Fragment shader gl_FragColor.a doesn't work in plugin.

[0x8888]

I've made a demo on my Github page, and it is almost copied and built from the plugin example.
The problem is when I modify the fragment shader into a simple example: 

varying vec2 vTextureCoord;

varying vec4 vColor;

 

uniform sampler2D uSampler;

uniform vec4 uTextureClamp;

uniform vec4 uColor;

 

void main(void)

{

    gl_FragColor = vec4(1.0, 1.0, 0.0, 0.0);

}

The expected effect is the sprite becoming transparent, however the actual result is the sprite become yellow.

And I tried to modify the gl_FragColor.a for several values between 0.0 and 1.0, none of these value works as expected.
Can anyone help to solve this problem. 

Note: I can change the alpha of the sprite for this simple demo (or even discard for every fragment), but some effects requires some pixel‘s alpha value being calculated, so I need to find out how to change the alpha value of a pixel.

Update: I guess the problem is premultipliedAlpha, adding the code below before drawElements and set base.premultipliedAlpha false will work. however I need to know if this has some side-effect.

this.renderer.state.setBlendMode(PIXI.utils.premultiplyBlendMode[Number(base.premultipliedAlpha)][sprite.blendMode]);

[ivan.popelyshev]

All colors are premultiplied. "gl_FragColor = vec4(a,a,a,a)" should work. Its required for linear filtering.

We use non-premuiltiplied alpha only in compressed-textures plugin. 
 

Basically, instead of (R,G,B,a) textures and output stores (Ra,Ga,Ba,a)