Many of the emitter node attributes are the same as the options available in the Emitter Options window. See Particles > Create Emitter.
Select from the following options:
When Emit From Dark is off, fully luminous parts of the texture emit at 100% of the specified emitter rate. Parts of the texture that lack luminance don't emit at all. Other parts emit between 0 and 100% in proportion to the luminance. In simple terms, bright colors emit more than dark colors.
When Emit From Dark is turned on, the opposite luminance scheme applies. Parts of the texture that lack luminance emit at 100% of the specified rate, while fully luminous parts don't emit at all.
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