shadow effect RFC (#3350)

dev-docs/RFCs/v7.x/shadow-for-directional-lights.md

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+# RFC: Shadow for directional lights
+
+* **Authors**: Jian Huang
+
+* **Date**: July. 2019
+
+* **Status**: For Review
+
+## Abstract
+Deck.gl has new basic lighting effect since 7.0, having shadow effects for directional lights can improve rendering quality.
+
+## Proposal
+### API
+`LightWithShadowEffect` class is the public interface to create shadow effects, it extends from `LightingEffect` and has one param.
+* `lights`(Object): collection of light sources
+
+New layer prop
+* `enableShadow`(Boolean): when this prop is true, layer casts and renders shadows
+
+### Example
+```js
+const ambientLight = new AmbientLight({
+  color: [255, 255, 255],
+  intensity: 1.0
+});
+
+const dirLight0 = new DirectionalLight({
+  color: [255, 255, 255],
+  intensity: 1.0,
+  direction: [10, -20, -30]
+});
+
+const dirLight1 = new DirectionalLight({
+  color: [255, 255, 255],
+  intensity: 1.0,
+  direction: [-10, -20, -30]
+});
+
+const lightWithShadowEffect = new LightWithShadowEffect({
+  ambientLight,
+  dirLight0,
+  dirLight1
+});
+const deckgl = new Deck({
+  canvas: 'my-deck-canvas',
+  effects: [lightWithShadowEffect],
+  layers: [
+    // building layer
+    new SolidPolygonLayer({
+      enableShadow: true,
+      ...
+    }),
+    // ground layer
+    new SolidPolygonLayer({
+      enableShadow: true,
+      ...
+    })
+  ]
+});
+```
+### Workflow
+* User creates `LightWithShadowEffect` instance which adds shadow modules into default shader modules
+* `LightWithShadowEffect` instance is passed into deck and create shadow passes based on light directions, there is one shadow pass for each directional light, for now totally two lights are supported
+* For each shadow pass, layers are rendered to a shadow map
+* After all the effects are processed, there is a regular layer rendering pass, shadow maps are used to render the final shadows.
+
+## Limitations
+With the current design, the user needs to create `LightWithShadowEffect` instance before Deck's initialization, so that the default modules can be modified before layer shaders are assembled.