Minor tweaks to size/resolution handling

I think the original code was correct but I've clarified it and used a value of 1 pixel per mm for rendering to make it clear that that doesn't matter.

CMakeLists.txt

@@ -16,8 +16,8 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS TRUE)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 
-# Set the CMAKE_PREFIX_PATH environment variable to (e.g) ~/Qt/5.8/clang_64 so it can find Qt.
-# CoreTools is for win/macdeployqt
+# Set the CMAKE_PREFIX_PATH environment variable to (e.g) ~/Qt/6.6.1/gcc_64/lib/cmake/
+# so it can find Qt. CoreTools is for win/macdeployqt.
 find_package(Qt6 REQUIRED COMPONENTS Core Widgets Svg CoreTools)
 
 qt_standard_project_setup()

MainWindow.cpp

@@ -231,10 +231,16 @@ void MainWindow::loadFile(QString filename)
 
 	StateFileLoaded loadedState;
 
-	// See
-	// https://code.woboq.org/qt5/qtsvg/src/svg/qsvghandler.cpp.html#_ZL15convertToPixelsdbN11QSvgHandler10LengthTypeE
-	// The default size is derived from the width="" height="" svg attribute tags
-	// assuming 90 DPI.
+	// When Qt renders an SVG it sets its width and height based on the
+	// width="" height="" svg attributes. These can be given in physical units
+	// e.g. width="25mm". In that case Qt renders at 90 DPI. However that is
+	// incorrect. SVG/CSS use 96 DPI.
+	//
+	// See `convertToPixels()` here: https://codebrowser.dev/qt6/qtsvg/src/svg/qsvghandler.cpp.html#932
+	//
+
+	// This is the size of the entire document in mm, calculated from the
+	// width/height attributes, using the correct conversion from pixels (96 DPI).
 	QSizeF mediaSize(render.widthMm, render.heightMm);
 
 	loadedState.mediaSize = mediaSize;
@@ -903,4 +909,3 @@ void MainWindow::on_actionExport_HPGL_triggered()
 		return;
 	}
 }
-

PathPaintDevice.cpp

@@ -18,20 +18,13 @@ size_t qHash(const QPolygonF& key)
 	return qHash(repr);
 }
 
-PathPaintDevice::PathPaintDevice(double widthInMm, double heightInMm, double pixelsPerMm)
+PathPaintDevice::PathPaintDevice(double width, double height)
+	: width(width), height(height)
 {
-	engine = nullptr;
-	width = widthInMm;
-	height = heightInMm;
-	ppm = pixelsPerMm;
-	if (ppm == 0.0)
-		ppm = 1.0;
 }
 
 PathPaintDevice::~PathPaintDevice()
 {
-	if (engine)
-		delete engine;
 }
 
 void PathPaintDevice::addPath(const QPolygonF& path)
@@ -45,8 +38,8 @@ void PathPaintDevice::addPath(const QPolygonF& path)
 QPaintEngine* PathPaintDevice::paintEngine() const
 {
 	if (!engine)
-		engine = new PathPaintEngine();
-	return engine;
+		engine = std::make_unique<PathPaintEngine>();
+	return engine.get();
 }
 
 QList<QPolygonF> PathPaintDevice::paths()
@@ -60,9 +53,9 @@ int PathPaintDevice::metric(PaintDeviceMetric metric) const
 	{
 	// Width in pixels.
 	case PdmWidth:
-		return static_cast<int>(width * ppm);
+		return static_cast<int>(width);
 	case PdmHeight:
-		return static_cast<int>(height * ppm);
+		return static_cast<int>(height);
 	case PdmWidthMM:
 		return static_cast<int>(width);
 	case PdmHeightMM:
@@ -72,19 +65,17 @@ int PathPaintDevice::metric(PaintDeviceMetric metric) const
 	case PdmDepth:
 		return 1;
 	case PdmDpiX:
-		return static_cast<int>(25.4 * ppm); // Convert to inches.
+		return static_cast<int>(25.4);
 	case PdmDpiY:
-		return static_cast<int>(25.4 * ppm);
+		return static_cast<int>(25.4);
 	case PdmPhysicalDpiX:
-		return static_cast<int>(25.4 * ppm);
+		return static_cast<int>(25.4);
 	case PdmPhysicalDpiY:
-		return static_cast<int>(25.4 * ppm);
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0))
+		return static_cast<int>(25.4);
 	case PdmDevicePixelRatio:
 		return 1;
 	case PdmDevicePixelRatioScaled:
 		return 1;
-#endif
 	}
 	return 0;
 }

PathPaintDevice.h

@@ -14,7 +14,8 @@ size_t qHash(const QPolygonF& key);
 class PathPaintDevice : public QPaintDevice
 {
 public:
-	PathPaintDevice(double widthInMm, double heightInMm, double pixelsPerMm = 90.0 / 25.4);
+	// Width/height in pixels.
+	PathPaintDevice(double width, double height);
 	~PathPaintDevice() override;
 
 	// Adds a path to the device.
@@ -31,7 +32,7 @@ class PathPaintDevice : public QPaintDevice
 	int metric(PaintDeviceMetric metric) const override;
 
 private:
-	mutable PathPaintEngine* engine;
+	mutable std::unique_ptr<PathPaintEngine> engine;
 
 	// The paths added.
 	QList<QPolygonF> pagePaths;
@@ -40,6 +41,4 @@ class PathPaintDevice : public QPaintDevice
 
 	double width;
 	double height;
-
-	double ppm;
 };

SvgRenderer.cpp

@@ -13,7 +13,10 @@
 namespace
 {
 
-// 1 px is 1/96th of an inch.
+// If the user specifies width="96px" then we assume 96 DPI and return
+// a size of 1 inch (this matches the latest versions of Inkscape).
+// Note Qt assumes 90 DPI when rendering and SVG but we account for that so
+// it has no effect.
 const double MM_PER_CM = 10.0;
 const double MM_PER_Q = 0.25;
 const double MM_PER_IN = 25.4;
@@ -184,10 +187,12 @@ SResult<SvgRender> svgToPaths(const QString& filename, bool searchForTspans)
 
 	render.viewBox = renderer.viewBoxF();
 
-	// So here we pretend that the viewbox is in mm.
+	// Give the size of the canvas. We just use 1 user unit per pixel and
+	// then convert later.
 	PathPaintDevice pg(render.viewBox.width(), render.viewBox.height());
 	QPainter p(&pg);
 
+	// Render, this will assume 1 user unit = 1px.
 	renderer.render(&p, render.viewBox);
 
 	// These are the paths in user units.

SvgRenderer.h

@@ -25,6 +25,7 @@ struct SvgRender
 	// from the width= and height= attributes, if they have physical units. If they
 	// are in user units then it is assumed to be mm. If they are in % or are not present
 	// then the view box size is used and is assumed to be in mm.
+	// If they are in pixels then they are converted at 96 DPI per the SVG spec.
 	double widthMm;
 	double heightMm;