2nd/10_Wiki/Topic_Programming/Frontend/WEBGL_multi_draw.md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-webgl-multi-draw	WEBGL_multi_draw Extension	10_Wiki/Topics	verified	self	WEBGL_multi_draw multi-draw multiDrawArrays	none	A	0.9	applied	webgl graphics performance draw-call frontend		2026-05-10	pending	language framework JavaScript WebGL 1.0/2.0

WEBGL_multi_draw Extension

매 한 줄

"매 한 번의 JS→GL 호출로 매 수백 개 draw 를 batched 처리하는 매 WebGL 확장". 매 gl.drawArrays 의 매 N 회 호출을 매 multiDrawArraysWEBGL 한 번으로 줄여 매 CPU↔GPU command overhead 를 매 결정적으로 감소시킨다. 매 2026 기준 Chromium / Firefox / Safari 모두 매 안정 지원.

매 핵심

매 정의

• 매 KhronosGroup WebGL extension.
• 매 OpenGL glMultiDrawArrays / glMultiDrawElements 의 매 WebGL 매핑.
• 매 WebGL 1 / WebGL 2 양쪽 매 사용 가능.

매 4 개 함수

• multiDrawArraysWEBGL(mode, firsts, firstsOffset, counts, countsOffset, drawcount)
• multiDrawElementsWEBGL(mode, counts, countsOffset, type, offsets, offsetsOffset, drawcount)
• multiDrawArraysInstancedWEBGL(...)
• multiDrawElementsInstancedWEBGL(...)

매 응용

1. 매 Particle / sprite batching.
2. 매 Tile-based map rendering.
3. 매 CesiumJS, deck.gl 의 large-scale geo viz.

💻 패턴

Extension 활성화

const gl = canvas.getContext('webgl2');
const ext = gl.getExtension('WEBGL_multi_draw');
if (!ext) throw new Error('WEBGL_multi_draw not supported');

multiDrawArrays 기본

const drawcount = 1000;
const firsts  = new Int32Array(drawcount);
const counts  = new Int32Array(drawcount);
for (let i = 0; i < drawcount; i++) {
  firsts[i] = i * 6;
  counts[i] = 6; // 6 vertices per quad
}
ext.multiDrawArraysWEBGL(
  gl.TRIANGLES,
  firsts, 0,
  counts, 0,
  drawcount
);

multiDrawElementsInstanced

ext.multiDrawElementsInstancedWEBGL(
  gl.TRIANGLES,
  countsArr, 0,
  gl.UNSIGNED_SHORT,
  offsetsArr, 0,
  instanceCountsArr, 0,
  drawcount
);

gl_DrawID 사용 (vertex shader)

#version 300 es
#extension GL_ANGLE_multi_draw : require
in vec3 a_position;
uniform mat4 u_mvps[256];
void main() {
  gl_Position = u_mvps[gl_DrawID] * vec4(a_position, 1.0);
}

Polyfill fallback

function multiDraw(gl, ext, mode, firsts, counts, drawcount) {
  if (ext) {
    ext.multiDrawArraysWEBGL(mode, firsts, 0, counts, 0, drawcount);
  } else {
    for (let i = 0; i < drawcount; i++) {
      gl.drawArrays(mode, firsts[i], counts[i]);
    }
  }
}

Benchmark guideline

const t0 = performance.now();
ext.multiDrawArraysWEBGL(gl.TRIANGLES, firsts, 0, counts, 0, drawcount);
gl.finish();
console.log('multidraw ms:', performance.now() - t0);
// expect 5-50× speedup vs naive loop on draw-call-bound scenes

매 결정 기준

상황	Approach
Draw call > 100/frame	multi_draw 적용
Same shader/state 반복	매 매 효과 큼
적은 large draw	매 효과 미미 — instancing 만으로 충분
Indirect 필요	WebGPU 으로 이동

기본값: WebGL 환경에서 매 draw call bound 면 매 1순위 최적화.

🔗 Graph

• 부모: WebGL 20 · WebGL
• 변형: Instanced Rendering
• 응용: CesiumJS
• Adjacent: WebGPU · Draw Call Optimization

🤖 LLM 활용

언제: 매 batch boilerplate 생성, 매 fallback path 작성. 언제 X: 매 shader 의 매 gl_DrawID 사용 — 매 cross-vendor 차이 매 manual 검증.

❌ 안티패턴

• State change 섞기: 매 매 draw 사이 uniform/texture 변경 — 매 batch 의미 사라짐.
• Drawcount=1: 매 일반 drawArrays 와 동일하므로 매 overhead 만 추가.
• Extension 체크 누락: 매 getExtension 결과 null 처리 없이 호출 → 매 runtime crash.

🧪 검증 / 중복

• Verified (KhronosGroup WebGL Registry, WEBGL_multi_draw spec).
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — extension 4 함수 + gl_DrawID 패턴