line drawing shader

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Normalized pixel coordinates (from 0 to 1)
    fragCoord = fragCoord / iResolution.xy;
    
    vec2 start = vec2(0.2, 0.2);
    vec2 end = iMouse.xy / iResolution.xy;
    
    vec2 direction = normalize(end - start);
    
    // project each frag coordinate into the direction.
    
    float t = dot(fragCoord - start, direction);
    float tEnd = dot(end - start, direction);
    

    float radius = 0.01;
    
    // get the distance from this point:
    
    float d = distance(start + t * direction, fragCoord);
    float startDist = distance(fragCoord, start);
    float endDist = distance(fragCoord, end);
    
    if ((d > radius || t > tEnd || t < 0.0) && (startDist > radius && endDist > radius))
    {
        fragColor = vec4(0.5, 0.5, 0.5, 1.0);
    }
    else
    {
        fragColor = vec4(1.0, 0.0, 0.0, 1.0);
    }
}