From ad4a258c87574f76397ed719cf4ac4a529ce7632 Mon Sep 17 00:00:00 2001
From: Tim Schaub <tim.schaub@gmail.com>
Date: Fri, 6 Oct 2017 09:12:52 -0600
Subject: [PATCH] Only clear the canvas when needed

---
 src/ol/renderer/canvas/tilelayer.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/ol/renderer/canvas/tilelayer.js b/src/ol/renderer/canvas/tilelayer.js
index ab7ad60e1d..d8fcd7a898 100644
--- a/src/ol/renderer/canvas/tilelayer.js
+++ b/src/ol/renderer/canvas/tilelayer.js
@@ -228,7 +228,9 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame = function(frameState, layer
         canvas.width = width;
         canvas.height = height;
       } else {
-        context.clearRect(0, 0, width, height);
+        if (this.renderedExtent_ && !ol.extent.equals(imageExtent, this.renderedExtent_)) {
+          context.clearRect(0, 0, width, height);
+        }
         oversampling = this.oversampling_;
       }
     }