opengl round rectangle

fragment shader

#version 330 core

uniform vec3      iResolution;           // viewport resolution (in pixels)
uniform float     iTime;                 // shader playback time (in seconds)
uniform float     iTimeDelta;            // render time (in seconds)
uniform float     iFrameRate;            // shader frame rate
uniform int       iFrame;                // shader playback frame
uniform float     iChannelTime[4];       // channel playback time (in seconds)
uniform vec3      iChannelResolution[4]; // channel resolution (in pixels)
uniform vec4      iMouse;                // mouse pixel coords. xy: current (if MLB down), zw: click
uniform vec4      iDate;                 // (year, month, day, time in seconds)
out vec4 FragColor;

// from https://iquilezles.org/articles/distfunctions
float roundedBoxSDF(vec2 CenterPosition, vec2 Size, float Radius)
{
    return length(max(abs(CenterPosition)-Size+Radius,0.0))-Radius;
}

void mainImage(out vec4 fragColor, in vec2 fragCoord)
{
    vec2  size           = vec2(300.0, 200.0);
    // vec2  location       = iMouse.xy;
    vec2  location       = vec2(150,150);
    float thickness      = 5.0;
    float shadowSoftness = 10.0f;
    vec2  shadowOffset   = vec2(10.0, -10.0);
    float edgeSoftness   = 1.0;
    float radius         = (sin(iTime*2.0) + 1.0) * 30.0 + thickness * 2.0;

    float distance       = roundedBoxSDF(location - fragCoord.xy, size/2.0, radius);
    float smoothedAlpha  = 1.0 - smoothstep(-edgeSoftness, edgeSoftness, abs(distance) - thickness);
    vec4  quadColor      = mix(vec4(1.0), vec4(0.0, 0.2, 1.0, smoothedAlpha), smoothedAlpha);

    float shadowDistance = roundedBoxSDF(location + shadowOffset - fragCoord.xy, size/2.0, radius);
    float shadowAlpha    = 1.0 - smoothstep(-shadowSoftness/2.0, shadowSoftness/2.0, abs(shadowDistance));
    vec4 shadowColor     = vec4(0.4, 0.4, 0.4, 1.0);
    fragColor            = mix(quadColor, shadowColor, shadowAlpha - smoothedAlpha);
}

void main() 
{
   mainImage(FragColor, gl_FragCoord.xy);
}