TDA4VM: [opengl] alpha is invalid

hello:

I have a question about alpha channel.

usually, when i don't want to show something, i can set alpha channel to 0, to make it transparent.

but when i do this on tda4, it has black shadow,

why is this? and how can i do to resolved it?

the code in shader (only circle part):    gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);

the effect:

it's ok on my pc:

thanks!