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Texture Luma Key is a very useful node designed to mask pixels based on a given threshold. In practice, it is used to create a composite based on the luminance levels of the incoming texture.
To use the node, you first have to insert it in the Layer Layouts. You can find it in the dedicated Search bar, under Utilities -> Texture collapsible.
Give the Texture Luma node a Texture input and connect it to an active lifeline.
All pixels from the incoming Texture whose brightness is above the Luminance value will be masked. This node is often used in combination with other Texture nodes, such as the Texture Blend node.