Constant BLUR_FRAG 

pub const BLUR_FRAG: &str = r#"
#version 330 core

in  vec2 v_uv;
out vec4 frag_color;

uniform sampler2D u_texture;
uniform vec2      u_texel_size;
uniform vec2      u_direction;
uniform float     u_sigma;

// Fixed 9-tap kernel weights + offsets (radius 4, precomputed for sigma≈1.5)
// For variable sigma, you'd compute these on the CPU and upload as uniforms.
const int  N_TAPS    = 5;
const float OFFSETS[5] = float[](0.0, 1.3846153846, 3.2307692308, 5.0769230769, 6.9230769231);
const float WEIGHTS[5] = float[](0.2270270270, 0.3162162162, 0.0702702703, 0.0162162162, 0.0054054054);

void main() {
    vec4 result = texture(u_texture, v_uv) * WEIGHTS[0];
    for (int i = 1; i < N_TAPS; ++i) {
        vec2 off = u_direction * u_texel_size * OFFSETS[i] * (u_sigma / 1.5);
        result += texture(u_texture, v_uv + off) * WEIGHTS[i];
        result += texture(u_texture, v_uv - off) * WEIGHTS[i];
    }
    frag_color = result;
}
"#;

GLSL fragment shader source for the separable Gaussian blur pass. Expects: u_texture: sampler2D — input texture u_texel_size: vec2 — 1/resolution u_direction: vec2 — (1,0) for H, (0,1) for V u_sigma: float — Gaussian sigma in pixels