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


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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.

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