fastuidraw_align.vert.glsl.hpp

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/*!
 * \file fastuidraw_align.vert.glsl.hpp
 * \brief file fastuidraw_align.vert.glsl.hpp
 *
 * Copyright 2018 by Intel.
 *
 * Contact: kevin.rogovin@gmail.com
 *
 * This Source Code Form is subject to the
 * terms of the Mozilla Public License, v. 2.0.
 * If a copy of the MPL was not distributed with
 * this file, You can obtain one at
 * http://mozilla.org/MPL/2.0/.
 *
 * \author Kevin Rogovin <kevin.rogovin@gmail.com>
 *
 */


/*!\addtogroup GLSLVertCode
 * @{
 */

///@cond
void
fastuidraw_align_compute_Q_adjoint_Q(in vec3 pclip_p,
                                     out mat2 Q, out mat2 adjQ)
{
  vec3 clip;
  clip = vec3(fastuidraw_viewport_pixels, 1.0) * pclip_p;

  vec2 c0, c1;
  c0.x = clip.z * fastuidraw_item_matrix[0].x - clip.x * fastuidraw_item_matrix[0].z;
  c0.y = clip.z * fastuidraw_item_matrix[0].y - clip.y * fastuidraw_item_matrix[0].z;
  c1.x = clip.z * fastuidraw_item_matrix[1].x - clip.x * fastuidraw_item_matrix[1].z;
  c1.y = clip.z * fastuidraw_item_matrix[1].y - clip.y * fastuidraw_item_matrix[1].z;

  Q = mat2(c0, c1);
  adjQ = mat2( Q[1][1], -Q[0][1], -Q[1][0], Q[0][0]);
}
///@endcond

/*!
 * Given a normal vector in local coordinate and a position
 * in -clip- coordinates, return the vector N in local coordinates
 * so that N transformed to screen coordiantes is perpindicular
 * to the vector T transformed to screen coordinate where T
 * is the vector perpindicular to the input normal vector.
 * The magnitude of N is not necessarily the same as the magnitude
 * of the input vector n, only the nature of its direction is
 * guarnateed.
 * \param clip_p position in CLIP coordinates
 * \param n normal vector in LOCAL coordinates
 */
vec2
fastuidraw_align_normal_to_screen(in vec3 clip_p, in vec2 n)
{
  mat2 Q, adjQ;

  fastuidraw_align_compute_Q_adjoint_Q(clip_p, Q, adjQ);

  vec2 n_screen, t_screen, t;

  t = vec2(-n.y, n.x);
  t_screen = fastuidraw_viewport_pixels * (Q * t);
  n_screen = vec2(t_screen.y, -t_screen.x);

  /* NOTE!
   *   We do NOT divide by the det of Q, this is because
   *   we only care about the -direction- of return
   *   value, not the magnitude, thus the adjoint is
   *   good enough [and it avoids us needing to deal
   *   with a the case if the determinant is 0].
   */
  return adjQ * (fastuidraw_viewport_recip_pixels * n_screen);
}
/*! @} */