[away3d] Scaling problem when replacing faces on mesh

[JamesK]

I'm having trouble when trying to create BitmapMaterials from scaled
bitmap images.

The source images are of different sizes and scales. I'm trying to map
them onto a 400*300 mesh, adding empty space to the sides of the image
when the scales differ, so for example an overwide image would be
scaled down to have width of 400 and then blank space at the top and
bottom of the image.

I have a function which allows me to create a mesh and add two faces
using the material which works perfectly.

Later, when I use a near identical function to remove the faces and
replace them with new faces using a different image, the scale is
wrong. The faces end up displaying the zoomed center of the source
image.

Can anybody tell me where I'm going wrong here?

Here's the code I'm using:

//WIDTH = 400 , HEIGHT = 300

var loadedBitmap:Bitmap = event.currentTarget.content as Bitmap;

var newBitmapData:BitmapData =  new BitmapData( WIDTH, HEIGHT, true,
0xff );
var targetRect : Rectangle =  new Rectangle( 0, 0, loadedBitmap.width,
loadedBitmap.height );

var startX : int = ( WIDTH - loadedBitmap.width ) / 2;
var startY : int = ( HEIGHT - loadedBitmap.height ) / 2;
var startPoint : Point = new Point( startX, startY );

newBitmapData.copyPixels( loadedBitmap.bitmapData, targetRect,
startPoint );

var artMaterial:BitmapMaterial = new BitmapMaterial(newBitmapData,
{smooth:true, debug:false});

var uva:UV = new UV(1, 1);
var uvb:UV = new UV(0, 1);
var uvc:UV = new UV(0, 0);
var uvd:UV = new UV(1, 0);

var v000a:Vertex = new Vertex(- WIDTH /2, 0, 0);
var v010a:Vertex = new Vertex(- WIDTH / 2, +HEIGHT, 0);
var v100a:Vertex = new Vertex(+ WIDTH/2, 0, 0);
var v110a:Vertex = new Vertex(+ WIDTH/2, +HEIGHT, 0);

//only doing this when replacing the faces
for ( var i : int = _shape.faces.length-1; i >= 0; i-- )
{
        var currentFace : Face = _shape.faces[i];
        _shape.removeFace( currentFace );
}

_shape.addFace(new Face(v000a, v100a, v010a, artMaterial, uvc, uvd,
uvb));
_shape.addFace(new Face(v100a, v110a, v010a, artMaterial, uvd, uva,
uvb));

_shape.movePivot(0, HEIGHT/2, 0);