Fix Placemark rotation and tilt (WorldWindEarth#31):

1) Change order of unitSquareTransform matrix operations to fix image stretching on rotation and apply correct pivot point.
2) Normalize unitSquareTransform matrix Z-range to prevent texture clipping on tilting.

Author: EMaksymenko

worldwind/src/main/java/gov/nasa/worldwind/shape/Placemark.java

@@ -808,48 +808,56 @@ protected void determineActiveTexture(RenderContext rc) {
         // offset is defined with its origin at the image's bottom-left corner and axes that extend up and to the right
         // from the origin point. When the placemark has no active texture the image scale defines the image size and no
         // other scaling is applied.
+        double offsetX, offsetY, scaleX, scaleY;
         if (this.activeTexture != null) {
             int w = this.activeTexture.getWidth();
             int h = this.activeTexture.getHeight();
             double s = this.activeAttributes.imageScale * visibilityScale;
             this.activeAttributes.imageOffset.offsetForSize(w, h, offset);
-
-            unitSquareTransform.multiplyByTranslation(
-                screenPlacePoint.x - offset.x * s,
-                screenPlacePoint.y - offset.y * s,
-                screenPlacePoint.z);
-
-            unitSquareTransform.multiplyByScale(w * s, h * s, 1);
+            offsetX = offset.x * s;
+            offsetY = offset.y * s;
+            scaleX = w * s;
+            scaleY = h * s;
         } else {
             // This branch serves both non-textured attributes and also textures that haven't been loaded yet.
             // We set the size for non-loaded textures to the typical size of a contemporary "small" icon (24px)
             double size = this.activeAttributes.imageSource != null ? 24 : this.activeAttributes.imageScale;
             size *= visibilityScale;
             this.activeAttributes.imageOffset.offsetForSize(size, size, offset);
+            offsetX = offset.x;
+            offsetY = offset.y;
+            scaleX = scaleY = size;
+        }
 
-            unitSquareTransform.multiplyByTranslation(
-                screenPlacePoint.x - offset.x,
-                screenPlacePoint.y - offset.y,
+        // Position image on screen
+        unitSquareTransform.multiplyByTranslation(
+                screenPlacePoint.x,
+                screenPlacePoint.y,
                 screenPlacePoint.z);
 
-            unitSquareTransform.multiplyByScale(size, size, 1);
+        // Divide Z by 2^24 to prevent texture clipping when tilting (where 24 is depth buffer bit size).
+        // Doing so will limit depth range to (diagonal length)/2^24 and make its value within 0..1 range.
+        unitSquareTransform.multiplyByScale(1, 1, 1d / (1 << 24) );
+
+        // Perform the tilt so that the image tilts back from its base into the view volume
+        if (this.imageTilt != 0) {
+            double tilt = this.imageTiltReference == WorldWind.RELATIVE_TO_GLOBE ?
+                    rc.camera.tilt + this.imageTilt : this.imageTilt;
+            unitSquareTransform.multiplyByRotation(-1, 0, 0, tilt);
         }
 
-        // ... perform image rotation
+        // Perform image rotation
         if (this.imageRotation != 0) {
             double rotation = this.imageRotationReference == WorldWind.RELATIVE_TO_GLOBE ?
-                rc.camera.heading - this.imageRotation : -this.imageRotation;
-            unitSquareTransform.multiplyByTranslation(0.5, 0.5, 0);
+                    rc.camera.heading - this.imageRotation : -this.imageRotation;
             unitSquareTransform.multiplyByRotation(0, 0, 1, rotation);
-            unitSquareTransform.multiplyByTranslation(-0.5, -0.5, 0);
         }
 
-        // ... and perform the tilt so that the image tilts back from its base into the view volume.
-        if (this.imageTilt != 0) {
-            double tilt = this.imageTiltReference == WorldWind.RELATIVE_TO_GLOBE ?
-                rc.camera.tilt + this.imageTilt : this.imageTilt;
-            unitSquareTransform.multiplyByRotation(-1, 0, 0, tilt);
-        }
+        // Apply pivot translation
+        unitSquareTransform.multiplyByTranslation(-offsetX, -offsetY, 0);
+
+        // Apply scale
+        unitSquareTransform.multiplyByScale(scaleX, scaleY, 1);
     }
 
     /**