alpha blending fix in scatterplots (#2033)

* explicitly specify alpha blending; rename NaN flag to accurately describe function

* fix incorret dest alpha blending function

client/src/components/scatterplot/drawPointsRegl.js

@@ -13,7 +13,7 @@ export default function drawPointsRegl(regl) {
     uniform float nPoints;
     uniform float minViewportDimension;
 
-    varying vec4 fragColor;
+    varying lowp vec4 fragColor;
 
     const float zBottom = 0.99;
     const float zMiddle = 0.;
@@ -26,22 +26,22 @@ export default function drawPointsRegl(regl) {
     ${glPointSize}
 
     void main() {
-      bool isNaN, isSelected, isHighlight;
-      getFlags(flag, isNaN, isSelected, isHighlight);
+      bool isBackground, isSelected, isHighlight;
+      getFlags(flag, isBackground, isSelected, isHighlight);
 
       gl_PointSize = pointSize(nPoints, minViewportDimension, isSelected, isHighlight);
 
-      float z = isNaN ? zBottom : (isHighlight ? zTop : zMiddle);
+      float z = isBackground ? zBottom : (isHighlight ? zTop : zMiddle);
       vec3 xy = projection * vec3(position, 1.);
       gl_Position = vec4(xy.xy, z, 1.);
 
-      float alpha = isNaN ? 0.9 : 1.0;
+      float alpha = isBackground ? 0.9 : 1.0;
       fragColor = vec4(color, alpha);
     }`,
 
     frag: `
     precision mediump float;
-    varying vec4 fragColor;
+    varying lowp vec4 fragColor;
     void main() {
       if (length(gl_PointCoord.xy - 0.5) > 0.5) {
         discard;
@@ -64,5 +64,15 @@ export default function drawPointsRegl(regl) {
     count: regl.prop("count"),
 
     primitive: "points",
+
+    blend: {
+      enable: true,
+      func: {
+        srcRGB: "src alpha",
+        srcAlpha: 1,
+        dstRGB: 0,
+        dstAlpha: "zero",
+      },
+    },
   });
 }

client/src/components/scatterplot/scatterplot.js

@@ -16,10 +16,11 @@ import {
   createColorQuery,
 } from "../../util/stateManager/colorHelpers";
 import renderThrottle from "../../util/renderThrottle";
-
-const flagSelected = 1;
-const flagNaN = 2;
-const flagHighlight = 4;
+import {
+  flagBackground,
+  flagSelected,
+  flagHighlight,
+} from "../../util/glHelpers";
 
 function createProjectionTF(viewportWidth, viewportHeight) {
   /*
@@ -135,8 +136,9 @@ class Scatterplot extends React.PureComponent {
     const flags = new Float32Array(nObs);
     if (colorByData) {
       for (let i = 0, len = flags.length; i < len; i += 1) {
-        if (!Number.isFinite(colorByData[i])) {
-          flags[i] = flagNaN;
+        const val = colorByData[i];
+        if (typeof val === "number" && !Number.isFinite(val)) {
+          flags[i] = flagBackground;
         }
       }
     }