Farbrausch-like loaderbar in a single shader

loader.inc.cpp

namespace {
//works!
///@return 0 if everything succeeds 1 otherwise (or throw an exception)
int initLoader(){
	this->loader.s_loader= new FWrender::Shader();
	this->loader.s_loader->createFromMemory( /* create(&vss, &fss) */
        "#version 330 core\n"
        "layout(location = 0) in vec3 "SHADER_ATTRIB_VERTEX";"
        "void main(){"
		    "gl_Position = vec4("SHADER_ATTRIB_VERTEX",1.);"
	    "}", 
#if 0 /*old loaderbar*/
		"#version 330 core\n"
		"uniform float transition;\n"
		"uniform vec2 "SHADER_UNIFORM_RESOLUTION";\n"
		"void main(void)\n"
		"{\n"
			"vec2 uv = gl_FragCoord.xy / "SHADER_UNIFORM_RESOLUTION".xy; uv -=.5;\n"
			//"vec3 color = vec3(1.,0.,0.);"
			"vec3 color;\n"
			"if (abs(uv.y) < 0.05 && (uv.x+.5)<transition) color = vec3(1.);\n"
			"gl_FragColor = vec4(color,1.0);\n"
		"}\n"
#else /*new loaderbar*/
		"#version 330 core\n"
		"uniform float transition;\n"
		"uniform vec2 "SHADER_UNIFORM_RESOLUTION";\n"

		"#define LIM .001\n"
		"#define THICK .1\n"
		"#define BORDER .01\n"

		"float udBox(vec2 p, vec2 b){return length(max(abs(p)-b,0.0));} \n"

		"vec4 getLoader(vec2 uv){\n"
			"vec4 color; \n"
	
			"float f  = udBox(uv + vec2(1.-transition, 0.) , vec2(transition - BORDER*2., THICK - BORDER*3.));\n"
			"float f0 = udBox(uv , vec2(1., THICK + BORDER)); \n"
			"float f1 = udBox(uv , vec2(1. - BORDER, THICK - BORDER));\n"
   
			"if (f0<LIM && f1>LIM) color = vec4(vec3(0.),1.);\n"
			"if (f<LIM) color = vec4(vec3(0.),1.);\n"
	
			"return color;\n"
		"}\n"

		"void main()\n"
		"{\n"
			"vec2 uv = gl_FragCoord.xy / "SHADER_UNIFORM_RESOLUTION".xy;\n"
			"gl_FragColor = vec4(uv,0.5+0.5*sin(transition),1.0);\n"	//< textura goez here
			
			"uv -=.5; uv *=2.;\n"	// < here you can transform it as your flavour. Note that 0.0 is the center, and the space is [-1..1] 
			"vec4 color = getLoader(uv*1.1);\n"
			 
			"gl_FragColor = vec4(mix(gl_FragColor.rgb, color.rgb, color.a),1.); \n"
		"}\n"
#endif 
	);
	return 0;
}

}

loader.inc.glsl

// sightly optimized version

#version 330 core

uniform float transition;
uniform vec2 resolution;

#define LIM .001
#define THICK .1
#define BORDER .01

const vec4 barColor    = vec4(0.,0.,0.,1.);
const vec4 borderColor = vec4(0.,0.,0.,1.);

float udBox(vec2 p, vec2 b){return length(max(abs(p)-b,0.0));}

vec2 sincos( float x ){return vec2(sin(x), cos(x));}
vec2 rotate2d(vec2 uv, float phi){vec2 t = sincos(phi); return vec2(uv.x*t.y-uv.y*t.x, uv.x*t.x+uv.y*t.y);}

vec4 getLoader(vec2 uv){
    if (udBox(uv , vec2(1., THICK + BORDER))<LIM && udBox(uv , vec2(1. - BORDER, THICK - BORDER))>LIM) return borderColor;
    if (udBox(uv + vec2(1.-transition, 0.) , vec2(transition - BORDER*2., THICK - BORDER*3.))<LIM) return barColor;
    return vec4(0.);
}

void main()
{
    vec2 uv = gl_FragCoord.xy / resolution.xy;
	gl_FragColor = vec4(uv,0.5+0.5*sin(transition),1.0);
	
    uv -=.5; uv *=2.;
	vec4 color = getLoader(uv * 1.1);
	
    gl_FragColor = vec4(mix(gl_FragColor.rgb, color.rgb, color.a),1.);
}