/* -*- mode: c++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
/*
 * ======================================================================
 *  AUTHOR:  Wei Qiao <qiaow@purdue.edu>
 *           Purdue Rendering and Perceptualization Lab (PURPL)
 *
 *  Copyright (c) 2004-2012  HUBzero Foundation, LLC
 *
 *  See the file "license.terms" for information on usage and
 *  redistribution of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 * ======================================================================
 */

// update velocity based on acceleration

#include "common.cg"

float4 main(in float2 uv : TEXCOORD0,
            uniform samplerRECT pos_tex,
            uniform samplerRECT vel_tex,
            uniform sampler2D terrain_tex,            
            uniform float timestep = 0.01,
            uniform float damping = 0.99,
            uniform float3 gravity = float3(0, -1, 0),
            uniform float3 spherePos,
            uniform float3 sphereVel) : COLOR
{
    const float3 terrain_scale = float3(8.0, 2.0, 8.0);
    const float3 terrain_offset = float3(-4.0, 0.01, -4.0);

    float3 pos = texRECT(pos_tex, uv).xyz;
    float3 vel = texRECT(vel_tex, uv).xyz;

    float3 pos_next = pos + vel * timestep;  // predicted position next timestep

    float3 force = gravity;
    // float3 force = 0.0;
    // Gravitation(pos, spherePos, force, 0.1);

    SphereCollide(pos_next, vel, spherePos, 1.0, sphereVel, 0.5, force);
    // FloorCollide(pos_next, vel, 0.0, 0.5, force);
    TerrainCollide(pos_next, vel, terrain_tex, terrain_scale, terrain_offset, 0.5);

    const float inv_mass = 1.0;    
    vel += force * inv_mass * timestep; // F = ma      
    vel *= damping;

    return float4(vel, 1.0);
}