The reason the block is shaded like that is because the block is an opaque block, this problem won't occur if you use a transparent block.
There's no way to stop this from happening on opaque blocks.
As for multi-part models, yes, you can separate several conditions with commas, all of the conditions will need to be met by the block in order for the new model to be applied:
Is there a way to combat shading from a block placed above another one (or sides or bottom).
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Also, what can I do as far as the "multipart" in blockstates in concerned. Can you list several conditions, and all of them need to be met?
The reason the block is shaded like that is because the block is an opaque block, this problem won't occur if you use a transparent block.
There's no way to stop this from happening on opaque blocks.
As for multi-part models, yes, you can separate several conditions with commas, all of the conditions will need to be met by the block in order for the new model to be applied:
I do tutorials all about values encountered within block modelling on my website