// ===========================================================================
// Copyright 2018 Autodesk, Inc. All rights reserved.
//
// Use of this software is subject to the terms of the Autodesk license
// agreement provided at the time of installation or download, or which
// otherwise accompanies this software in either electronic or hard copy form.
// ===========================================================================
//
// leafDroop
//
// this is a paint effect runtime mel function
// It applies a negative y force( gravity ) to leaves 

global proc float []
leafDroop(

// current segment count on base brushstroke
int	  $step,
// this next variable is true if this is the first segment created 
int   $isStartTube,

// The following 3 vectors define a coordinate system
// for the brushstroke, in a similar fashion to a matrix
// One can use them to create forces and offsets that
// are in the "brushstroke space"
// Unlike a normal coordinate frame, these vectors change
// along the brushstroke, and will mirror any deformations to
// the base brushstroke( or curve on surface ).

// vector in direction of brushstroke 
float $vX, float $vY, float $vZ,
// surface normal at current brushstroke segment
float $nX, float $nY, float $nZ,
// vector across direction of brushstroke ( crossProduct of v and n above )
float $uX, float $uY, float $uZ,

// a random seed that is unique for this tube 
int   $randSeed,

// the growth section: 0 branch, 1 twig, 2 leaf, 3 flower
int	  $tubeType,	

int	  $maxSegments,		// the number of segments the longest tubes have	
int	  $segments,		// the number of segments this tube has	
int	  $remainingSegments, // the number of segments yet to be created	
int	  $branchDepth,		// The branching level this segment is on( 0 is the base )
int   $branchId,        // Index for the segment, each segment on each level is unique (0..n)
int   $parentId,        // BranchID for the parent of this segment
int   $siblingCnt,      // The number of sibling segments at this level, n-1

// the start and end points of the current tube segment in worldspace
float $sX, float $sY, float $sZ,	     
float $eX, float $eY, float $eZ,	     

// the current length per segment
float $segmentLength,

// the start and end widths of the current tube segment in worldspace
float $startWidth, float $endWidth,

float $flatness, // current flatness of tube

// current twist  of segment about tube axis
float $twistX, float $twistY, float $twistZ,

// color, transparency, and incandescence of current tube segment
float $colorR, float $colorG, float $colorB,
float $transparencyR, float $transparencyG, float $transparencyB,
float $incandescenceR, float $incandescenceG, float $incandescenceB,

// name of brush and stroke
string $brushName, string $strokeName
)
{
	float $tubes[];

//------------------------------------------------------------
// Add any Customizations Here:
//------------------------------------------------------------
	if( $tubeType == 2 ) // if is a leaf segment
	{
		float $droopAmount = 1.0;
		// move the segment end down relative to the segment length
		$eY -= $segmentLength * $droopAmount; 
	}
	else
	{
		// else do nothing
		$tubes[0] = 0;
		return( $tubes );
	}
//------------------------------------------------------------

	// We set the outputs for the current segment.
	// Note that all these values must be set here, not
	// only the modified ones.
    $tubes[0] = 1; // 1 indicates that we have modified the segment 
	$tubes[1] = $tubeType; 
	$tubes[2] = $segments; 
	$tubes[3] = $remainingSegments; // set this to zero to terminate a tube
	$tubes[4] = $sX;
	$tubes[5] = $sY;
	$tubes[6] = $sZ;
	$tubes[7] = $eX;
	$tubes[8] = $eY;
	$tubes[9] = $eZ;
	$tubes[10] = $endWidth;
	$tubes[11] = $segmentLength;
	$tubes[12] = $flatness;
	$tubes[13] = $twistX;
	$tubes[14] = $twistY;
	$tubes[15] = $twistZ;
	$tubes[16] = $colorR;
	$tubes[17] = $colorG;
	$tubes[18] = $colorB;
	$tubes[19] = $transparencyR;
	$tubes[20] = $transparencyG;
	$tubes[21] = $transparencyB;
	$tubes[22] = $incandescenceR;
	$tubes[23] = $incandescenceG;
	$tubes[24] = $incandescenceB;

 	$tubes[25] = 0;
 	return( $tubes );
}