Fix exp perf regression

src/scene/renderer/renderer.js

@@ -1,7 +1,7 @@
 import { Debug, DebugHelper } from '../../core/debug.js';
 import { now } from '../../core/time.js';
 import { BlueNoise } from '../../core/math/blue-noise.js';
 import { FloatPacking } from '../../core/math/float-packing.js';
 import { Vec2 } from '../../core/math/vec2.js';
 import { Vec3 } from '../../core/math/vec3.js';
 import { Vec4 } from '../../core/math/vec4.js';
@@ -15,9 +15,9 @@
     SHADERSTAGE_VERTEX, SHADERSTAGE_FRAGMENT,
     CULLFACE_BACK, CULLFACE_FRONT, CULLFACE_NONE,
     BINDGROUP_MESH_UB,
     PIXELFORMAT_R16F,
     FILTER_LINEAR,
     ADDRESS_CLAMP_TO_EDGE
 } from '../../platform/graphics/constants.js';
 import { DebugGraphics } from '../../platform/graphics/debug-graphics.js';
 import { UniformBuffer } from '../../platform/graphics/uniform-buffer.js';
@@ -32,7 +32,7 @@
 } from '../constants.js';
 import { LightCube } from '../graphics/light-cube.js';
 import { getBlueNoiseTexture } from '../graphics/noise-textures.js';
 import { Texture } from '../../platform/graphics/texture.js';
 import { LightTextureAtlas } from '../lighting/light-texture-atlas.js';
 import { Material } from '../materials/material.js';
 import { ShadowMapCache } from './shadow-map-cache.js';
@@ -104,30 +104,6 @@
 const _tempMeshInstances = [];
 const _tempMeshInstancesSkinned = [];
 
-// construct the exponent lookup table used in gaussian splat rendering
-const createExpTableTexture = (device) => {
-    const expTableSize = 32;
-    const expTable = new Uint16Array(expTableSize);
-    for (let i = 0; i < expTableSize; ++i) {
-        const value = Math.exp(-4.0 * i / (expTableSize - 1));
-        const nvalue = (value - Math.exp(-4.0)) / (1.0 - Math.exp(-4.0));
-        expTable[i] = FloatPacking.float2Half(nvalue);
-    }
-
-    return new Texture(device, {
-        name: 'internal-expTable',
-        width: expTableSize,
-        height: 1,
-        format: PIXELFORMAT_R16F,
-        mipmaps: false,
-        minFilter: FILTER_LINEAR,
-        magFilter: FILTER_LINEAR,
-        addressU: ADDRESS_CLAMP_TO_EDGE,
-        addressV: ADDRESS_CLAMP_TO_EDGE,
-        levels: [expTable]
-    });
-};
-
 /**
  * The base renderer functionality to allow implementation of specialized renderers.
  *
@@ -273,7 +249,6 @@
         this.blueNoiseJitterData = new Float32Array(4);
         this.blueNoiseJitterId = scope.resolve('blueNoiseJitter');
         this.blueNoiseTextureId = scope.resolve('blueNoiseTex32');
-        this.expTableTextureId = scope.resolve('expTable');
 
         this.alphaTestId = scope.resolve('alpha_ref');
         this.opacityMapId = scope.resolve('texture_opacityMap');
@@ -289,8 +264,6 @@
         // a single instance of light cube
         this.lightCube = new LightCube();
         this.constantLightCube = scope.resolve('lightCube[0]');
-
-        this.expTableTexture = createExpTableTexture(this.device);
     }
 
     destroy() {
@@ -309,9 +282,6 @@
 
         this.gsplatDirector?.destroy();
         this.gsplatDirector = null;
-
-        this.expTableTexture?.destroy();
-        this.expTableTexture = null;
     }
 
     /**
@@ -1232,7 +1202,6 @@
     updateFrameUniforms() {
         // blue noise texture
         this.blueNoiseTextureId.setValue(getBlueNoiseTexture(this.device));
-        this.expTableTextureId.setValue(this.expTableTexture);
     }
 
     /**

src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplat.js

@@ -22,15 +22,20 @@ export default /* glsl */`
 varying mediump vec2 gaussianUV;
 varying mediump vec4 gaussianColor;
 
-uniform sampler2D expTable;
+#define EXP4        0.018315638888734       // exp(-4)
+#define INV_EXP4    1.018657360363774       // 1 / (1 - exp(-4))
+
+float normExp(float x) {
+    return (exp(x * -4.0) - EXP4) * INV_EXP4;
+}
 
 void main(void) {
     mediump float A = dot(gaussianUV, gaussianUV);
     if (A > 1.0) {
         discard;
     }
 
-    mediump float alpha = texture2DLod(expTable, vec2(A, 0.5), 0.0).r * gaussianColor.a;
+    mediump float alpha = normExp(A) * gaussianColor.a;
 
     #if defined(SHADOW_PASS) || defined(PICK_PASS) || defined(PREPASS_PASS)
         if (alpha < alphaClip) {

src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplat.js

@@ -19,12 +19,16 @@ export default /* wgsl */`
     #include "floatAsUintPS"
 #endif
 
+const EXP4      = exp(-4.0);
+const INV_EXP4  = 1.0 / (1.0 - EXP4);
+
+fn normExp(x: f32) -> f32 {
+    return (exp(x * -4.0) - EXP4) * INV_EXP4;
+}
+
 varying gaussianUV: vec2f;
 varying gaussianColor: vec4f;
 
-var expTable: texture_2d<f32>;
-var expTableSampler: sampler;
-
 @fragment
 fn fragmentMain(input: FragmentInput) -> FragmentOutput {
     var output: FragmentOutput;
@@ -36,7 +40,7 @@ fn fragmentMain(input: FragmentInput) -> FragmentOutput {
     }
 
     // evaluate alpha
-    var alpha = textureSampleLevel(expTable, expTableSampler, vec2f(A, 0.5), 0).r * gaussianColor.a;
+    var alpha = normExp(A) * gaussianColor.a;
 
     #if defined(SHADOW_PASS) || defined(PICK_PASS) || defined(PREPASS_PASS)
         if (alpha < uniform.alphaClip) {