## FCurveKey.LeftTanX

### Description

Sets or returns the left tangent X (in frames) of the fcurve key relative to the FCurveKey.Time value as a Double.
The left tan x value may be a negative value between a distance which is one third the distance between 2 consecutive key and 0; this guarantees that the curve is valid.
The left tangent of the first key and the right tangent of the last key cannot be changed. If you try to set the tangent for one of these the property does nothing and returns S_FALSE in C++.
Tangent values only apply to standard function curves (where FCurve.Type = siStandardFCurve), the tangent value 0 is returned for all other function curve types defined by siFCurveType.

### C# Syntax

    // get accessor
    Object rtn = FCurveKey.LeftTanX;
    // set accessor
    FCurveKey.LeftTanX = Object;

### Examples

#### 1. JScript Example

    /*
            This example illustrates how to create an fcurve and set the tangents on a specific key
    */
    // Create new scene
    NewScene( "", false );
    // Create a null
    var nullobj = Application.ActiveSceneRoot.AddNull();
    // Get the posx parameter from the null
    var posx = nullobj.posx;
    // Create an fcurve on posx
    var fc = posx.AddFCurve2();
    // Define some keys (3 key-value pairs)
    var keyvalues = new Array( 0,0, 50,25, 100,50 );
    fc.SetKeys( keyvalues );
    // Get the 1st, 2nd & 3rd FCurveKey objects
    var key0 = fc.Keys(0);
    var key1 = fc.Keys(1);
    var key2 = fc.Keys(2);
    // Make the key non-G1 continuous so it can have different left and right values
    key1.Constraint( siG1ContinuousConstraint ) = false;
    key1.LeftTanX = 0;
    key1.LeftTanY = Math.abs( key1.Value - key0.Value );
    key1.RightTanX = key2.Time - key1.Time;
    key1.RightTanY = 0;
    PrintKeys( fc );
    // Convenience function to print the FCurveKeys
    function PrintKeys( in_fcrv )
    {
            for ( var i=0; i<in_fcrv.Keys.Count; i++ ) {
                    var fckey = in_fcrv.Keys(i);
                    var str = "key : " + fckey.Index;
                    str += "\n\ttime : " + fckey.Time;
                    str += "\n\tvalue : " + fckey.Value;
                    str += "\n\ttangents : " + fckey.LeftTanX + "," + fckey.LeftTanY + "," + fckey.RightTanX + "," + fckey.RightTanY;
                    str += "\n\tconstraints : " + GetConstraintType( fckey );
                    Application.LogMessage( str );
            }
    }
    // Convenience function to get the user-friendly string describing the contraint type
    function GetConstraintType( in_fcrvkey )
    {
            var ConstraintTypes = new Array (
                    new Array (siParameterConstraint,                       "LockParameter"),
                    new Array (siLeftRightValuesConstraint,                 "LeftRightParameter"),
                    new Array (siG1ContinuousConstraint,                    "G1Continuous"),
                    new Array (siLeftRightTangentDirectionConstraint,       "LeftRightTangentDirection"),
                    new Array (siLeftRightTangentLengthConstraint,          "LeftRightTangentLength"),
                    new Array (siLockConstraint,                            "LockAll"),
                    new Array (siHorizontalTangentConstraint,               "HorizontalTangent"),
                    new Array (siExtremumHorizontalTangentConstraint,       "ExtremumHorizontalTangent"),
                    new Array (siAdjustedTangentConstraint,                 "AdjustedTangent"),
                    new Array (siZeroLengthTangentConstraint,               "ZeroLengthTangent"),
                    new Array (siSameLengthTangentConstraint,               "SameLengthTangent"),
                    new Array (siNeighborTangentConstraint,                 "NeighborTangent"),
                    new Array (siMirrorTangentConstraint,                   "MirrorTangent"),
                    new Array (siAutoPlateauTangentConstraint,              "AutoPlateauTangent")
            )
            var str = "";
            for ( var i=0; i<ConstraintTypes.length; i++ ) {
                    if ( in_fcrvkey.Constraint( ConstraintTypes[i][0] ) ) {
                            str += ConstraintTypes[i][1] + " ";
                    }
            }
            return str;
    }
    // Expected results:
    //INFO : key : 0
    //      time : 0
    //      value : 0
    //      tangents : -16.666666666666668,0,16.666666666666668,0
    //      constraints : G1Continuous
    //INFO : key : 1
    //      time : 50
    //      value : 25
    //      tangents : 0,25,50,0
    //      constraints :
    //INFO : key : 2
    //      time : 100
    //      value : 50
    //      tangents : -16.666666666666664,0,16.666666666666664,0
    //      constraints : G1Continuous

#### 2. VBScript Example

    '
    ' This example illustrates how to create an fcurve and set the tangents on a specific key
    '
    ' Create new scene
    NewScene , false
    ' Create a null
    set nullobj = Application.ActiveSceneRoot.AddNull()
    ' Get the posx parameter from the null
    set posx = nullobj.posx
    ' Create an fcurve on posx
    set fc = posx.AddFCurve2()
    ' Define some keys (3 key-value pairs)
    keyvalues = array( 0,0, 50,25, 100,50 )
    fc.SetKeys keyvalues
    ' Get the 1st, 2nd & 3rd FCurveKey objects
    set key0 = fc.Keys(0)
    set key1 = fc.Keys(1)
    set key2 = fc.Keys(2)
    ' Make the key non-G1 continuous so it can have different left and right values
    key1.Constraint( siG1ContinuousConstraint ) = false
    key1.LeftTanX = 0
    key1.LeftTanY = Abs( key1.Value - key0.Value )
    key1.RightTanX = key2.Time - key1.Time
    key1.RightTanY = 0
    PrintKeys fc
    ' Convenience routine to print the FCurveKeys
    sub PrintKeys( in_fcrv )
            for each fckey in in_fcrv.Keys
                    str = "key : " & fckey.Index
                    str = str & vbCrLf & vbTab & "time : " & fckey.Time
                    str = str & vbCrLf & vbTab & "value : " & fckey.Value
                    str = str & vbCrLf & vbTab & "tangents : " & fckey.LeftTanX & "," & fckey.LeftTanY & "," & fckey.RightTanX & "," & fckey.RightTanY
                    str = str & vbCrLf & vbTab & "constraints : " & GetConstraintType( fckey )
                    Application.LogMessage str
            next
    end sub
    ' Convenience function to get the user-friendly string describing the contraint type
    function GetConstraintType( in_fcrvkey )
            set ConstraintTypes = CreateObject( "Scripting.Dictionary" )
            ConstraintTypes.Add siParameterConstraint,                      "LockParameter"
            ConstraintTypes.Add siLeftRightValuesConstraint,                "LeftRightParameter"
            ConstraintTypes.Add siG1ContinuousConstraint,                   "G1Continuous"
            ConstraintTypes.Add siLeftRightTangentDirectionConstraint,      "LeftRightTangentDirection"
            ConstraintTypes.Add siLeftRightTangentLengthConstraint,         "LeftRightTangentLength"
            ConstraintTypes.Add siLockConstraint,                           "LockAll"
            ConstraintTypes.Add siHorizontalTangentConstraint,              "HorizontalTangent"
            ConstraintTypes.Add siExtremumHorizontalTangentConstraint,      "ExtremumHorizontalTangent"
            ConstraintTypes.Add siAdjustedTangentConstraint,                "AdjustedTangent"
            ConstraintTypes.Add siZeroLengthTangentConstraint,              "ZeroLengthTangent"
            ConstraintTypes.Add siSameLengthTangentConstraint,              "SameLengthTangent"
            ConstraintTypes.Add siNeighborTangentConstraint,                "NeighborTangent"
            ConstraintTypes.Add siMirrorTangentConstraint,                  "MirrorTangent"
            ConstraintTypes.Add siAutoPlateauTangentConstraint,             "AutoPlateauTangent"
            str = ""
            for each k in ConstraintTypes.Keys
                    if in_fcrvkey.Constraint( k ) then
                            str = str & ConstraintTypes(k) & " "
                    end if
            next
            GetConstraintType = str
    end function
    ' Expected results:
    'INFO : key : 0
    '       time : 0
    '       value : 0
    '       tangents : -16.6666666666667,0,16.6666666666667,0
    '       constraints : G1Continuous
    'INFO : key : 1
    '       time : 50
    '       value : 25
    '       tangents : 0,25,50,0
    '       constraints :
    'INFO : key : 2
    '       time : 100
    '       value : 50
    '       tangents : -16.6666666666667,0,16.6666666666667,0
    '       constraints : G1Continuous

#### 3. Python Example

    #
    # This example illustrates how to create an fcurve and set the tangents on a specific key
    #
    from win32com.client import constants as c
    # Convenience function to get the user-friendly string describing the contraint type
    def GetConstraintType( in_fcrvkey ) :
            ConstraintTypes = {
                    c.siParameterConstraint:"LockParameter",
                    c.siLeftRightValuesConstraint:"LeftRightParameter",
                    c.siG1ContinuousConstraint:"G1Continuous",
                    c.siLeftRightTangentDirectionConstraint:"LeftRightTangentDirection",
                    c.siLeftRightTangentLengthConstraint:"LeftRightTangentLength",
                    c.siLockConstraint:"LockAll",
                    c.siHorizontalTangentConstraint:"HorizontalTangent",
                    c.siExtremumHorizontalTangentConstraint:"ExtremumHorizontalTangent",
                    c.siAdjustedTangentConstraint:"AdjustedTangent",
                    c.siZeroLengthTangentConstraint:"ZeroLengthTangent",
                    c.siSameLengthTangentConstraint:"SameLengthTangent",
                    c.siNeighborTangentConstraint:"NeighborTangent",
                    c.siMirrorTangentConstraint:"MirrorTangent",
                    c.siAutoPlateauTangentConstraint:"AutoPlateauTangent"
            }
            str = ""
            for k in ConstraintTypes.keys() :
                    if ( in_fcrvkey.Constraint(k) ) :
                            str += ConstraintTypes[k] + " "
            return str
    # Convenience function to print the FCurveKeys
    def PrintKeys( in_fcrv ) :
            for fckey in in_fcrv.Keys :
                    str = "key : %d" % (fckey.Index)
                    str = str + "\n\ttime : %d" % (fckey.Time)
                    str = str + "\n\tvalue : %d" % (fckey.Value)
                    str = str + "\n\ttangents : %d,%d,%d,%d" % (fckey.LeftTanX,fckey.LeftTanY,fckey.RightTanX,fckey.RightTanY)
                    str = str + "\n\tconstraints : " + GetConstraintType( fckey )
                    Application.LogMessage( str )
            return None
    # Create new scene
    Application.NewScene( "", 0 )
    # Create a null
    nullobj = Application.ActiveSceneRoot.AddNull()
    # Get the posx parameter from the null
    posx = nullobj.posx
    # Create an fcurve on posx
    fc = posx.AddFCurve2()
    # Define some keys (3 key-value pairs)
    keyvalues = [ 0,0, 50,25, 100,50 ]
    fc.SetKeys( keyvalues )
    # Get the 1st, 2nd & 3rd FCurveKey objects
    key0 = fc.Keys(0)
    key1 = fc.Keys(1)
    key2 = fc.Keys(2)
    # Make the key non-G1 continuous so it can have different left and right values
    # (Python can't use the Constraint property as-is, so use SetConstraint as an alias)
    key1.SetConstraint( c.siG1ContinuousConstraint, 0 )
    key1.LeftTanX = 0
    key1.LeftTanY = abs( key1.Value - key0.Value )
    key1.RightTanX = key2.Time - key1.Time
    key1.RightTanY = 0
    PrintKeys( fc )
    # Expected results:
    # INFO : key : 0
    #       time : 0
    #       value : 0
    #       tangents : -16,0,16,0
    #       constraints : G1Continuous
    # INFO : key : 1
    #       time : 50
    #       value : 25
    #       tangents : 0,25,50,0
    #       constraints :
    # INFO : key : 2
    #       time : 100
    #       value : 50
    #       tangents : -16,0,16,0
    #       constraints : G1Continuous

### See Also

FCurve.SetKeyTangents FCurveKey.LeftTanY FCurveKey.RightTanY FCurveKey.RightTanX