fix dot grid fragment shader for android

shaders/dot_grid.frag

@@ -1,17 +1,19 @@
 #include <flutter/runtime_effect.glsl>
 precision mediump float;
 
-uniform vec2 uScreenOrigin;      // screenOffset in pixels
-uniform float uScaledSpacing;     // spacing * scale in pixels
-uniform vec2 uPhasePx;            // phase shift in pixels: mod(-canvasOffset * scale, scaledSpacing)
-uniform float uRadiusPx;          // dot radius in pixels: size * scale
+uniform vec2 uScreenOrigin;      // screenOffset in logical pixels
+uniform float uScaledSpacing;     // spacing * scale in logical pixels
+uniform float uInvScaledSpacing;  // 1.0 / uScaledSpacing
+uniform vec2 uPhasePx;            // phase shift in logical pixels
+uniform float uRadiusPx;          // dot radius in logical pixels
 uniform vec4 uDotColor;           // RGBA 0..1
 uniform vec4 uBgColor;            // RGBA 0..1
 
 out vec4 fragColor;
 
 void main() {
-  vec2 p = FlutterFragCoord().xy - uScreenOrigin; // local pixel coords within the painted rect
+  // Work entirely in logical pixels
+  vec2 p = FlutterFragCoord().xy - uScreenOrigin; // local logical pixel coords within the painted rect
 
   // LOD: if dots become sub-pixel or spacing too dense, draw only background
   if (uScaledSpacing < 1.0 || uRadiusPx < 0.25) {
@@ -19,14 +21,18 @@ void main() {
     return;
   }
 
-  // Periodic grid in screen space with phase to account for canvasOffset
-  vec2 rem = mod(p + uPhasePx, uScaledSpacing);
-  vec2 d = min(rem, vec2(uScaledSpacing) - rem);
-  float dist = length(d);
+  // Fractional position within the lattice, centered around each grid point
+  vec2 cell = (p + uPhasePx) * uInvScaledSpacing; // multiply by inverse spacing to avoid division
+  vec2 f = fract(cell) - 0.5;                     // [-0.5, 0.5)
 
-  // Analytic AA around the circle edge (~1px transition)
-  float edge = uRadiusPx;
-  float alpha = 1.0 - smoothstep(edge - 1.0, edge + 1.0, dist);
+  // Use squared distance to avoid sqrt
+  float s2 = uScaledSpacing * uScaledSpacing;
+  float dist2 = dot(f, f) * s2;
+
+  // Analytic AA using squared thresholds (~1 logical px transition)
+  float e0 = max(0.0, uRadiusPx - 1.0);
+  float e1 = uRadiusPx + 1.0;
+  float alpha = 1.0 - smoothstep(e0 * e0, e1 * e1, dist2);
 
   fragColor = mix(uBgColor, uDotColor, alpha);
 }