feat: add support for vec4

Co-Authored-By: Jenya Tra <91479375+jenyatra@users.noreply.github.com>

README.md

@@ -136,6 +136,18 @@ The `new MapboxInterpolateHeatmapLayer()` function has the following parameters:
   }
   ```
 
+- `valueToColor4`: Same as `valueToColor`, but with alpha channel support. The function name and signature must be defined as:
+
+  `vec4 valueToColor4(float value, float defaultOpacity)`
+
+  Default value:
+
+  ```glsl
+  vec4 valueToColor4(float value, float defaultOpacity) {
+      return vec4(valueToColor(value), defaultOpacity);
+  }
+  ```
+
 ## Technical explanation
 
 The color is computed using the [Inverse Distance Weighting](https://en.wikipedia.org/wiki/Inverse_distance_weighting) (IDW) algorithm:

src/layer.ts

@@ -13,10 +13,15 @@ class MapboxInterpolateHeatmapLayer implements CustomLayerInterface {
   aoi?: { lat: number; lon: number }[] = [];
   textureCoverSameAreaAsROI: boolean;
   valueToColor?: string = `
-  vec3 valueToColor(float value) {
+    vec3 valueToColor(float value) {
       return vec3(max((value-0.5)*2.0, 0.0), 1.0 - 2.0*abs(value - 0.5), max((0.5-value)*2.0, 0.0));
-  }
-`;
+    }
+  `;
+  valueToColor4?: string = `
+    vec4 valueToColor4(float value, float defaultOpacity) {
+        return vec4(valueToColor(value), defaultOpacity);
+    }
+  `;
   points: number[][] = [];
   // Custom Props
   aPositionComputation?: number;
@@ -91,8 +96,8 @@ class MapboxInterpolateHeatmapLayer implements CustomLayerInterface {
               void main(void) {
                   vec4 data = texture2D(u_ComputationTexture, vec2(gl_FragCoord.x/u_ScreenSize.x, gl_FragCoord.y/u_ScreenSize.y));
                   float u = data.x/data.y;
-                  gl_FragColor.rgb = valueToColor(u);
-                  gl_FragColor.a = u_Opacity;
+                  u += u_Opacity*0.00000001; // force WebGL to use u_Opacity. This might not be the case depending on valueToColor4
+                  gl_FragColor = valueToColor4(u, u_Opacity);
               }
           `;
     const computationVertexSource = `