How to make my puddles flat?

Development Rendering
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1

Hello!
I’ve began to add puddles to my ground based on the heighmap.
It works but not relly as intended : here are 2 screenshots with different puddle level (puddles are just blue for the moment I’m just trying to place it)


I think puddles should be flat that is the problem

I’ve tried something with vertex normal instead of heightmap but I’m not sure if it’s the good direction and not sure how to use it, so any help or advice would be very nice !

2

I’ve finally made a mask with vertex normals for puddles not being on slopes, and transformed normals of puddle to 0.0.1 (from tangent to ws)