Squares

Solid square

vec4 square(vec2 uv, float width)
{
	uv = uv * 2.0 - 1.0;
	
	vec2 abs_uv = abs(uv.xy);
	float square = step(width, max(abs_uv.x, abs_uv.y));
	return vec4(vec3(square), 1.0);
}

Outline square

vec4 square_stroke(vec2 uv, float width, float stroke_width)
{
	uv = uv * 2.0 - 1.0;
	
	vec2 abs_uv = abs(uv.xy);
	float dist = max(abs_uv.x, abs_uv.y);
	vec3 stroke = 1.0 - vec3( step(width, dist) - step(width + stroke_width, dist) );
	return vec4(vec3(stroke), 1.0);
}

Rounded square

vec4 square_rounded(vec2 uv, float width, float radius){
	uv = uv * 2.0 - 1.0;
	
	radius *= width; // make radius go from 0-1 instead of 0-width
	vec2 abs_uv = abs(uv.xy) - radius;
	vec2 dist = vec2(max(abs_uv.xy, 0.0));
	float square = step(width - radius, length(dist));
	return vec4(vec3(square), 1.0);
}