[]请问为什么我在ps内设置gl_FragColor的Alpha值，并没有效果呢

        var ps: string ="#ifdef FSHIGHPRECISION\nprecision highp float;\n" +

        "#else\nprecision mediump float;\n" + 

        "#endif\n\n" + 

        "#include \"LightHelper.glsl\";\n\n" +

            "uniform sampler2D u_texture;\n" +

            "varying vec2 v_Texcoord0;\n" +

            "void main(){\n" +

            "gl_FragColor=vec4(0.0,1.0,1.0,1.0);\n" +

            "}";