From 841da657265749648578b0a647f4822c2a70dcb3 Mon Sep 17 00:00:00 2001
From: Xiaoji Chen <cxiaoji@gmail.com>
Date: Wed, 1 May 2024 10:21:16 -0700
Subject: [PATCH] Simplify ArcLayer

---
 .../src/arc-layer/arc-layer-vertex.glsl.ts    | 15 ++++++++----
 modules/layers/src/arc-layer/arc-layer.ts     | 24 ++++---------------
 2 files changed, 16 insertions(+), 23 deletions(-)

diff --git a/modules/layers/src/arc-layer/arc-layer-vertex.glsl.ts b/modules/layers/src/arc-layer/arc-layer-vertex.glsl.ts
index a01c41fb017..f666ea49e30 100644
--- a/modules/layers/src/arc-layer/arc-layer-vertex.glsl.ts
+++ b/modules/layers/src/arc-layer/arc-layer-vertex.glsl.ts
@@ -22,7 +22,6 @@ export default `\
 #version 300 es
 #define SHADER_NAME arc-layer-vertex-shader
 
-in vec3 positions;
 in vec4 instanceSourceColors;
 in vec4 instanceTargetColors;
 in vec3 instanceSourcePositions;
@@ -137,7 +136,15 @@ void main(void) {
   geometry.worldPosition = instanceSourcePositions;
   geometry.worldPositionAlt = instanceTargetPositions;
 
-  float segmentIndex = positions.x;
+  /*
+  *  --(i, -1)-----------_(i+1, -1)--
+  *       |          _,-"  |
+  *       o      _,-"      o
+  *       |  _,-"          |
+  *  --(i, 1)"-------------(i+1, 1)--
+  */
+  float segmentIndex = float(gl_VertexID / 2);
+  float segmentSide = mod(float(gl_VertexID), 2.) == 0. ? -1. : 1.;
   float segmentRatio = getSegmentRatio(segmentIndex);
   float prevSegmentRatio = getSegmentRatio(max(0.0, segmentIndex - 1.0));
   float nextSegmentRatio = getSegmentRatio(min(numSegments - 1.0, segmentIndex + 1.0));
@@ -147,7 +154,7 @@ void main(void) {
   float indexDir = mix(-1.0, 1.0, step(segmentIndex, 0.0));
   isValid = 1.0;
 
-  uv = vec2(segmentRatio, positions.y);
+  uv = vec2(segmentRatio, segmentSide);
   geometry.uv = uv;
   geometry.pickingColor = instancePickingColors;
 
@@ -244,7 +251,7 @@ void main(void) {
 
   // extrude
   vec3 offset = vec3(
-    getExtrusionOffset((next.xy - curr.xy) * indexDir, positions.y, widthPixels),
+    getExtrusionOffset((next.xy - curr.xy) * indexDir, segmentSide, widthPixels),
     0.0);
   DECKGL_FILTER_SIZE(offset, geometry);
   DECKGL_FILTER_GL_POSITION(curr, geometry);
diff --git a/modules/layers/src/arc-layer/arc-layer.ts b/modules/layers/src/arc-layer/arc-layer.ts
index 3d21382c9c8..97cf59c2736 100644
--- a/modules/layers/src/arc-layer/arc-layer.ts
+++ b/modules/layers/src/arc-layer/arc-layer.ts
@@ -34,7 +34,6 @@ import {
   DefaultProps
 } from '@deck.gl/core';
 
-import {Geometry} from '@luma.gl/engine';
 import {Model} from '@luma.gl/engine';
 
 import vs from './arc-layer-vertex.glsl';
@@ -231,11 +230,14 @@ export default class ArcLayer<DataT = any, ExtraPropsT extends {} = {}> extends
     super.updateState(opts);
     const {props, oldProps, changeFlags} = opts;
     // Re-generate model if geometry changed
-    if (changeFlags.extensionsChanged || props.numSegments !== oldProps.numSegments) {
+    if (changeFlags.extensionsChanged) {
       this.state.model?.destroy();
       this.state.model = this._getModel();
       this.getAttributeManager()!.invalidateAll();
     }
+    if (props.numSegments !== oldProps.numSegments) {
+      this.state.model!.setVertexCount(props.numSegments * 2);
+    }
   }
 
   draw({uniforms}) {
@@ -257,28 +259,12 @@ export default class ArcLayer<DataT = any, ExtraPropsT extends {} = {}> extends
 
   protected _getModel(): Model {
     const {numSegments} = this.props;
-    let positions: number[] = [];
-    /*
-     *  (0, -1)-------------_(1, -1)
-     *       |          _,-"  |
-     *       o      _,-"      o
-     *       |  _,-"          |
-     *   (0, 1)"-------------(1, 1)
-     */
-    for (let i = 0; i < numSegments; i++) {
-      positions = positions.concat([i, 1, 0, i, -1, 0]);
-    }
 
     const model = new Model(this.context.device, {
       ...this.getShaders(),
       id: this.props.id,
       bufferLayout: this.getAttributeManager()!.getBufferLayouts(),
-      geometry: new Geometry({
-        topology: 'triangle-strip',
-        attributes: {
-          positions: {size: 3, value: new Float32Array(positions)}
-        }
-      }),
+      topology: 'triangle-strip',
       isInstanced: true
     });