Calculate the dot product between the RGB and the vector (300, 300, 300) and clamp to 0 … 1.
This will make black transparent, but all other colors (including very dark gray) opaque.
If you want a more feathered edge, reduce the values to (30,30,30).
If you want dark-gray to also be transparent, you can subtract some small amount from the value before you clamp to 0 … 1.
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