Leaderboard

I think the error means this part, then. Change from <SlabBlock> to <Block>

Glass is rendered in a different layer than ice by default. It probably works for glass since the pixel is either completely completely opaque or transparent. Ice works differently since it has variable opacity. However, your pixels seem to be only grabbing the pixel colors and not the alpha. Another scenario is that you are grabbing the alpha; however, setRenderLayer was not set. The fix I showed uses the tint index alpha channel to set pixels. It could be that this value must be 1.0f to be multiplied with the current texture alpha channel if the render layer is set. However, this is mainly speculation.

1.0f sets the alpha channel if I'm not mistaken. However, if the block is in the solid render type, it will render through the entire ground. You would need to put it in the correct render layer as well (in this case the ice would be in RenderType#TRANSLUCENT).

I would recommend trying and seeing if it works anyways just for the heck of it. Forge is in beta right now for 1.16, so some issues are to be expected. They're working quite hard to get things figured out, but it may take a little while.