From VFXPedia

pCustom [pCu]
	The pCustom tool is used to create custom expressions that affect the properties of particles. This tool is almost identical to the Custom tool, except that the calculations affect properties of the particles rather than the properties of a pixel.

The following particle properties are exposed to the pCustom control :

px, py, pz	particle position on the x, y, and z axis
vx, vy, vz	particle velocity on the x, y and z axis
rx, ry, rz	particle rotation on the x, y, and z axis
sx, sy, sz	particle spin on the x, y, and z axis
pxi1, pyi1	the 2D position of a particle, corrected for image 1's aspect
pxi2, pyi2	the 2D position of a particle, corrected for image 2's aspect
mass	not currently used by anything
size	The current size of a particle
id	(5.2) The particle's identifier
r, g, b, a	The particles Red, Green, Blue and Alpha color values
rgnhit	This value is 1 if the particle hit the pCustom tools defined region
rgndist	This variable contains the particles distance from the region
condscale	The "strength" of the region at the particle's position
rgnix, rgniy, rgniz	Values representing where on the region the particle hit
rgnnx, rgnny, rgnnz	region surface normal of the particle when it hit the region
w1, h1	Image 1 width and height
w2 h2	Image 2 width and height
i1, i2, i3, i4	The result of the intermediate calculations 1 through 4
s1, s2, s3, s4	The result of the setup calculations 1 through 4
n1..n8	The values of numeric inputs 1 through 8
p1x, p1y, p1z .. p4x, p4y, p4z	The values of position inputs 1 through 4
time	The current time or frame of the compositions
age	The current age of the particle
lifespan	The lifespan of the current particle

Additional information on the custom class of tools can be found in documentation for the Custom tool. All of the operators, functions and conditional statements described for that tool apply to pCustom as well, including pixel-read functions for the two image inputs (e.g. getr1w(x,y), getz2b(x,y) etc).

Tips for pCustom (edit)

Soft particle clipping with pCustom (Last Updated: Month 7th, 2006)

Particles just disappear when they intersect the clipping plane of the camera. This causes a visible pop in the image which can be distracting. This tip demonstrates how the pCustom tool can be used to create a soft clip region for the camera, by fading out the particles before they reach the clipping plane.

Particle ID

When emitting one particle per frame you can use the lifespan and age to calculate a unique ID for each particle:

(age*lifespan-time)*-1

To better visualize what the particle ID does, create a pEmitter with the emission rate set to 1 per frame. In a pCustom tool enter id/50 as red expression and if(id=4,0.3,0.1) as size expression. (source: this pigsfly post by Gringo)

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