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The Alpha channel, which takes into account the mask provided through the mask input.

outNumChannels

The number of channels output by this layer. This is the number of textures defined in channelsFilenames. This should always be connected to PxrProjectionStack's layersNumChannels parameter so it may unpack the channel data.

outChannelsRGB

The RGB channel data, packed in a single struct output. PxrProjectionStack needs to know will get the number of available channels through a connection with outNumChannels to be able to unpack the datalayers from the struct's numChannels member.