env_spritetrail
env_spritetrail is a point entity available in all 
Source games.
It frequently emits a sprite, intended to create a trail effect when moving.
In code, it is represented by theCSpriteTrailclass, defined in theSpriteTrail.cppfile.
Todo: Add applicable properties from env_sprite. Not all work, such as scale, and the alpha input appears to be a boolean, rendering it invisible for values less than 1.
Keyvalues
- Lifetime
(lifetime)<float> - How long the beam is shown.
- Start Width
(startwidth)<float> - The width of the beam to the beginning.
- End Width
(endwidth)<float> - The width of the beam when it has full expanded.
- Sprite Name
(spritename)<string> - The material to draw.
- Animated
(animate)<boolean> !FGD 
Confirm:Supposed to tell the sprite to animate if it can, however only framerate appears to control this.
RenderFields:
- Render Mode
(rendermode)<choices> - Set a non-standard rendering mode on this entity.
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- Render FX
(renderfx)<choices> - Various somewhat legacy alpha effects. See render effects
- Render Amount / Transparency
(renderamt)<integer 0–255> - Transparency amount, requires a Render Mode other than Normal. 0 is invisible, 255 is fully visible.
- Render Color (R G B)
(rendercolor)<color255> - Color tint.
- Disable Receiving Shadows
(disablereceiveshadows)<boolean> - Prevent the entity from receiving shadows on itself.
- Name
(targetname)<string> - The name that other entities refer to this entity by, via Inputs/Outputs or other keyvalues (e.g.
parentnameortarget).
Also displayed in Hammer's 2D views and Entity Report.See also: Generic Keyvalues, Inputs and Outputs available to all entities
Inputs
RenderFields:
Alpha<integer 0–255>- Sets the entity's transparency to a number from 0 (invisible) to 255 (fully visible). Requires the entity to have its
rendermodeset to a number other than0.
Color<color255>- Sets an RGB color for the entity.
