GLSL shader example
Metaballs with a Smooth SDF Union
The left half uses a hard union and the right half a smooth union. Blend width changes the neck geometry rather than blurring final color.
Live example
Edit the code and the preview updates as you type.
What to observe
Metaballs with a Smooth SDF Union.
Mechanism
The left half uses a hard union and the right half a smooth union. Blend width changes the neck geometry rather than blurring final color.
The controlling code
float a=length(p-vec2(-.3,.1*sin(u_time)))-.28,b=length(p-vec2(.3,-.1*sin(u_time)))-.28;float d=p.x<0.?min(a,b):smoothUnion(a,b,.05+.55*u_parameter);c=mix(c,vec3(.2,.8,.95),1.-smoothstep(-aa,aa,d));
Experiment
Set the smooth-union width .05+.55*u_parameter to .1, then .5. The neck between the right-hand droplets grows at the larger value, while the hard union on the left does not change.
Debugging and cost
Two circle distances are combined before one antialiased color mix, so this is a geometry-field blend. Replace smoothUnion(a,b,.05+.55*u_parameter) with min(a,b) to make both halves use the same hard boundary and remove the smooth bridge.