Layer Transformations

Hello !

Because i can see several experts of ActionDescriptor() on this forum, i guess that my unanswered question is not very clear !

So i will try to do better (sorry for my english).

I am looking for a script function that can do a "perspective" transformation of a rectangle in a layer.

The following code is given by the scriptlistener for this action: In this example, only the bottom edge of the rectangle is resized at about 50%.

(I just add comments and remove duplicated definitions).

    var idTrnf = charIDToTypeID( "Trnf" );
    var idnull = charIDToTypeID( "null" );
    var idHrzn = charIDToTypeID( "Hrzn" );
    var idVrtc = charIDToTypeID( "Vrtc" );
    var idRlt = charIDToTypeID( "#Rlt" );
    var idPrc = charIDToTypeID( "#Prc" );
   
    var desc3 = new ActionDescriptor();
    var ref2 = new ActionReference();
    var idLyr = charIDToTypeID( "Lyr " );
    var idOrdn = charIDToTypeID( "Ordn" );
    var idTrgt = charIDToTypeID( "Trgt" );
    ref2.putEnumerated( idLyr, idOrdn, idTrgt );
    desc3.putReference( idnull, ref2 );
    var idFTcs = charIDToTypeID( "FTcs" );
    var idQCSt = charIDToTypeID( "QCSt" );
    var idQcsa = charIDToTypeID( "Qcsa" );
    desc3.putEnumerated( idFTcs, idQCSt, idQcsa );
   
    var desc4 = new ActionDescriptor();
    desc4.putUnitDouble( idHrzn, idRlt, 0.000000 ); // Translate Horiz
    desc4.putUnitDouble( idVrtc, idRlt, 17.171411 ); // Translate Verti
    var idOfst = charIDToTypeID( "Ofst" );
    desc3.putObject( idOfst, idOfst, desc4 );
    var idWdth = charIDToTypeID( "Wdth" );
    desc3.putUnitDouble( idWdth, idPrc, 73.076923 ); // Resize horiz (%)
   
    var desc5 = new ActionDescriptor();
    desc5.putUnitDouble( idHrzn, idPrc, -0.000000 ); // Rectangle->Trapezium  Horiz
    desc5.putUnitDouble( idVrtc, idPrc, 0.422127 ); // Rectangle->Trapezium Verti
    var idPnt = charIDToTypeID( "Pnt " );
    var idUsng = charIDToTypeID( "Usng" );
    desc3.putObject( idUsng, idPnt, desc5 );
    executeAction( idTrnf, desc3, DialogModes.NO );

One can see that the parameter values of this transformation are not very easy to understand.

The main part (that is the transformation rectangle->trapezium) is done at the end with only one value here : 0.422127.

The other values in that script are only used to correct the result, that needs to be resized  at the same size (top edge) and translated at the same position. 

The image below shows how this mysterious transformation (without resizing nor translating corrections) modifies a black rectangle.

Does somebody can explain what is the geometric calculation that produces such a result with a single value ?

This knowledge seems to be required to compute the correction factors.

Thanks for your answers.