/*
  Part of the Processing project - http://processing.org

  Copyright (c) 2012-15 The Processing Foundation
  Copyright (c) 2004-12 Ben Fry and Casey Reas
  Copyright (c) 2001-04 Massachusetts Institute of Technology

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation, version 2.1.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General
  Public License along with this library; if not, write to the
  Free Software Foundation, Inc., 59 Temple Place, Suite 330,
  Boston, MA  02111-1307  USA
*/

uniform mat4 projectionMatrix;
uniform mat4 modelviewMatrix;
 
uniform vec4 viewport;
uniform int perspective; 
 
attribute vec4 position;
attribute vec4 color;
attribute vec2 offset;

varying vec4 vertColor;

void main() {
  vec4 pos = modelviewMatrix * position;
  vec4 clip = projectionMatrix * pos;

  // Perspective ---
  // convert from world to clip by multiplying with projection scaling factor
  // invert Y, projections in Processing invert Y
  vec2 perspScale = (projectionMatrix * vec4(1, -1, 0, 0)).xy;

  // formula to convert from clip space (range -1..1) to screen space (range 0..[width or height])
  // screen_p = (p.xy/p.w + <1,1>) * 0.5 * viewport.zw

  // No Perspective ---
  // multiply by W (to cancel out division by W later in the pipeline) and
  // convert from screen to clip (derived from clip to screen above)
  vec2 noPerspScale = clip.w / (0.5 * viewport.zw);

  gl_Position.xy = clip.xy + offset.xy * mix(noPerspScale, perspScale, float(perspective > 0));
  gl_Position.zw = clip.zw;
  
  vertColor = color;
}