//version 130

uniform vec2 iResolution; // viewport resolution (in pixels)
out vec2 texCoord;

void main() {
       texCoord = vec2( (gl_VertexID << 1) & 2, gl_VertexID & 2 );
       gl_Position = vec4( texCoord * 2.0 - 1.0, 0.0, 1.0 );
       texCoord = texCoord * iResolution;
       texCoord.y = iResolution.y - texCoord.y; // flip Y
}