def downLOD(context, ratio):
    scene = bpy.context.scene
    bpy.ops.object.mode_set(mode='OBJECT')

    bpy.context.scene.objects.active = None

    model = None
    for obj in scene.objects:
        obj.select = False
        if obj.name != "Camera" and obj.name != "Lamp":
            if model != None:
                raise Exception("model already found")
            model = obj

    scene.objects.active = model
    model.select = True

    bpy.ops.object.mode_set(mode='EDIT')
    bpy.ops.mesh.select_all(action='SELECT')
    bpy.ops.mesh.quads_convert_to_tris()
    bpy.context.scene.update()
    bpy.ops.object.mode_set(mode='OBJECT')

    bpy.ops.object.modifier_add(type='DECIMATE')
    bpy.context.object.modifiers['Decimate'].ratio=ratio
    bpy.ops.object.modifier_apply(modifier="Decimate")