如何在OpenGL中设置顶点的不透明度？

in vec4 a_vertexColor;
in float a_opacity;

out vec4 outColor;

outColor = vec4(a_vertexColor.rgb, a_vertexColor.a * a_opacity);

glVertexAttribPointer(a_vertexColor, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, color));
glVertexAttribPointer(a_opacity, 1, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, opacity));

#version 330 core
layout (location = 0) in vec3 a_position;
layout (location = 1) in vec4 a_vertexColor;
layout (location = 2) in float a_opacity;

out vec4 v_vertexColor;
out float v_opacity;

uniform mat4 u_modelViewProjection;

void main()
{
    gl_Position = u_modelViewProjection * vec4(a_position, 1.0);
    v_vertexColor = a_vertexColor;
    v_opacity = a_opacity;
}

#version 330 core
in vec4 v_vertexColor;
in float v_opacity;

out vec4 outColor;

void main()
{
    outColor = vec4(v_vertexColor.rgb, v_vertexColor.a * v_opacity);
}