GPU-based volume raycasting

[qlizhi]

hello, i have download an source code of GPU-based volume raycasting,which is based on Qt. And i upload it to my google code as below:
http://code.google.com/p/qt-color-de...downloads/list
I have read several times of the source code, but in some place i still cannot understand.
in frag_shader

vec2 intersectBox(vec4 texel_zero, vec4 texel_dir_norm)
{
	float t1,t2;
	t1=(0.0-texel_zero.x)/texel_dir_norm.x;
	t2=(1.0-texel_zero.x)/texel_dir_norm.x;
	float tminX=min(t1,t2);
	float tmaxX=max(t1,t2);
	t1=(0.0-texel_zero.y)/texel_dir_norm.y;
	t2=(1.0-texel_zero.y)/texel_dir_norm.y;
	float tminY=min(t1,t2);
	float tmaxY=max(t1,t2);
	t1=(0.0-texel_zero.z)/texel_dir_norm.z;
	t2=(1.0-texel_zero.z)/texel_dir_norm.z;
	float tminZ=min(t1,t2);
	float tmaxZ=max(t1,t2);
	float largest_tmin = max(max(tminX, tminY), max(tminX, tminZ));
	float smallest_tmax = min(min(tmaxX, tmaxY), min(tmaxX, tmaxZ));
	vec2 result=vec2(largest_tmin,smallest_tmax);
	return result;
}

Can somebody tell me how this function get the near and far .