layout (quads, equal_spacing, ccw) in;

void main() {
  float u = gl_TessCoord.x;
  float omu = 1 - u;
  float v = gl_TessCoord.y;
  float omv = 1 - v;

  gl_Position =
    omu * omv * gl_in[0].gl_Position +
    u   * omv * gl_in[1].gl_Position +
    u   * v   * gl_in[2].gl_Position +
    omu * v   * gl_in[3].gl_Position;
}