c++ - 2D Ambient lighting in OpenGL -

im making 2d side scroller game , implementing lights. lights light gradient texture rendered on top of terrain multiplied make brighten area. however, dont know how nor understand how ambient lighting. following picture sums have , bottom part want.

i open answers regarding shaders know how use them.

i ended creating fbo texture size of screen, clearing color of ambience , drawing in nearby lights. then, passed through shader made takes in 2 textures uniforms. texture draw , light fbo itself. shader multiplies textures being drawn fbo , came out nicely.

ambience.frag

uniform sampler2d texture1; uniform sampler2d texture2; varying vec2 texcoord;  void main( void ) {     vec4 color1 = vec4(texture2d(texture1, gl_texcoord[0].st));     vec4 color2 = vec4(texture2d(texture2, texcoord));     gl_fragcolor = color1*vec4(color2.r,color2.g,color2.b,1.0); } 

ambience.vs

varying vec2 texcoord; uniform vec2 screen; uniform vec2 camera;  void main(){     gl_position = ftransform();     gl_texcoord[0] = gl_multitexcoord0;     vec2 temp = vec2(gl_vertex.x,gl_vertex.y)-camera;     texcoord = temp/screen; }