From 2bd635461a79d98a3c1c3ae505fb70b79168577c Mon Sep 17 00:00:00 2001
From: Denis Rouzaud <denis.rouzaud@gmail.com>
Date: Fri, 27 Sep 2024 08:49:42 +0200
Subject: [PATCH] [mvt] fix opacity incorrectly for low scale (#58882)

* [mvt] fix opacity incorrectly for low scale

if the opactiy was defined by expression on the lower scale bound, it was incorrectly set to 0

* simplify test
---
 .../vectortile/qgsmapboxglstyleconverter.cpp  | 22 +++++++++++++------
 tests/src/python/test_qgsmapboxglconverter.py | 12 ++++++++++
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/src/core/vectortile/qgsmapboxglstyleconverter.cpp b/src/core/vectortile/qgsmapboxglstyleconverter.cpp
index af2d74e50156..229b4334d367 100644
--- a/src/core/vectortile/qgsmapboxglstyleconverter.cpp
+++ b/src/core/vectortile/qgsmapboxglstyleconverter.cpp
@@ -2458,10 +2458,12 @@ QgsProperty QgsMapBoxGlStyleConverter::parseInterpolateByZoom( const QVariantMap
   QString scaleExpression;
   if ( stops.size() <= 2 )
   {
-    scaleExpression = interpolateExpression( stops.value( 0 ).toList().value( 0 ).toDouble(),
-                      stops.last().toList().value( 0 ).toDouble(),
-                      stops.value( 0 ).toList().value( 1 ),
-                      stops.last().toList().value( 1 ), base, multiplier, &context );
+    scaleExpression = interpolateExpression(
+                        stops.value( 0 ).toList().value( 0 ).toDouble(), // zoomMin
+                        stops.last().toList().value( 0 ).toDouble(), // zoomMax
+                        stops.value( 0 ).toList().value( 1 ), // valueMin
+                        stops.last().toList().value( 1 ), // valueMax
+                        base, multiplier, &context );
   }
   else
   {
@@ -2532,10 +2534,16 @@ QString QgsMapBoxGlStyleConverter::parseOpacityStops( double base, const QVarian
                             base, 1, &context ) );
   }
 
+
+  bool numeric = false;
+  const QVariant vv = stops.last().toList().value( 1 );
+  double dv = vv.toDouble( &numeric );
+
   caseString += QStringLiteral( " WHEN @vector_tile_zoom >= %1 "
-                                "THEN set_color_part(@symbol_color, 'alpha', %2) END" )
-                .arg( stops.last().toList().value( 0 ).toString() )
-                .arg( stops.last().toList().value( 1 ).toDouble() * maxOpacity );
+                                "THEN set_color_part(@symbol_color, 'alpha', %2) END" ).arg(
+                  stops.last().toList().value( 0 ).toString(),
+                  numeric ? QString::number( dv * maxOpacity ) : QString( "(%1) * %2" ).arg( parseValue( vv, context ) ).arg( maxOpacity )
+                );
   return caseString;
 }
 
diff --git a/tests/src/python/test_qgsmapboxglconverter.py b/tests/src/python/test_qgsmapboxglconverter.py
index c0175a6a3858..0cfcd343f2c3 100644
--- a/tests/src/python/test_qgsmapboxglconverter.py
+++ b/tests/src/python/test_qgsmapboxglconverter.py
@@ -385,6 +385,18 @@ def testInterpolateOpacityByZoom(self):
                                                                                  conversion_context).expressionString(),
                          "set_color_part(@symbol_color, 'alpha', 25.5)")
 
+        self.assertEqual(QgsMapBoxGlStyleConverter.parseInterpolateOpacityByZoom({'base': 2,
+                                                                                  'stops': [
+                                                                                      [10, 0],
+                                                                                      [11, ["match", ["get", "class"], ["path"], 0.5, 0]],
+                                                                                      [13, ["match", ["get", "class"], ["path"], 1, 0.5]]]
+                                                                                  }, 255,
+                                                                                 conversion_context).expressionString(),
+                         ('''CASE WHEN @vector_tile_zoom < 10 THEN set_color_part(@symbol_color, 'alpha', 0) '''
+                          '''WHEN @vector_tile_zoom >= 10 AND @vector_tile_zoom < 11 THEN set_color_part(@symbol_color, 'alpha', ((0) * 255) + ((2^(@vector_tile_zoom - 10) - 1) / (2^(11 - 10) - 1)) * (((CASE WHEN "class" = 'path' THEN 0.5 ELSE 0 END) * 255) - ((0) * 255))) '''
+                          '''WHEN @vector_tile_zoom >= 11 AND @vector_tile_zoom < 13 THEN set_color_part(@symbol_color, 'alpha', ((CASE WHEN "class" = 'path' THEN 0.5 ELSE 0 END) * 255) + ((2^(@vector_tile_zoom - 11) - 1) / (2^(13 - 11) - 1)) * (((CASE WHEN "class" = 'path' THEN 1 ELSE 0.5 END) * 255) - ((CASE WHEN "class" = 'path' THEN 0.5 ELSE 0 END) * 255))) '''
+                          '''WHEN @vector_tile_zoom >= 13 THEN set_color_part(@symbol_color, 'alpha', (CASE WHEN "class" = 'path' THEN 1 ELSE 0.5 END) * 255) END'''))
+
     def testInterpolateListByZoom(self):
         conversion_context = QgsMapBoxGlStyleConversionContext()
         prop, default_color, default_val = QgsMapBoxGlStyleConverter.parseInterpolateListByZoom([