/* -*- mode: c++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
/*
 * Copyright (c) 2004-2013  HUBzero Foundation, LLC
 *
 * Authors:
 *   Wei Qiao <qiaow@purdue.edu>
 */

// constrain particle to be outside volume of a sphere
void SphereConstraint(inout float3 x, float3 center, float r)
{
    float3 delta = x - center;
    float dist = length(delta);
    if (dist < r) {
        x = center + delta*(r / dist);
    }
}

// constrain particle to be above floor
void FloorConstraint(inout float3 x, float level)
{
    if (x.y < level) {
        x.y = level;
    }
}

// constrain particle to heightfield stored in texture
void TerrainConstraint(inout float3 pos, uniform sampler2D terrain_tex,
                       float3 scale, float3 offset)
{
    float2 uv = (pos.xz - offset.xz) / scale.xz;
    float h = tex2D(terrain_tex, uv).r*scale.y + offset.y;
    if (pos.y < h) {
        pos.y = h;
    }
}

void SphereCollide(inout float3 x, inout float3 vel, float3 center, float r,
                   float3 sphere_vel, float friction, inout float3 force)
{
    float3 delta = x - center;
    float dist = length(delta);
    if (dist < r) {
        // x = center + delta*(r / dist);    
        vel += (delta / dist) * friction;
        vel += sphere_vel;
    }
}

void FloorCollide(inout float3 x, inout float3 vel, float level,
                  float friction, inout float3 force)
{
    if (x.y < level) {
        // x.y = level;
        // force.y += -vel.y*friction;
        vel.y += -vel.y*friction;
    }
}

void Gravitation(float3 pos, float3 mass_pos, inout float3 force, float epsilon)
{
    float3 delta = mass_pos - pos;
    float dist = length(delta);
    float3 dir = delta / dist;
    force += dir * (1.0 / (epsilon + dist*dist));
}

void TerrainCollide(float3 pos, inout float3 vel, uniform sampler2D terrain_tex,
                    float3 scale, float3 offset, float friction)
{
    const float2 texelSize = float2( 1.0 / 256.0, 1.0 / 256.0 );
    float2 uv = (pos.xz - offset.xz) / scale.xz;
    float h0 = tex2D(terrain_tex, uv).r;
    float h = h0*scale.y + offset.y;
    if (pos.y < h) {
        // calculate normal (could precalc this)
        float h1 = tex2D(terrain_tex, uv + texelSize*float2(1, 0) ).r;
        float h2 = tex2D(terrain_tex, uv + texelSize*float2(0, 1) ).r;
        float3 N = cross(float3(scale.x*texelSize.x, (h1-h0)*scale.y, 0),
                         float3(0, (h2-h0)*scale.y, scale.z*texelSize.y));
        N = normalize(N);
        vel = reflect(vel, N);
        vel *= friction;
    }
}

struct a2v
{
    //For semantics, see pages 230-240 of Cg_Toolkit.pdf
    float4 Position : POSITION;
    float4 TexCoord : TEXCOORD0;
};

struct v2f
{
    float4 HPosition: HPOS;		//Clip space pos. Not readable in frag prog
    float4 TexCoord : TEXCOORD0;
    float3 EyeVector: TEXCOORD1;	//world space eye vector
    float3 Light    : TEXCOORD2;    //world space light vector 
};

struct PixelOut {
    float4 Color:	COLOR0;
    //float Depth:	DEPTH;
};