Can morphtargets be used additively ?

[teub]

Hello,

I quickly looked at the samples and documentations and I'm not sure if I can use morphtargets for additive animation.
Can blend shapes be used additively ? Would it require a custom shader ?

I've seen another discussion post where someone implemented blendshapes from scratch without filament's morphtargets and I'm not sure which way I should go about it.

Any help to get me started on the subject would be appreciated !