Properly size the canvas based on image size and rotation angle

examples/icon.js

@@ -13,6 +13,7 @@ import GoogleLayer from 'olgm/layer/Google.js';
 import {defaults as defaultInteractions} from 'olgm/interaction.js';
 
 const center = [-7908084, 6177492];
+const left = [-7928084, 6177492];
 
 const googleLayer = new GoogleLayer();
 
@@ -36,6 +37,26 @@ feature.setStyle(new Style({
   }))
 }));
 source.addFeature(feature);
+
+const bigIcon = new Feature(new Point(left));
+bigIcon.setStyle(new Style({
+  image: new Icon(({
+    anchor: [132, 468],
+    anchorXUnits: 'pixels',
+    anchorYUnits: 'pixels',
+    src: 'data/linux.png',
+    opacity: 0.5
+  }))
+}));
+
+source.addFeature(bigIcon);
+
+setInterval(function() {
+  const image = bigIcon.getStyle().getImage();
+  image.setRotation(image.getRotation() + Math.PI / 180.0);
+  bigIcon.changed();
+}, 30);
+
 const vector = new VectorLayer({
   source: source
 });

src/olgm/gm/MapIcon.js

@@ -54,6 +54,14 @@ class MapIcon extends MapElement {
     }
   }
 
+  /**
+   * Rotate a point around the origin by a given angle expressed as a (cos, sin) pair.
+   * @private
+   */
+
+  rotate_(cosTheta, sinTheta, x, y) {
+    return [x * cosTheta - y * sinTheta, x * sinTheta + y * cosTheta];
+  }
 
   /**
    * Draws the icon to the canvas 2d context.
@@ -74,17 +82,39 @@ class MapIcon extends MapElement {
 
     style.zIndex = /** @type {number} */ (this.get('zIndex'));
 
-    const ctx = canvas.getContext('2d');
-    ctx.clearRect(0, 0, canvas.width, canvas.height);
-
-    const anchor = this.imageStyle_.getAnchor() || [0, 0];
     const scale = this.imageStyle_.getScale() || 1;
+    const anchor = this.imageStyle_.getAnchor() || [0, 0];
     const rotation = this.imageStyle_.getRotation() || 0;
     const opacity = this.imageStyle_.getOpacity() || 1;
 
     const offsetX = anchor[0] * scale;
     const offsetY = anchor[1] * scale;
 
+    let w_2 = image.width * scale * 0.5;
+    let h_2 = image.height * scale * 0.5;
+
+    w_2 += Math.abs(w_2 - offsetX);
+    h_2 += Math.abs(h_2 - offsetY);
+
+    const cosTheta = Math.cos(rotation);
+    const sinTheta = Math.sin(rotation);
+
+    const p1 = this.rotate_(cosTheta, sinTheta, -w_2, -h_2);
+    const p2 = this.rotate_(cosTheta, sinTheta, +w_2, -h_2);
+    const p3 = this.rotate_(cosTheta, sinTheta, -w_2, +h_2);
+    const p4 = this.rotate_(cosTheta, sinTheta, +w_2, +h_2);
+
+    const minX = Math.min(p1[0], p2[0], p3[0], p4[0]);
+    const maxX = Math.max(p1[0], p2[0], p3[0], p4[0]);
+    const minY = Math.min(p1[1], p2[1], p3[1], p4[1]);
+    const maxY = Math.max(p1[1], p2[1], p3[1], p4[1]);
+
+    canvas.width = Math.round(maxX - minX);
+    canvas.height = Math.round(maxY - minY);
+
+    const ctx = canvas.getContext('2d');
+    ctx.clearRect(0, 0, canvas.width, canvas.height);
+
     const x = canvas.width / 2 - offsetX;
     const y = canvas.height / 2 - offsetY;
 
@@ -97,7 +127,6 @@ class MapIcon extends MapElement {
       image.width * scale, image.height * scale);
   }
 
-
   /**
    * Manage feature being added to the map
    * @api