change to enumeration

Author: almarklein

src/traces/image/attributes.js

@@ -52,13 +52,14 @@ module.exports = extendFlat({
         ].join(' ')
     },
     interpolate: {
-        valType: 'boolean',
-        dflt: false,
+        valType: 'enumerated',
+        values: ['nearest', 'linear'],
         role: 'info',
         editType: 'plot',
         description: [
-            'Whether to render the image with bilinear interpolation.',
-            'Default false (nearest neighbor interpolation).'
+            'The type of interpolation for rendering the image.',
+            'Default `nearest` (nearest neighbor interpolation).',
+            'Can also be set to `linear` (bilinear interpolation).'
         ].join(' ')
     },
     zmin: {

src/traces/image/defaults.js

@@ -47,7 +47,7 @@ module.exports = function supplyDefaults(traceIn, traceOut) {
         traceOut.zmax = cm.zmaxDflt;
     }
 
-    coerce('interpolate', false);
+    coerce('interpolate', 'nearest');
     coerce('text');
     coerce('hovertext');
     coerce('hovertemplate');

src/traces/image/plot.js

@@ -140,7 +140,7 @@ module.exports = function plot(gd, plotinfo, cdimage, imageLayer) {
         // http://phrogz.net/tmp/canvas_image_zoom.html
         // https://developer.mozilla.org/en-US/docs/Web/CSS/image-rendering
         var style = '';
-        if(!trace.interpolate) {
+        if(trace.interpolate === 'nearest') {
             style += 'image-rendering: optimizeSpeed; image-rendering: -moz-crisp-edges; image-rendering: -o-crisp-edges; image-rendering: -webkit-optimize-contrast; image-rendering: optimize-contrast; image-rendering: crisp-edges; image-rendering: pixelated;';
         }
         if(fastImage) {