Screen-Space Refraction

const float GLASS_FAR = 8.0;
const float CAMERA_FOCAL = 1.8;

vec3 cameraRay(vec2 fragCoord) {
    vec2 screen = (2.0 * fragCoord - iResolution.xy) / iResolution.y;
    return normalize(vec3(screen, -CAMERA_FOCAL));
}

float sphereHit(vec3 rayDirection, vec3 center, float radius) {
    vec3 offset = -center;
    float b = dot(offset, rayDirection);
    float c = dot(offset, offset) - radius * radius;
    float h = b * b - c;
    if (h < 0.0) return GLASS_FAR;
    float root = sqrt(h);
    float nearHit = -b - root;
    return nearHit > 0.0 ? nearHit : GLASS_FAR;
}

vec3 backgroundScene(vec2 uv, float time) {
    vec2 centered = uv - 0.5;
    vec3 color = mix(vec3(0.035, 0.07, 0.12), vec3(0.22, 0.44, 0.62), uv.y);
    vec2 gridUv = centered * vec2(iResolution.x / iResolution.y, 1.0) * 8.0;
    vec2 gridLine = smoothstep(vec2(0.46), vec2(0.49), abs(fract(gridUv + 0.5) - 0.5));
    float grid = max(gridLine.x, gridLine.y);
    color = mix(color, vec3(0.82, 0.68, 0.34), grid * 0.45);
    vec2 lightPosition = vec2(0.24 * sin(time * 0.45), 0.13 * cos(time * 0.62));
    float lightShape = 1.0 - smoothstep(0.06, 0.075, length(centered - lightPosition));
    color += vec3(1.8, 0.72, 0.24) * lightShape;
    float bar = 1.0 - smoothstep(0.018, 0.028, abs(centered.x + 0.31));
    color = mix(color, vec3(0.08, 0.8, 0.72), bar * smoothstep(-0.4, 0.3, centered.y));
    return color;
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 uv = fragCoord / iResolution.xy;
    fragColor = vec4(backgroundScene(uv, iTime), 1.0);
}