fix: Fix coordinate system to match Flutter implementation

- Fix y-coordinate inversion between UI (top-left origin) and WebGL (bottom-left origin)
- Convert UI coordinates to WebGL coordinates when passing to shader
- Invert drag vector y-direction to match coordinate system
- Add getResolution() method to ThreeScene for coordinate conversion
- Update shader to use normalized drag vectors directly

This resolves the issue where distortion appeared at opposite y-position from clicked point.

src/components/ImageDistortion.tsx

@@ -143,22 +143,27 @@ export const ImageDistortion: React.FC<ImageDistortionProps> = ({
   useEffect(() => {
     if (!sceneRef.current || !isReady) return;
 
+    // 현재 해상도 가져오기
+    const resolution = sceneRef.current.getResolution();
+
     // 포인트 배열 생성
+    // UI는 좌상단 (0,0), WebGL은 좌하단 (0,0)이므로 y 좌표를 반전
     const points = new Float32Array(SHADER_CONFIG.MAX_POINTS * 2);
     currentAreas.forEach((area, areaIndex) => {
       area.basePoints.forEach((point, pointIndex) => {
         const index = (areaIndex * 4 + pointIndex) * 2;
         points[index] = point.x;
-        points[index + 1] = point.y;
+        points[index + 1] = 1.0 - point.y; // y 좌표 반전
       });
     });
 
     // 드래그 벡터 배열 생성
+    // dragVector도 y 좌표계를 맞춰야 하므로 y를 반전
     const dragVectors = new Float32Array(SHADER_CONFIG.MAX_DRAG_VECTORS * 2);
     currentAreas.forEach((area, index) => {
       const baseIndex = index * 2;
       dragVectors[baseIndex] = area.dragVector.x;
-      dragVectors[baseIndex + 1] = area.dragVector.y;
+      dragVectors[baseIndex + 1] = -area.dragVector.y; // y 방향 반전
     });
 
     // 강도 배열 생성

src/engine/ThreeScene.ts

@@ -115,6 +115,16 @@ export class ThreeScene {
     this.renderer.render(this.scene, this.camera);
   }
 
+  /**
+   * 현재 해상도 가져오기
+   */
+  public getResolution(): { x: number; y: number } {
+    return {
+      x: this.uniforms.u_resolution.value.x,
+      y: this.uniforms.u_resolution.value.y,
+    };
+  }
+
   /**
    * 리소스 정리
    */

src/shaders/distortion.frag.glsl

@@ -1,8 +1,8 @@
 uniform vec2 u_resolution;
 uniform sampler2D u_texture;
-uniform vec2 u_points[32];        // 최대 8영역 × 4포인트 (정규화된 좌표)
+uniform vec2 u_points[32];        // 최대 8영역 × 4포인트 (정규화된 좌표 0-1)
 uniform int u_numAreas;
-uniform vec2 u_dragVectors[8];    // 정규화된 좌표
+uniform vec2 u_dragVectors[8];    // 드래그 벡터 (정규화된 좌표 0-1)
 uniform float u_distortionStrengths[8];
 
 varying vec2 vUv;
@@ -72,10 +72,9 @@ void main() {
 
       if (distToCenter < maxUvRadius) {
         float influence = 1.0 - smoothstep(0.0, maxUvRadius, distToCenter);
-        // dragVector도 정규화된 좌표이므로 픽셀로 변환
-        vec2 distortion = (u_dragVectors[i] * u_resolution) * influence * u_distortionStrengths[i];
-        // texCoord는 이미 정규화된 좌표이므로 정규화된 왜곡 적용
-        texCoord += distortion / u_resolution;
+        // dragVector는 정규화된 좌표(0-1)이므로 바로 사용 (Flutter와 동일한 결과)
+        vec2 distortion = u_dragVectors[i] * influence * u_distortionStrengths[i];
+        texCoord += distortion;
         texCoord = clamp(texCoord, 0.0, 1.0);
       }
       found = true;