shader for opacity of both layer

[AndrewChan2022]

what is shader for base.a < 1 and overlay.a < 1

[felixturner]

This works for transparent images:

float blendOpac = blendLayerOpacity * blend.a;
vec4 outCol;
outCol.rgb = blendScreen(base.rgb, blend.rgb, blendOpac); // works for any blendMode
outCol.a =  blendOpac + base.a;

[tgrajewski]

Any tips how to use it when both base and blend colors come from textures with premultiplied alpha?