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[decorations] Use a proper ellipsoidal length calculation when

calculating the size of the scalebar decoration

Avoids a misleading Cartesian-based scalebar (unless project is
set to always use cartesian measurements, that is!)

Fixes #28407
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nyalldawson committed Jun 10, 2020
1 parent df46fba commit af19cea5acc90a6cc19d116dd35739059d0c5f83
Showing with 42 additions and 22 deletions.
  1. +40 −22 src/app/decorations/qgsdecorationscalebar.cpp
  2. +2 −0 src/app/decorations/qgsdecorationscalebar.h
@@ -206,27 +206,41 @@ void QgsDecorationScaleBar::setupScaleBar()
}
mSettings.setLabelBarSpace( 1.8 );
}

double QgsDecorationScaleBar::mapWidth( const QgsMapSettings &settings ) const
{
const QgsRectangle mapExtent = settings.extent();
if ( mSettings.units() == QgsUnitTypes::DistanceUnknownUnit )
{
return mapExtent.width();
}
else
{
QgsDistanceArea da;
da.setSourceCrs( settings.destinationCrs(), QgsProject::instance()->transformContext() );
da.setEllipsoid( QgsProject::instance()->ellipsoid() );

QgsUnitTypes::DistanceUnit units = da.lengthUnits();

// we measure the horizontal distance across the vertical center of the map
const double yPosition = 0.5 * ( mapExtent.yMinimum() + mapExtent.yMaximum() );
double measure = da.measureLine( QgsPointXY( mapExtent.xMinimum(), yPosition ),
QgsPointXY( mapExtent.xMaximum(), yPosition ) );

measure /= QgsUnitTypes::fromUnitToUnitFactor( mSettings.units(), units );
return measure;
}
}

void QgsDecorationScaleBar::render( const QgsMapSettings &mapSettings, QgsRenderContext &context )
{
if ( !enabled() )
return;

//Get canvas dimensions
QPaintDevice *device = context.painter()->device();
int deviceHeight = device->height() / device->devicePixelRatioF();
int deviceWidth = device->width() / device->devicePixelRatioF();

//Get map units per pixel. This can be negative at times (to do with
//projections) and that just confuses the rest of the code in this
//function, so force to a positive number.
double scaleBarUnitsPerPixel = std::fabs( context.mapToPixel().mapUnitsPerPixel() );

// Exit if the canvas width is 0 or layercount is 0 or QGIS will freeze
if ( mapSettings.layers().isEmpty() || !deviceWidth || !scaleBarUnitsPerPixel )
return;

double unitsPerSegment = mPreferredSize;

const int deviceHeight = device->height() / device->devicePixelRatioF();
const int deviceWidth = device->width() / device->devicePixelRatioF();
QgsSettings settings;
bool ok = false;
QgsUnitTypes::DistanceUnit preferredUnits = QgsUnitTypes::decodeDistanceUnit( settings.value( QStringLiteral( "qgis/measure/displayunits" ) ).toString(), &ok );
@@ -235,10 +249,16 @@ void QgsDecorationScaleBar::render( const QgsMapSettings &mapSettings, QgsRender

QgsUnitTypes::DistanceUnit scaleBarUnits = mapSettings.mapUnits();

// Adjust units meter/feet/... or vice versa
scaleBarUnitsPerPixel *= QgsUnitTypes::fromUnitToUnitFactor( scaleBarUnits, preferredUnits );
//Get map units per pixel
const double scaleBarUnitsPerPixel = ( mapWidth( mapSettings ) / mapSettings.outputSize().width() ) * QgsUnitTypes::fromUnitToUnitFactor( mSettings.units(), preferredUnits );
scaleBarUnits = preferredUnits;

// Exit if the canvas width is 0 or layercount is 0 or QGIS will freeze
if ( mapSettings.layers().isEmpty() || !deviceWidth || !scaleBarUnitsPerPixel )
return;

double unitsPerSegment = mPreferredSize;

//Calculate size of scale bar for preferred number of map units
double scaleBarWidth = mPreferredSize / scaleBarUnitsPerPixel;

@@ -258,19 +278,17 @@ void QgsDecorationScaleBar::render( const QgsMapSettings &mapSettings, QgsRender

// Work out the exponent for the number - e.g, 1234 will give 3,
// and .001234 will give -3
double powerOf10 = std::floor( std::log10( unitsPerSegment ) );
const double powerOf10 = std::floor( std::log10( unitsPerSegment ) );

// snap to integer < 10 times power of 10
if ( mSnapping )
{
double scaler = std::pow( 10.0, powerOf10 );
const double scaler = std::pow( 10.0, powerOf10 );
unitsPerSegment = std::round( unitsPerSegment / scaler ) * scaler;
scaleBarWidth = unitsPerSegment / scaleBarUnitsPerPixel;
}

//Get type of map units and set scale bar unit label text
double mapSize = unitsPerSegment * QgsUnitTypes::fromUnitToUnitFactor( scaleBarUnits, mapSettings.mapUnits() );
double segmentSize = context.convertFromMapUnits( mapSize, QgsUnitTypes::RenderMillimeters );
const double segmentSizeInMm = scaleBarWidth / context.convertToPainterUnits( 1, QgsUnitTypes::RenderMillimeters );

QString scaleBarUnitLabel;
switch ( scaleBarUnits )
@@ -348,7 +366,7 @@ void QgsDecorationScaleBar::render( const QgsMapSettings &mapSettings, QgsRender
mSettings.setLabelHorizontalPlacement( mPlacement == TopCenter || mPlacement == BottomCenter ? QgsScaleBarSettings::LabelCenteredSegment : QgsScaleBarSettings::LabelCenteredEdge );

QgsScaleBarRenderer::ScaleBarContext scaleContext;
scaleContext.segmentWidth = mStyleIndex == 3 ? segmentSize / 2 : segmentSize;
scaleContext.segmentWidth = mStyleIndex == 3 ? segmentSizeInMm / 2 : segmentSizeInMm;
scaleContext.scale = mapSettings.scale();

//Calculate total width of scale bar and label
@@ -76,6 +76,8 @@ class APP_EXPORT QgsDecorationScaleBar: public QgsDecorationItem
int mMarginHorizontal = 0;
int mMarginVertical = 0;

double mapWidth( const QgsMapSettings &settings ) const;

friend class QgsDecorationScaleBarDialog;
};

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