diff --git a/emcclib.js b/emcclib.js
index 5c1d039..dd56191 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -224,7 +224,7 @@ mergeInto(LibraryManager.library, {
      * Prepare to do some drawing on the canvas.
      */
     js_canvas_start_draw: function() {
-        ctx = offscreen_canvas.getContext('2d');
+        ctx = offscreen_canvas.getContext('2d', { alpha: false });
         update_xmin = update_xmax = update_ymin = update_ymax = undefined;
     },
 
@@ -257,7 +257,8 @@ mergeInto(LibraryManager.library, {
      */
     js_canvas_end_draw: function() {
         if (update_xmin !== undefined) {
-            var onscreen_ctx = onscreen_canvas.getContext('2d');
+            var onscreen_ctx =
+                onscreen_canvas.getContext('2d', { alpha: false });
             onscreen_ctx.drawImage(offscreen_canvas,
                                    update_xmin, update_ymin,
                                    update_xmax - update_xmin,
@@ -403,14 +404,14 @@ mergeInto(LibraryManager.library, {
             return midpoint_cache[font];
 
         // Find the width of the string
-        var ctx1 = onscreen_canvas.getContext('2d');
+        var ctx1 = onscreen_canvas.getContext('2d', { alpha: false });
         ctx1.font = font;
         var width = (ctx1.measureText(midpoint_test_str).width + 1) | 0;
 
         // Construct a test canvas of appropriate size, initialise it to
         // black, and draw the string on it in white
         var measure_canvas = document.createElement('canvas');
-        var ctx2 = measure_canvas.getContext('2d');
+        var ctx2 = measure_canvas.getContext('2d', { alpha: false });
         ctx2.canvas.width = width;
         ctx2.canvas.height = 2*height;
         ctx2.fillStyle = "#000000";
@@ -493,7 +494,7 @@ mergeInto(LibraryManager.library, {
      * the screen.
      */
     js_canvas_copy_to_blitter: function(id, x, y, w, h) {
-        var blitter_ctx = blitters[id].getContext('2d');
+        var blitter_ctx = blitters[id].getContext('2d', { alpha: false });
         blitter_ctx.drawImage(offscreen_canvas,
                               x, y, w, h,
                               0, 0, w, h);