 ) );
}
else
{
// For geographic CRS and for when there are no layers, set the
// direction back to the default
mRotationInt = 0;
}
}
return goodDirn;
}
