use "power of two" tile size

older/mobile OpenGL (without GL_ARB_texture_non_power_of_two) requires textures with size of power of two
we should provide tiles with required size to avoid scaling in QOpenGLTextureCache::bindTexture

libosmscout-client-qt/src/osmscoutclientqt/TiledMapRenderer.cpp

@@ -419,15 +419,17 @@ void TiledMapRenderer::onLoadJobFinished(QMap<QString,QMap<osmscout::TileKey,osm
             (double)loadXFrom + (double)width/2.0,
             (double)loadYFrom + (double)height/2.0);
 
-    /** tiles are up-scaled almost two times, when zoom level is near its next integer value (for example 16.99)
-     * to avoid blured result, we render tiles two times bigger than its minimum visual size.
-     * Another possible upscale (screenPixelRatio) is for HiDPI screens - when there is ratio 2.0, 100px on Qt canvas
-     * is displayed as 200px on the screen. To provide best results on HiDPI screen, we upscale tiles by this pixel ratio.
-     */
-    double finalDpi = mapDpi * 2 * this->screenPixelRatio;
+    // For HiDPI screens (screenPixelRatio > 1) tiles as up-scaled before displaying. When there is ratio 2.0, 100px on Qt canvas
+    // is displayed as 200px on the screen. To provide best results on HiDPI screen, we upscale tiles by this pixel ratio.
+    double finalDpi = mapDpi * this->screenPixelRatio;
 
     uint32_t tileDimension = double(OSMTile::osmTileOriginalWidth()) * (finalDpi / OSMTile::tileDPI()); // pixels
 
+    // older/mobile OpenGL (without GL_ARB_texture_non_power_of_two) requires textures with size of power of two
+    // we should provide tiles with required size to avoid scaling in QOpenGLTextureCache::bindTexture
+    tileDimension = qNextPowerOfTwo(tileDimension - 1);
+    finalDpi = (double(tileDimension) / double(OSMTile::osmTileOriginalWidth())) * OSMTile::tileDPI();
+
     QImage canvas(width * tileDimension,
                   height * tileDimension,
                   QImage::Format_RGBA8888_Premultiplied);