c++ - Dynamic 2d shadows - Blending issue -

good day dear community.

i'm working on dynamic shadows game shall work on, happens bring problem, in hope (i'm actually) help.

this right now:

notice red square, want gradually fade away light source moves out of sight. check if point of polygon inside circle's radius, of course doesn't solve it; said want fade gradually until blacks out if light far away.

there's 1 idea on mind hope better one. not talk since it's last option , find 'brute force' technique.

this how render light:

    glbegin(gl_triangle_fan);     {         graphics::instance()->setcolor(r_,g_,b_,intensity_);         glvertex2f(posx_,posy_);          glcolor4f(0.f, 0.f, 0.f, 0.0f);          (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 how 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 all. if didn't made myself clear or if missed post relevant code, please , don't downvote.

how calculating range center of red square light sources center? normalise value suitable range , adjust transparency or colour of red square? 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 call calcintensity , feed in reletive positions of light , square , radius of light.

[edit] ...or more optomised version if you're not pre-checking it's within 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; }