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///////////////////////////////////////////
// Preliminary setup

// Load plugin nearestPointOnMesh (installed with Maya, just disabled by default)

// Set INSTANCER rotation order to YXZ

// add particle attributes:
// rotationPP			vector		map to rotation
// randomRotationY		float
// aimDir			vector		map to aim Direction
// aimUp			vector		map to aim Axis
// customScale			vector		map to Scale
// customID			float		map to objectID


///////////////////////////////////////////
// Particle's creation expression
// !!!!!! remember to replace bellow "particleShape1" and "pPlane1" properly. pPlane1 is emitting surface.

// Set the orient axis for the instance
particleShape1.aimUp = <<0,1,0>>;

// random rotation for Y axis
particleShape1.randomRotationY = rand(360);

//// Orient to normal for grass patches. Most plants you'll want growing according to world up.
// get surface normal at particle position (needs the plugin to be loaded)
vector $p = particleShape1.position;
vector $normal = `nearestPointOnMesh -ip ($p.x) ($p.y) ($p.z) -normal -q pPlane1`;
// orient the instance
particleShape1.aimDir = $normal;

// solve each angle, convert to degrees
float $rotX = rad_to_deg( atan2( ($normal.z), ($normal.y) ) );
float $rotZ = rad_to_deg( asin( -($normal.x) ) );

// assign random rotation in Y
particleShape1.rotationPP = << $rotX, particleShape1.randomRotationY, $rotZ >>;

// random scale if you will, set the values
particleShape1.customScale = rand(1,3);

// number of instance sources to assign (if 4 sources, use max of 3.999)
particleShape1.customID = rand(0, 3.999);