Conditionally render tiles to a separate tile canvas

Because clip geometries are anti-aliased in most browsers, there will be tiny
gaps between tiles. If tiles are rendered to a tile canvas which is then drawn
to the map canvas upon composition, these gaps can be avoided. For rotated
views, it is stil necessary to clip the tile, but in this case a 1-pixel
buffer is used.

This change also brings a huge performance improvement for panning, because
the fully rendered tiles can be reused.

Because of the added cost of using drawImage in addition to replaying the tile
replay group, we fall back to directly drawing to the map canvas when the tile
canvas would be too large, or during interaction/animation when resolution or
rotation change.

src/ol/render/canvas/canvasreplay.js

@@ -1995,32 +1995,35 @@ ol.render.canvas.ReplayGroup.prototype.isEmpty = function() {
  * @param {number} viewRotation View rotation.
  * @param {Object.<string, boolean>} skippedFeaturesHash Ids of features
  *     to skip.
+ * @param {boolean=} opt_clip Clip at `maxExtent`. Default is true.
  */
-ol.render.canvas.ReplayGroup.prototype.replay = function(
-    context, pixelRatio, transform, viewRotation, skippedFeaturesHash) {
+ol.render.canvas.ReplayGroup.prototype.replay = function(context, pixelRatio,
+    transform, viewRotation, skippedFeaturesHash, opt_clip) {
 
   /** @type {Array.<number>} */
   var zs = Object.keys(this.replaysByZIndex_).map(Number);
   zs.sort(ol.array.numberSafeCompareFunction);
 
-  // setup clipping so that the parts of over-simplified geometries are not
-  // visible outside the current extent when panning
-  var maxExtent = this.maxExtent_;
-  var minX = maxExtent[0];
-  var minY = maxExtent[1];
-  var maxX = maxExtent[2];
-  var maxY = maxExtent[3];
-  var flatClipCoords = [minX, minY, minX, maxY, maxX, maxY, maxX, minY];
-  ol.geom.flat.transform.transform2D(
-      flatClipCoords, 0, 8, 2, transform, flatClipCoords);
-  context.save();
-  context.beginPath();
-  context.moveTo(flatClipCoords[0], flatClipCoords[1]);
-  context.lineTo(flatClipCoords[2], flatClipCoords[3]);
-  context.lineTo(flatClipCoords[4], flatClipCoords[5]);
-  context.lineTo(flatClipCoords[6], flatClipCoords[7]);
-  context.closePath();
-  context.clip();
+  if (opt_clip !== false) {
+    // setup clipping so that the parts of over-simplified geometries are not
+    // visible outside the current extent when panning
+    var maxExtent = this.maxExtent_;
+    var minX = maxExtent[0];
+    var minY = maxExtent[1];
+    var maxX = maxExtent[2];
+    var maxY = maxExtent[3];
+    var flatClipCoords = [minX, minY, minX, maxY, maxX, maxY, maxX, minY];
+    ol.geom.flat.transform.transform2D(
+        flatClipCoords, 0, 8, 2, transform, flatClipCoords);
+    context.save();
+    context.beginPath();
+    context.moveTo(flatClipCoords[0], flatClipCoords[1]);
+    context.lineTo(flatClipCoords[2], flatClipCoords[3]);
+    context.lineTo(flatClipCoords[4], flatClipCoords[5]);
+    context.lineTo(flatClipCoords[6], flatClipCoords[7]);
+    context.closePath();
+    context.clip();
+  }
 
   var i, ii, j, jj, replays, replay;
   for (i = 0, ii = zs.length; i < ii; ++i) {

src/ol/renderer/canvas/canvasvectortilelayerrenderer.js

@@ -11,6 +11,7 @@ goog.require('ol.ViewHint');
 goog.require('ol.array');
 goog.require('ol.dom');
 goog.require('ol.extent');
+goog.require('ol.geom.flat.transform');
 goog.require('ol.layer.VectorTile');
 goog.require('ol.proj.Units');
 goog.require('ol.render.EventType');
@@ -51,6 +52,18 @@ ol.renderer.canvas.VectorTileLayer = function(layer) {
    */
   this.renderedTiles_ = [];
 
+  /**
+   * @private
+   * @type {number}
+   */
+  this.resolution_ = NaN;
+
+  /**
+   * @private
+   * @type {number}
+   */
+  this.rotation_ = NaN;
+
   /**
    * @private
    * @type {ol.Extent}
@@ -110,44 +123,101 @@ ol.renderer.canvas.VectorTileLayer.prototype.composeFrame =
   // see http://jsperf.com/context-save-restore-versus-variable
   var alpha = replayContext.globalAlpha;
   replayContext.globalAlpha = layerState.opacity;
+  var imageSmoothingEnabled = replayContext.imageSmoothingEnabled;
+  replayContext.imageSmoothingEnabled = false;
+
 
   var tilesToDraw = this.renderedTiles_;
   var tileGrid = source.getTileGrid();
 
-  var currentZ, i, ii, origin, tile, tileSize;
-  var tilePixelRatio, tilePixelResolution, tilePixelSize, tileResolution;
+  var currentZ, height, i, ii, insertPoint, insertTransform, origin, pixelScale;
+  var pixelSpace, replayState, rotatedTileExtent, rotatedTileSize, size, tile;
+  var tileCenter, tileContext, tileExtent, tilePixelResolution, tilePixelSize;
+  var tileResolution, tileSize, tileTransform, width;
   for (i = 0, ii = tilesToDraw.length; i < ii; ++i) {
     tile = tilesToDraw[i];
+    replayState = tile.getReplayState();
+    tileExtent = tileGrid.getTileCoordExtent(
+        tile.getTileCoord(), this.tmpExtent_);
     currentZ = tile.getTileCoord()[0];
-    tileSize = tileGrid.getTileSize(currentZ);
-    tilePixelSize = source.getTilePixelSize(currentZ, pixelRatio, projection);
-    tilePixelRatio = tilePixelSize[0] /
-        ol.size.toSize(tileSize, this.tmpSize_)[0];
+    tileSize = ol.size.toSize(tileGrid.getTileSize(currentZ), this.tmpSize_);
+    pixelSpace = tile.getProjection().getUnits() == ol.proj.Units.TILE_PIXELS;
+    size = frameState.size;
     tileResolution = tileGrid.getResolution(currentZ);
-    tilePixelResolution = tileResolution / tilePixelRatio;
-    if (tile.getProjection().getUnits() == ol.proj.Units.TILE_PIXELS) {
-      origin = ol.extent.getTopLeft(tileGrid.getTileCoordExtent(
-          tile.getTileCoord(), this.tmpExtent_));
-      transform = ol.vec.Mat4.makeTransform2D(this.tmpTransform_,
-          pixelRatio * frameState.size[0] / 2,
-          pixelRatio * frameState.size[1] / 2,
-          pixelRatio * tilePixelResolution / resolution,
-          pixelRatio * tilePixelResolution / resolution,
-          viewState.rotation,
-          (origin[0] - center[0]) / tilePixelResolution,
-          (center[1] - origin[1]) / tilePixelResolution);
+    tilePixelResolution = tileResolution / source.getTilePixelRatio();
+    pixelScale = pixelRatio / resolution;
+    scale = tileResolution / resolution;
+    offsetX = Math.round(pixelRatio * size[0] / 2);
+    offsetY = Math.round(pixelRatio * size[1] / 2);
+    width = tileSize[0] * pixelRatio * scale;
+    height = tileSize[1] * pixelRatio * scale;
+    if (width < 1 || width > size[0]) {
+      if (pixelSpace) {
+        origin = ol.extent.getTopLeft(tileExtent);
+        tileTransform = ol.vec.Mat4.makeTransform2D(this.tmpTransform_,
+            pixelRatio * size[0] / 2, pixelRatio * size[1] / 2,
+            pixelScale * tilePixelResolution,
+            pixelScale * tilePixelResolution,
+            rotation,
+            (origin[0] - center[0]) / tilePixelResolution,
+            (center[1] - origin[1]) / tilePixelResolution);
+      } else {
+        tileTransform = transform;
+      }
+      replayState.replayGroup.replay(replayContext, pixelRatio,
+          tileTransform, rotation, skippedFeatureUids);
+    } else {
+      rotatedTileExtent = ol.extent.getForViewAndSize(
+          ol.extent.getCenter(tileExtent), tileResolution, rotation, tileSize);
+      rotatedTileSize = [ol.extent.getWidth(rotatedTileExtent),
+            ol.extent.getHeight(rotatedTileExtent)];
+      tilePixelSize = source.getTilePixelSize(currentZ, pixelRatio, projection);
+      if (pixelSpace) {
+        tileTransform = ol.vec.Mat4.makeTransform2D(this.tmpTransform_,
+            width / 2, height / 2,
+            pixelScale * tilePixelResolution, pixelScale * tilePixelResolution,
+            rotation,
+            -tilePixelSize[0] / 2, -tilePixelSize[1] / 2);
+      } else {
+        tileCenter = ol.extent.getCenter(rotatedTileExtent);
+        tileTransform = ol.vec.Mat4.makeTransform2D(this.tmpTransform_,
+            width / 2, height / 2,
+            pixelScale, -pixelScale,
+            -rotation,
+            -tileCenter[0], -tileCenter[1]);
+      }
+      tileContext = tile.getContext();
+      if (replayState.resolution !== resolution ||
+          replayState.rotation !== rotation) {
+        replayState.resolution = resolution;
+        replayState.rotation = rotation;
+        tileContext.canvas.width = width + 0.5;
+        tileContext.canvas.height = height + 0.5;
+        replayState.replayGroup.replay(tileContext, pixelRatio,
+            tileTransform, rotation, skippedFeatureUids, rotation !== 0);
+      }
+      insertTransform = ol.vec.Mat4.makeTransform2D(this.tmpTransform_,
+          (pixelRatio * size[0] - width) / 2,
+          (pixelRatio * size[1] - height) / 2,
+          pixelScale, -pixelScale,
+          -rotation,
+          -center[0], -center[1]);
+      insertPoint = ol.geom.flat.transform.transform2D(
+          ol.extent.getCenter(rotatedTileExtent), 0, 1, 2, insertTransform);
+      replayContext.drawImage(tileContext.canvas,
+          insertPoint[0], insertPoint[1]);
     }
-    tile.getReplayState().replayGroup.replay(replayContext, pixelRatio,
-        transform, rotation, skippedFeatureUids);
   }
 
-  transform = this.getTransform(frameState, 0);
+  this.resolution_ = resolution;
+  this.rotation_ = rotation;
 
   if (replayContext != context) {
     this.dispatchRenderEvent(replayContext, frameState, transform);
     context.drawImage(replayContext.canvas, 0, 0);
   }
   replayContext.globalAlpha = alpha;
+  replayContext.imageSmoothingEnabled = imageSmoothingEnabled;
 
   this.dispatchPostComposeEvent(context, frameState, transform);
 };
@@ -179,16 +249,19 @@ ol.renderer.canvas.VectorTileLayer.prototype.createReplayGroup = function(tile,
       'Source is an ol.source.VectorTile');
   var tileGrid = source.getTileGrid();
   var tileCoord = tile.getTileCoord();
+  var buffer = 1;
+  var resolution = tileGrid.getResolution(tileCoord[0]);
   var pixelSpace = tile.getProjection().getUnits() == ol.proj.Units.TILE_PIXELS;
   var extent;
   if (pixelSpace) {
     var tilePixelSize = source.getTilePixelSize(tileCoord[0], pixelRatio,
         tile.getProjection());
-    extent = [0, 0, tilePixelSize[0], tilePixelSize[1]];
+    extent = [-buffer, -buffer,
+      tilePixelSize[0] + buffer, tilePixelSize[1] + buffer];
   } else {
-    extent = tileGrid.getTileCoordExtent(tileCoord);
+    extent = ol.extent.buffer(tileGrid.getTileCoordExtent(tileCoord),
+        buffer * resolution);
   }
-  var resolution = tileGrid.getResolution(tileCoord[0]);
   var tileResolution = pixelSpace ? source.getTilePixelRatio() : resolution;
   replayState.dirty = false;
   var replayGroup = new ol.render.canvas.ReplayGroup(0, extent,
@@ -234,6 +307,7 @@ ol.renderer.canvas.VectorTileLayer.prototype.createReplayGroup = function(tile,
   replayState.renderedRevision = revision;
   replayState.renderedRenderOrder = renderOrder;
   replayState.replayGroup = replayGroup;
+  replayState.resolution = NaN;
 };
 
 

src/ol/vectortile.js

@@ -4,6 +4,7 @@ goog.require('ol.Tile');
 goog.require('ol.TileCoord');
 goog.require('ol.TileLoadFunctionType');
 goog.require('ol.TileState');
+goog.require('ol.dom');
 goog.require('ol.proj.Projection');
 
 
@@ -33,6 +34,12 @@ ol.VectorTile =
 
   goog.base(this, tileCoord, state);
 
+  /**
+   * @private
+   * @type {CanvasRenderingContext2D}
+   */
+  this.context_ = ol.dom.createCanvasContext2D();
+
   /**
    * @private
    * @type {ol.format.Feature}
@@ -84,6 +91,14 @@ ol.VectorTile =
 goog.inherits(ol.VectorTile, ol.Tile);
 
 
+/**
+ * @return {CanvasRenderingContext2D}
+ */
+ol.VectorTile.prototype.getContext = function() {
+  return this.context_;
+};
+
+
 /**
  * @inheritDoc
  */