Fix pixel density scaling.

Author: alshan

demo/plot/compose-desktop/src/main/kotlin/demo/plot/pixeldensity/PixelDensityTestDemo.kt

@@ -60,7 +60,7 @@ fun main() = application {
                     // Plot panel that reacts to pixel density changes
                     PlotPanel(
                         figure = createTestFigure(),
-                        preserveAspectRatio = false,
+                        preserveAspectRatio = true,
                         modifier = Modifier.fillMaxSize(),
                         computationMessagesHandler = { messages ->
                             messages.forEach { println("Plot: $it") }

lets-plot-compose/src/desktopMain/kotlin/org/jetbrains/letsPlot/skia/compose/PlotPanel.kt

@@ -65,6 +65,7 @@ actual fun PlotPanel(
                 .weight(1f) // Take the remaining vertical space
                 .fillMaxWidth() // Fill available width
                 .onSizeChanged { newSize ->
+                    // Convert logical pixels (from Compose layout) to physical pixels (plot SVG pixels)
                     panelSize = DoubleVector(newSize.width / density, newSize.height / density)
                 }
                 .background(Color.LightGray)
@@ -104,12 +105,14 @@ actual fun PlotPanel(
                     val plotWidth = viewModel.svg.width().get() ?: panelSize.x
                     val plotHeight = viewModel.svg.height().get() ?: panelSize.y
 
+                    // Calculate centering position in physical pixels
+                    // Both panelSize and plot dimensions are in physical pixels
                     val position = DoubleVector(
                         maxOf(0.0, (panelSize.x - plotWidth) / 2.0),
                         maxOf(0.0, (panelSize.y - plotHeight) / 2.0)
                     )
 
-                    plotContainer.updateViewModel(viewModel, position)
+                    plotContainer.updateViewModel(viewModel, position, density.toFloat())
                 }
             }
 

lets-plot-compose/src/desktopMain/kotlin/org/jetbrains/letsPlot/skia/compose/desktop/PlotContainer.kt

@@ -13,12 +13,13 @@ class PlotContainer : Disposable {
     internal val svgView = SvgView()
     private var viewModel: ViewModel? = null
 
-    fun updateViewModel(viewModel: ViewModel, position: DoubleVector) {
+    fun updateViewModel(viewModel: ViewModel, position: DoubleVector, pixelDensity: Float) {
         this.viewModel = viewModel
 
         svgView.svg = viewModel.svg
         svgView.eventDispatcher = viewModel.eventDispatcher
         svgView.setPosition(position.x.toFloat(), position.y.toFloat())
+        svgView.setPixelDensity(pixelDensity)
     }
 
     override fun dispose() {

lets-plot-compose/src/desktopMain/kotlin/org/jetbrains/letsPlot/skia/compose/desktop/SvgView.kt

@@ -30,12 +30,20 @@ class SvgView : SkiaSvgView() {
     // Position offset for centering the plot
     private var offsetX: Float = 0f
     private var offsetY: Float = 0f
+    
+    private var pixelDensity: Float = 1.0f
 
     fun setPosition(x: Float, y: Float) {
+        // physical pixels (plot SVG pixels)
         offsetX = x
         offsetY = y
         needRedraw()
     }
+    
+    fun setPixelDensity(density: Float) {
+        pixelDensity = density
+        needRedraw()
+    }
 
     override fun needRedraw() {
         // Forward redraw requests to the Compose recomposition mechanism.
@@ -50,10 +58,15 @@ class SvgView : SkiaSvgView() {
         @Suppress("USELESS_CAST")
         val canvas = drawScope.drawContext.canvas.nativeCanvas as org.jetbrains.skia.Canvas
 
-        // Apply position offset for centering
+        // Apply density scaling and position offset
         canvas.save()
         try {
+            // Scale from physical pixels (plot SVG pixels) back to logical pixels (canvas pixels)
+            canvas.scale(pixelDensity, pixelDensity)
+            
+            // Apply position offset for centering (also in physical pixels, so gets scaled too)
             canvas.translate(offsetX, offsetY)
+            
             renderIntern(canvas)
         } finally {
             canvas.restore()
@@ -64,9 +77,11 @@ class SvgView : SkiaSvgView() {
         val change = pointerEvent.changes.first()
         val position = change.position
 
-        // Adjust coordinates to account for the position offset
-        val adjustedX = (position.x - offsetX).roundToInt()
-        val adjustedY = (position.y - offsetY).roundToInt()
+        // Convert logical pixel coordinates to physical pixel coordinates for SVG interaction
+        // 1. Scale down by density (logical → physical pixels)
+        // 2. Subtract position offset (which is also in physical pixels)
+        val adjustedX = ((position.x / pixelDensity) - offsetX).roundToInt()
+        val adjustedY = ((position.y / pixelDensity) - offsetY).roundToInt()
         val vector = Vector(adjustedX, adjustedY)
 
         // Translate PointerEvent to lets-plot MouseEvent
@@ -123,9 +138,9 @@ class SvgView : SkiaSvgView() {
     }
 
     fun handleClick(position: Offset, clickCount: Int) {
-        // Adjust coordinates to account for the position offset
-        val adjustedX = (position.x - offsetX).roundToInt()
-        val adjustedY = (position.y - offsetY).roundToInt()
+        // Convert logical pixel coordinates to physical pixel coordinates for SVG interaction
+        val adjustedX = ((position.x / pixelDensity) - offsetX).roundToInt()
+        val adjustedY = ((position.y / pixelDensity) - offsetY).roundToInt()
         val vector = Vector(adjustedX, adjustedY)
         val mouseEvent = MouseEvent.leftButton(vector)
 

lets-plot-compose/src/desktopMain/kotlin/org/jetbrains/letsPlot/skia/compose/desktop/SvgViewPanel.kt

@@ -54,6 +54,7 @@ fun SvgViewPanel(
         modifier = modifier
             .pointerHoverIcon(PointerIcon(Cursor(Cursor.CROSSHAIR_CURSOR)))
             .onSizeChanged { size ->
+                // Convert canvas logical pixels (from Compose layout) to physical pixels (plot SVG pixels)
                 val width = (size.width / density).toInt()
                 val height = (size.height / density).toInt()
                 canvasSize = IntSize(width, height)