///////////////////////////////////////////////////////////////////////////
//
//		   fragment program in charge
//		   of position texture update
//
///////////////////////////////////////////////////////////////////////////



float4 main(float2 coords	   : TEXCOORD0,
            uniform float tslf,
            uniform float bbposTexSide,
            uniform float partCount,
            uniform float3 randomMove,
            uniform float3 lastMove,
            uniform float3 fallVect,
	    uniform sampler2D oldPos : TEXUNIT0) : COLOR
{
  float4 newPos;

    newPos = tex2D(oldPos, coords);
    newPos.a =1;

    // compute the newt position, based on fall direction and speed, and user movement to compensate
    newPos.xyz += fallVect * tslf - lastMove;

    // keep the particles within the box
    if (newPos.y <= 0.05)
    {
	    newPos.y += 0.9;
	    newPos.xz += randomMove.xz;
    }
    else if (newPos.y >= 0.95)
    {
	    newPos.y -= 0.9;
    }

    if (newPos.x <= 0.05)
    {
	    newPos.x += 0.9;
    }
    else if (newPos.x >= 0.95)
    {
	    newPos.x -= 0.9;
    }

    if (newPos.z <= 0.05)
    {
	    newPos.z += 0.9;
    }
    else if (newPos.z >= 0.95)
    {
	    newPos.z -= 0.9;
    }

  return newPos;
}