python - How do I use setFilmSize in panda3d to achieve the correct view?

I'm working with Panda3d and recently switched my game to isometric rendering. I moved the virtual camera accordingly and set an orthographic lens. Then I implemented the classes "Map" and "Canvas".

A canvas is a dynamically generated mesh: a flat quad. I'm using it to render the ingame graphics. Since the game itself is still set in a 3d coordinate system I'm planning to rely on these canvases to draw sprites. I could have named this class "Tile" but as I'd like to use it for non-tile sketches (enemies, environment) as well I thought canvas would describe it's function better.

Map does exactly what it's name suggests. Its constructor receives the number of rows and columns and then creates a standard isometric map. It uses the canvas class for tiles. I'm planning to write a map importer that reads a file to create maps on the fly.

Here's the canvas implementation:

class Canvas:
def __init__(self, texture, vertical=False, width=1,height=1):

    # create the mesh         
    format=GeomVertexFormat.getV3t2()
    format = GeomVertexFormat.registerFormat(format)

    vdata=GeomVertexData("node-vertices", format,  Geom.UHStatic)

    vertex = GeomVertexWriter(vdata, 'vertex')
    texcoord = GeomVertexWriter(vdata, 'texcoord')

    # add the vertices for a flat quad
    vertex.addData3f(1, 0, 0)
    texcoord.addData2f(1, 0)
    vertex.addData3f(1, 1, 0)
    texcoord.addData2f(1, 1)

    vertex.addData3f(0, 1, 0)
    texcoord.addData2f(0, 1)
    vertex.addData3f(0, 0, 0)
    texcoord.addData2f(0, 0)

    prim = GeomTriangles(Geom.UHStatic)
    prim.addVertices(0, 1, 2)
    prim.addVertices(2, 3, 0)

    self.geom = Geom(vdata)

    self.geom.addPrimitive(prim)

    self.node = GeomNode('node')
    self.node.addGeom(self.geom)

    # this is the handle for the canvas
    self.nodePath=NodePath(self.node)

    self.nodePath.setSx(width)
    self.nodePath.setSy(height)


    if vertical:
        self.nodePath.setP(90)

    # the most important part: "Drawing" the image
    self.texture=loader.loadTexture(""+texture+".png")
    self.nodePath.setTexture(self.texture)

Now the code for the Map class

class Map:

def __init__(self,rows,columns,size):
    self.grid=[]
    for i in range(rows):
        self.grid.append([])
        for j in range(columns):

            # create a canvas for the tile. For testing the texture is preset
            tile=Canvas(texture="../assets/textures/flat_concrete",width=size,height=size)
            x=(i-1)*size
            y=(j-1)*size


            # set the tile up for rendering
            tile.nodePath.reparentTo(render)
            tile.nodePath.setX(x)
            tile.nodePath.setY(y)

            # and store it for later access
            self.grid[i].append(tile)

And finally the usage

    def loadMap(self):
        self.map=Map(10, 10, 1)

this function is called within the constructor of the World class. The instantiation of world is the entry point to the execution.

The code is pretty straightforward and runs good. Sadly the output is not as expected:

Please note: The problem is not the white rectangle, it's my player object. The problem is that although the map should have equal width and height it's stretched weirdly. With orthographic rendering I expected the map to be a perfect square.

What did I do wrong ?

UPDATE:

I've changed the viewport. This is how I set up the orthographic camera:

    lens = OrthographicLens()
    lens.setFilmSize(40, 20)
    base.cam.node().setLens(lens)

You can change the "aspect" by modifying the parameters of setFilmSize. I don't know exactly how they are related to window size and screen resolution but after testing a little the values above seem to work for me.

Now everything is rendered correctly as long as I don't resize the window. Every change of the window's size as well as switching to fullscreen destroys the correct rendering. I know that implementing a listener for resize events is not in the scope of this question. However I wonder why I need to make the Film's height two times bigger than its width. My window is quadratic !

Can you tell me how to find out correct setting for the FilmSize ?

UPDATE 2:

I can imagine that it's hard to envision the behaviour of the game. At first glance the obvious solution is to pass the window's width and height in pixels to setFilmSize. There are two problems with that approach.

1. The parameters for setFilmSize are ingame units. You'll get a way to big view if you pass the pixel size


2. For some strange reason the image is distorted if you pass equal values for width and height.

Here's the output for setFilmSize(800,800) You'll have to stress your eyes but you'll see what I mean