语法：

surface_get_target_ext(index);

参数	描述
index	The render target index to check (from 0 to 3).

返回：

Integer (-1 for no surface target, or a surface ID value)

描述

This function will retrieve the surface ID assigned to one of the 4 render targets available to surfaces. You supply the index of the render target to check, and the function will return -1 if no surface is assigned, or an integer value >= 0, representing the ID of the surface assigned (as returned by the function surface_create()).

例如：

if surface_get_target_ext(0) == -1
    {
    surface_set_target_ext(0, global.Surf);
    }

The above code will first check and see if the shader render target 0 has been set to a surface, and if not, then one is assigned.