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The Transparency property determines the transparency of all active particles over their individual lifetimes. It works similar to ParticleEmitter/Size in how it affects particles over time. In terms of rendering, it works like the BasePart/Transparency of a part on a scale of 0 to 1, where 0 is completely visible (opaque), and a value of 1 is completely invisible (not rendered at all).

Pictured below are two default ParticleEmitters. The right emitter has a Transparency set to a DataType/NumberSequence that interpolates from 0 to 1.
Two default ParticleEmitters; the right emitter Transparency fades over the lifetime of the particles

A particle’s present transparency is determined by linearly interpolating on this NumberSequence using the particle’s age and the particle’s total lifetime. For example, if a particle spawned 2 seconds ago and has a 4 second lifetime, the transparency will be whatever is 50% of the way through the DataType/NumberSequence. For any DataType/NumberSequenceKeypoint with a nonzero envelop value, a random value in the envelop range is chosen for each keypoint for each particle when it spawns.

Changing this property applies changes to all particles present in the system. This is because the transparency of a particle is determined using its present lifetime and this NumberSequence (the Transparency at the time the particle was emit is not stored on a per-particle basis).

Code Samples

Creating a Particle Emitter from Scratch

This rather lengthy code sample shows how every property of a ParticleEmitter can be set, including DataType/NumberRange, DataType/NumberSequence and DataType/ColorSequence properties. Below is how the ParticleEmitter should after every property is set. Try playing around with the different properties to customize how the effect looks!

The final product