How to get the color of the texture of the current fragment in glsl?

Question

The problem

I'm trying to make shaders for my game. In the fragment shaders, I don't want to set the color of the fragment but add to it. How do I do so? I'm new to this, so sorry for any mistakes.

The code

varying vec3 vN;
varying vec3 v; 
varying vec4 color;
#define MAX_LIGHTS 1 
void main (void) 
{ 
   vec3 N = normalize(vN);
   vec4 finalColor = vec4(0.0, 0.0, 0.0, 0.0);
   
   for (int i=0;i<MAX_LIGHTS;i++)
   {
      vec3 L = normalize(gl_LightSource[i].position.xyz - v); 
      vec3 E = normalize(-v); // we are in Eye Coordinates, so EyePos is (0,0,0) 
      vec3 R = normalize(-reflect(L,N)); 
   
      vec4 Iamb = gl_LightSource[i].ambient; 
      vec4 Idiff = gl_LightSource[i].diffuse * max(dot(N,L), 0.0);
      Idiff = clamp(Idiff, 0.0, 1.0); 
      vec4 Ispec = gl_LightSource[i].specular * pow(max(dot(R,E),0.0),0.3*gl_FrontMaterial.shininess);
      Ispec = clamp(Ispec, 0.0, 1.0); 
   
      finalColor += Iamb + Idiff + Ispec;
   }
    gl_FragColor = color * finalColor ; 
}

Thanks in advance!

Answer 1

You can not get the color of the fragment you're writing to. In simple scenarios (like yours) what you can do is to enable blending and set the blending functions to achieve additive blending.

For more complex logic you'd create a framebuffer object, attach a texture to it, render your input(e.g. scene) to that texture, then switch to and render with another framebuffer(or the default "screen" one), this way you can sample your scene from the texture and add the lighting on top. Read how to do this in detail on WebGLFundamentals.com