Improve our alignment to physical pixel boundaries.

Previously we aligned elements to integral logical pixel boundaries, and relied
on the ratio between logical and physical pixels to be integral. This is no
longer the case in the wild world of zillions of screen sizes and densities.

Our new approach is to leave logical position/size alone until validation, at
which point we ensure that our boundaries (top, left, bottom, right) fall on
integral physical pixels and then translate those back to an updated logical
position and size which are used when laying out deeper children.

core/src/main/java/tripleplay/ui/Element.java

@@ -18,6 +18,7 @@
 import react.UnitSlot;
 import react.ValueView;
 
+import playn.core.Scale;
 import playn.scene.GroupLayer;
 import playn.scene.Layer;
 
@@ -417,6 +418,21 @@ protected UnitSlot invalidateSlot (final boolean styles) {
      */
     protected void validate () {
         if (!isSet(Flag.VALID)) {
+            // prior to laying ourselves out, ensure that our visual boundaries fall on physical
+            // pixels; this avoids rendering artifacts on devices where the scale factor between
+            // virtual and physical pixels is non-integral
+            Root root = root();
+            if (root != null) {
+                Scale scale = root.iface.plat.graphics().scale();
+                float x = layer.tx(), y = layer.ty();
+                float rx = scale.roundToNearestPixel(x), ry = scale.roundToNearestPixel(y);
+                float rr = scale.roundToNearestPixel(x + _size.width);
+                float rb = scale.roundToNearestPixel(y + _size.height);
+                layer.setTranslation(rx, ry);
+                _size.setSize(rr-rx, rb-ry);
+            }
+
+            // now that our boundaries are adjusted, we can layout our children (if any)
             layout();
             set(Flag.VALID, true);
             wasValidated();
@@ -473,7 +489,7 @@ protected IDimension preferredSize (float hintX, float hintY) {
      * Configures the location of this element, relative to its parent.
      */
     protected void setLocation (float x, float y) {
-        layer.setTranslation(MathUtil.round(x), MathUtil.round(y));
+        layer.setTranslation(x, y);
     }
 
     /**

pom.xml

@@ -4,7 +4,7 @@
   <parent>
     <groupId>io.playn</groupId>
     <artifactId>playn-project</artifactId>
-    <version>2.0.4</version>
+    <version>2.1-SNAPSHOT</version>
   </parent>
 
   <groupId>com.threerings</groupId>
@@ -70,7 +70,7 @@
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-    <playn.version>2.0.3</playn.version>
+    <playn.version>2.1-SNAPSHOT</playn.version>
   </properties>
 
   <modules>