Q:

Dynamic 2d shadows - Blending issue

Good day my dear community.
I'm working on dynamic shadows for a game I shall work on, but as it usually happens I bring you a problem, in hope (I'm certain actually) that someone will help.
This is where I am right now:
Notice the red square, I want it to gradually fade away as the light source moves out of the sight. I do check if a point of a polygon is inside circle's radius, but that of course doesn't solve it; as I said I want it to fade gradually until it completely blacks out If the light is too far away. 
There's one idea on my mind but I hope for a better one. I will not talk about it since it's really the last option and I find it to be a 'brute force' technique. 
This is how I render my light:
    glBegin(GL_TRIANGLE_FAN);
    {
        Graphics::Instance()->SetColor(r_,g_,b_,intensity_);
        glVertex2f(posX_,posY_);

        glColor4f(0.f, 0.f, 0.f, 0.0f);

        for (angle_=0.0; angle_<=3.14159265*2; angle_+=((3.14159265*2)/64.0f) )
        {
            glVertex2f(range_*(float)cos(angle_) + posX_,
                       range_*(float)sin(angle_) + posY_);
        }

        glVertex2f(posX_+range_, posY_);
    }

And this is how I blend it: 
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
l0->Render();

glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
l0->ProjectShadow(*mmm);
l0->ProjectShadow(*bb);

That is all. If I didn't made myself clear or If I missed to post relevant code, please do say so and don't downvote. 

A:

How about calculating the range to the center of your red square from the light sources center? Normalise that value to a suitable range and adjust the transparency or colour of the red square? Something like this:
double Range(double x1, double y1, double x2, double y2)
{
    double xDist = x1-x2;
    double yDist = y1-y2;
    return math::sqrt(xDist*xDist+yDist*yDist);
}

double CalcIntensity(double lightX, double lightY, double lightRadius, double objectX, double objectY)
{
    double range = Range(lightX, lightY, objectX, objectY);
    double intensity;
    if( range > lightRadius )
    {
        intensity = 0.0;
    }
    else
    {
        intensity = range/lightRadius;
    }
    return intensity;
}

Then just call CalcIntensity and feed in the reletive positions of the light and the square and the radius of the light. 
[Edit] ...or this would be a slightly more optomised version if you're not pre-checking it's within the lights radius:
double CalcIntensity(double lightX, double lightY, double lightRadius, double objectX, double objectY)
{
    double intensity = 0.0;
    double xDist = lightX-objectX;
    if( xDist < lightRadius )
    {
        yDist = lightY-objectY;
        if( yDist < lightRadius )
        {
            double range = math::sqrt(xDist*xDist+yDist*yDist);
            intensity = range/lightRadius;
        }
    }

    return intensity;
}

