分析一下这段代码： // texture2d to texture #if __version__ >= 300 // version 300 code #ifdef msaa_framebuffer_enabled #define _centroid centroid #else #define _centroid #endif _centroid in vec2 uv; #define varying in #define texture2d texture out vec4 fragcolor; #define gl_fragcolor fragcolor #else // version 100 code varying vec2 uv; #endif uniform vec4 current_color; uniform sampler2d texture_0; void main() { vec4 diffuse = texture2d( texture_0, uv ); #ifdef alpha_test if(diffuse.a < 0.5)