io_export_md3.py

# ##### BEGIN GPL LICENSE BLOCK #####
#
#  This program is free software; you can redistribute it and/or
#  modify it under the terms of the GNU General Public License
#  as published by the Free Software Foundation; either version 2
#  of the License, or (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#  You should have received a copy of the GNU General Public License
#  along with this program; if not, write to the Free Software Foundation,
#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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# ##### END GPL LICENSE BLOCK #####

bl_info = {
    "name": "Quake Model 3 (.md3)",
    "author": "Xembie",
    "version": (0, 7),
    "blender": (2, 53, 0),
    "location": "File > Export",
    "description": "Save a Quake Model 3 File)",
    "warning": "", # used for warning icon and text in addons panel
    "wiki_url": "",
    "tracker_url": "https://developer.blender.org/maniphest/task/edit/form/2/",
    "category": "Import-Export"}


import bpy,struct,math,os

MAX_QPATH = 64

MD3_IDENT = "IDP3"
MD3_VERSION = 15
MD3_MAX_TAGS = 16
MD3_MAX_SURFACES = 32
MD3_MAX_FRAMES = 1024
MD3_MAX_SHADERS = 256
MD3_MAX_VERTICES = 4096
MD3_MAX_TRIANGLES = 8192
MD3_XYZ_SCALE = 64.0

class md3Vert:
    xyz = []
    normal = 0
    binaryFormat = "<3hH"

    def __init__(self):
        self.xyz = [0.0, 0.0, 0.0]
        self.normal = 0

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    # copied from PhaethonH <phaethon@linux.ucla.edu> md3.py
    def Decode(self, latlng):
        lat = (latlng >> 8) & 0xFF;
        lng = (latlng) & 0xFF;
        lat *= math.pi / 128;
        lng *= math.pi / 128;
        x = math.cos(lat) * math.sin(lng)
        y = math.sin(lat) * math.sin(lng)
        z =                 math.cos(lng)
        retval = [ x, y, z ]
        return retval

    # copied from PhaethonH <phaethon@linux.ucla.edu> md3.py
    def Encode(self, normal):
        x = normal[0]
        y = normal[1]
        z = normal[2]
        # normalize
        l = math.sqrt((x*x) + (y*y) + (z*z))
        if l == 0:
            return 0
        x = x/l
        y = y/l
        z = z/l

        if (x == 0.0) & (y == 0.0) :
            if z > 0.0:
                return 0
            else:
                return (128 << 8)

        lng = math.acos(z) * 255 / (2 * math.pi)
        lat = math.atan2(y, x) * 255 / (2 * math.pi)
        retval = ((int(lat) & 0xFF) << 8) | (int(lng) & 0xFF)
        return retval

    def Save(self, file):
        tmpData = [0] * 4
        tmpData[0] = int(self.xyz[0] * MD3_XYZ_SCALE)
        tmpData[1] = int(self.xyz[1] * MD3_XYZ_SCALE)
        tmpData[2] = int(self.xyz[2] * MD3_XYZ_SCALE)
        tmpData[3] = self.normal
        data = struct.pack(self.binaryFormat, tmpData[0], tmpData[1], tmpData[2], tmpData[3])
        file.write(data)

class md3TexCoord:
    u = 0.0
    v = 0.0

    binaryFormat = "<2f"

    def __init__(self):
        self.u = 0.0
        self.v = 0.0

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    def Save(self, file):
        tmpData = [0] * 2
        tmpData[0] = self.u
        tmpData[1] = 1.0 - self.v
        data = struct.pack(self.binaryFormat, tmpData[0], tmpData[1])
        file.write(data)

class md3Triangle:
    indexes = []

    binaryFormat = "<3i"

    def __init__(self):
        self.indexes = [ 0, 0, 0 ]

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    def Save(self, file):
        tmpData = [0] * 3
        tmpData[0] = self.indexes[0]
        tmpData[1] = self.indexes[2] # reverse
        tmpData[2] = self.indexes[1] # reverse
        data = struct.pack(self.binaryFormat,tmpData[0], tmpData[1], tmpData[2])
        file.write(data)

class md3Shader:
    name = ""
    index = 0

    binaryFormat = "<%dsi" % MAX_QPATH

    def __init__(self):
        self.name = ""
        self.index = 0

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    def Save(self, file):
        tmpData = [0] * 2
        tmpData[0] = self.name
        tmpData[1] = self.index
        data = struct.pack(self.binaryFormat, tmpData[0], tmpData[1])
        file.write(data)

class md3Surface:
    ident = ""
    name = ""
    flags = 0
    numFrames = 0
    numShaders = 0
    numVerts = 0
    numTriangles = 0
    ofsTriangles = 0
    ofsShaders = 0
    ofsUV = 0
    ofsVerts = 0
    ofsEnd = 0
    shaders = []
    triangles = []
    uv = []
    verts = []

    binaryFormat = "<4s%ds10i" % MAX_QPATH  # 1 int, name, then 10 ints

    def __init__(self):
        self.ident = ""
        self.name = ""
        self.flags = 0
        self.numFrames = 0
        self.numShaders = 0
        self.numVerts = 0
        self.numTriangles = 0
        self.ofsTriangles = 0
        self.ofsShaders = 0
        self.ofsUV = 0
        self.ofsVerts = 0
        self.ofsEnd
        self.shaders = []
        self.triangles = []
        self.uv = []
        self.verts = []

    def GetSize(self):
        sz = struct.calcsize(self.binaryFormat)
        self.ofsTriangles = sz
        for t in self.triangles:
            sz += t.GetSize()
        self.ofsShaders = sz
        for s in self.shaders:
            sz += s.GetSize()
        self.ofsUV = sz
        for u in self.uv:
            sz += u.GetSize()
        self.ofsVerts = sz
        for v in self.verts:
            sz += v.GetSize()
        self.ofsEnd = sz
        return self.ofsEnd

    def Save(self, file):
        self.GetSize()
        tmpData = [0] * 12
        tmpData[0] = self.ident
        tmpData[1] = self.name
        tmpData[2] = self.flags
        tmpData[3] = self.numFrames
        tmpData[4] = self.numShaders
        tmpData[5] = self.numVerts
        tmpData[6] = self.numTriangles
        tmpData[7] = self.ofsTriangles
        tmpData[8] = self.ofsShaders
        tmpData[9] = self.ofsUV
        tmpData[10] = self.ofsVerts
        tmpData[11] = self.ofsEnd
        data = struct.pack(self.binaryFormat, tmpData[0],tmpData[1],tmpData[2],tmpData[3],tmpData[4],tmpData[5],tmpData[6],tmpData[7],tmpData[8],tmpData[9],tmpData[10],tmpData[11])
        file.write(data)

        # write the tri data
        for t in self.triangles:
            t.Save(file)

        # save the shader coordinates
        for s in self.shaders:
            s.Save(file)

        # save the uv info
        for u in self.uv:
            u.Save(file)

        # save the verts
        for v in self.verts:
            v.Save(file)

class md3Tag:
    name = ""
    origin = []
    axis = []

    binaryFormat="<%ds3f9f" % MAX_QPATH

    def __init__(self):
        self.name = ""
        self.origin = [0, 0, 0]
        self.axis = [0, 0, 0, 0, 0, 0, 0, 0, 0]

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    def Save(self, file):
        tmpData = [0] * 13
        tmpData[0] = self.name
        tmpData[1] = float(self.origin[0])
        tmpData[2] = float(self.origin[1])
        tmpData[3] = float(self.origin[2])
        tmpData[4] = float(self.axis[0])
        tmpData[5] = float(self.axis[1])
        tmpData[6] = float(self.axis[2])
        tmpData[7] = float(self.axis[3])
        tmpData[8] = float(self.axis[4])
        tmpData[9] = float(self.axis[5])
        tmpData[10] = float(self.axis[6])
        tmpData[11] = float(self.axis[7])
        tmpData[12] = float(self.axis[8])
        data = struct.pack(self.binaryFormat, tmpData[0],tmpData[1],tmpData[2],tmpData[3],tmpData[4],tmpData[5],tmpData[6], tmpData[7], tmpData[8], tmpData[9], tmpData[10], tmpData[11], tmpData[12])
        file.write(data)

class md3Frame:
    mins = 0
    maxs = 0
    localOrigin = 0
    radius = 0.0
    name = ""

    binaryFormat="<3f3f3ff16s"

    def __init__(self):
        self.mins = [0, 0, 0]
        self.maxs = [0, 0, 0]
        self.localOrigin = [0, 0, 0]
        self.radius = 0.0
        self.name = ""

    def GetSize(self):
        return struct.calcsize(self.binaryFormat)

    def Save(self, file):
        tmpData = [0] * 11
        tmpData[0] = self.mins[0]
        tmpData[1] = self.mins[1]
        tmpData[2] = self.mins[2]
        tmpData[3] = self.maxs[0]
        tmpData[4] = self.maxs[1]
        tmpData[5] = self.maxs[2]
        tmpData[6] = self.localOrigin[0]
        tmpData[7] = self.localOrigin[1]
        tmpData[8] = self.localOrigin[2]
        tmpData[9] = self.radius
        tmpData[10] = self.name
        data = struct.pack(self.binaryFormat, tmpData[0],tmpData[1],tmpData[2],tmpData[3],tmpData[4],tmpData[5],tmpData[6],tmpData[7], tmpData[8], tmpData[9], tmpData[10])
        file.write(data)

class md3Object:
    # header structure
    ident = ""            # this is used to identify the file (must be IDP3)
    version = 0            # the version number of the file (Must be 15)
    name = ""
    flags = 0
    numFrames = 0
    numTags = 0
    numSurfaces = 0
    numSkins = 0
    ofsFrames = 0
    ofsTags = 0
    ofsSurfaces = 0
    ofsEnd = 0
    frames = []
    tags = []
    surfaces = []

    binaryFormat="<4si%ds9i" % MAX_QPATH  # little-endian (<), 17 integers (17i)

    def __init__(self):
        self.ident = 0
        self.version = 0
        self.name = ""
        self.flags = 0
        self.numFrames = 0
        self.numTags = 0
        self.numSurfaces = 0
        self.numSkins = 0
        self.ofsFrames = 0
        self.ofsTags = 0
        self.ofsSurfaces = 0
        self.ofsEnd = 0
        self.frames = []
        self.tags = []
        self.surfaces = []

    def GetSize(self):
        self.ofsFrames = struct.calcsize(self.binaryFormat)
        self.ofsTags = self.ofsFrames
        for f in self.frames:
            self.ofsTags += f.GetSize()
        self.ofsSurfaces += self.ofsTags
        for t in self.tags:
            self.ofsSurfaces += t.GetSize()
        self.ofsEnd = self.ofsSurfaces
        for s in self.surfaces:
            self.ofsEnd += s.GetSize()
        return self.ofsEnd

    def Save(self, file):
        self.GetSize()
        tmpData = [0] * 12
        tmpData[0] = self.ident
        tmpData[1] = self.version
        tmpData[2] = self.name
        tmpData[3] = self.flags
        tmpData[4] = self.numFrames
        tmpData[5] = self.numTags
        tmpData[6] = self.numSurfaces
        tmpData[7] = self.numSkins
        tmpData[8] = self.ofsFrames
        tmpData[9] = self.ofsTags
        tmpData[10] = self.ofsSurfaces
        tmpData[11] = self.ofsEnd

        data = struct.pack(self.binaryFormat, tmpData[0],tmpData[1],tmpData[2],tmpData[3],tmpData[4],tmpData[5],tmpData[6],tmpData[7], tmpData[8], tmpData[9], tmpData[10], tmpData[11])
        file.write(data)

        for f in self.frames:
            f.Save(file)

        for t in self.tags:
            t.Save(file)

        for s in self.surfaces:
            s.Save(file)


def message(log,msg):
  if log:
    log.write(msg + "\n")
  else:
    print(msg)

class md3Settings:
  def __init__(self,
               savepath,
               name,
               logpath,
               overwrite=True,
               dumpall=False,
               ignoreuvs=False,
               scale=1.0,
               offsetx=0.0,
               offsety=0.0,
               offsetz=0.0):
    self.savepath = savepath
    self.name = name
    self.logpath = logpath
    self.overwrite = overwrite
    self.dumpall = dumpall
    self.ignoreuvs = ignoreuvs
    self.scale = scale
    self.offsetx = offsetx
    self.offsety = offsety
    self.offsetz = offsetz

def print_md3(log,md3,dumpall):
  message(log,"Header Information")
  message(log,"Ident: " + str(md3.ident))
  message(log,"Version: " + str(md3.version))
  message(log,"Name: " + md3.name)
  message(log,"Flags: " + str(md3.flags))
  message(log,"Number of Frames: " + str(md3.numFrames))
  message(log,"Number of Tags: " + str(md3.numTags))
  message(log,"Number of Surfaces: " + str(md3.numSurfaces))
  message(log,"Number of Skins: " + str(md3.numSkins))
  message(log,"Offset Frames: " + str(md3.ofsFrames))
  message(log,"Offset Tags: " + str(md3.ofsTags))
  message(log,"Offset Surfaces: " + str(md3.ofsSurfaces))
  message(log,"Offset end: " + str(md3.ofsEnd))
  if dumpall:
    message(log,"Frames:")
    for f in md3.frames:
      message(log," Mins: " + str(f.mins[0]) + " " + str(f.mins[1]) + " " + str(f.mins[2]))
      message(log," Maxs: " + str(f.maxs[0]) + " " + str(f.maxs[1]) + " " + str(f.maxs[2]))
      message(log," Origin(local): " + str(f.localOrigin[0]) + " " + str(f.localOrigin[1]) + " " + str(f.localOrigin[2]))
      message(log," Radius: " + str(f.radius))
      message(log," Name: " + f.name)

    message(log,"Tags:")
    for t in md3.tags:
      message(log," Name: " + t.name)
      message(log," Origin: " + str(t.origin[0]) + " " + str(t.origin[1]) + " " + str(t.origin[2]))
      message(log," Axis[0]: " + str(t.axis[0]) + " " + str(t.axis[1]) + " " + str(t.axis[2]))
      message(log," Axis[1]: " + str(t.axis[3]) + " " + str(t.axis[4]) + " " + str(t.axis[5]))
      message(log," Axis[2]: " + str(t.axis[6]) + " " + str(t.axis[7]) + " " + str(t.axis[8]))

    message(log,"Surfaces:")
    for s in md3.surfaces:
      message(log," Ident: " + s.ident)
      message(log," Name: " + s.name)
      message(log," Flags: " + str(s.flags))
      message(log," # of Frames: " + str(s.numFrames))
      message(log," # of Shaders: " + str(s.numShaders))
      message(log," # of Verts: " + str(s.numVerts))
      message(log," # of Triangles: " + str(s.numTriangles))
      message(log," Offset Triangles: " + str(s.ofsTriangles))
      message(log," Offset UVs: " + str(s.ofsUV))
      message(log," Offset Verts: " + str(s.ofsVerts))
      message(log," Offset End: " + str(s.ofsEnd))
      message(log," Shaders:")
      for shader in s.shaders:
        message(log,"  Name: " + shader.name)
        message(log,"  Index: " + str(shader.index))
      message(log," Triangles:")
      for tri in s.triangles:
        message(log,"  Indexes: " + str(tri.indexes[0]) + " " + str(tri.indexes[1]) + " " + str(tri.indexes[2]))
      message(log," UVs:")
      for uv in s.uv:
        message(log,"  U: " + str(uv.u))
        message(log,"  V: " + str(uv.v))
      message(log," Verts:")
      for vert in s.verts:
        message(log,"  XYZ: " + str(vert.xyz[0]) + " " + str(vert.xyz[1]) + " " + str(vert.xyz[2]))
        message(log,"  Normal: " + str(vert.normal))

  shader_count = 0
  vert_count = 0
  tri_count = 0
  for surface in md3.surfaces:
    shader_count += surface.numShaders
    tri_count += surface.numTriangles
    vert_count += surface.numVerts

  if md3.numTags >= MD3_MAX_TAGS:
    message(log,"!Warning: Tag limit reached! " + str(md3.numTags))
  if md3.numSurfaces >= MD3_MAX_SURFACES:
    message(log,"!Warning: Surface limit reached! " + str(md3.numSurfaces))
  if md3.numFrames >= MD3_MAX_FRAMES:
    message(log,"!Warning: Frame limit reached! " + str(md3.numFrames))
  if shader_count >= MD3_MAX_SHADERS:
    message(log,"!Warning: Shader limit reached! " + str(shader_count))
  if vert_count >= MD3_MAX_VERTICES:
    message(log,"!Warning: Vertex limit reached! " + str(vert_count))
  if tri_count >= MD3_MAX_TRIANGLES:
    message(log,"!Warning: Triangle limit reached! " + str(tri_count))

def save_md3(settings):
  if settings.logpath:
    if settings.overwrite:
      log = open(settings.logpath,"w")
    else:
      log = open(settings.logpath,"a")
  else:
    log = 0
  message(log,"##########Exporting MD3##########")
  bpy.ops.object.mode_set(mode='OBJECT')
  md3 = md3Object()
  md3.ident = MD3_IDENT
  md3.version = MD3_VERSION
  md3.name = settings.name
  md3.numFrames = (bpy.context.scene.frame_end + 1) - bpy.context.scene.frame_start

  for obj in bpy.context.selected_objects:
    if obj.type == 'MESH':
      nsurface = md3Surface()
      nsurface.name = obj.name
      nsurface.ident = MD3_IDENT

      vertlist = []

      for f,face in enumerate(obj.data.faces):
        ntri = md3Triangle()
        if len(face.verts) != 3:
          message(log,"Found a nontriangle face in object " + obj.name)
          continue

        for v,vert_index in enumerate(face.verts):
          uv_u = round(obj.data.active_uv_texture.data[f].uv[v][0],5)
          uv_v = round(obj.data.active_uv_texture.data[f].uv[v][1],5)

          match = 0
          match_index = 0
          for i,vi in enumerate(vertlist):
            if vi == vert_index:
              if settings.ignoreuvs:
                match = 1#there is a uv match for all
                match_index = i
              else:
                if nsurface.uv[i].u == uv_u and nsurface.uv[i].v == uv_v:
                  match = 1
                  match_index = i

          if match == 0:
            vertlist.append(vert_index)
            ntri.indexes[v] = nsurface.numVerts
            ntex = md3TexCoord()
            ntex.u = uv_u
            ntex.v = uv_v
            nsurface.uv.append(ntex)
            nsurface.numVerts += 1
          else:
            ntri.indexes[v] = match_index
        nsurface.triangles.append(ntri)
        nsurface.numTriangles += 1

      if obj.data.active_uv_texture:
        nshader = md3Shader()
        nshader.name = obj.data.active_uv_texture.name
        nshader.index = nsurface.numShaders
        nsurface.shaders.append(nshader)
        nsurface.numShaders += 1
      if nsurface.numShaders < 1: #we should add a blank as a placeholder
        nshader = md3Shader()
        nshader.name = "NULL"
        nsurface.shaders.append(nshader)
        nsurface.numShaders += 1

      for frame in range(bpy.context.scene.frame_start,bpy.context.scene.frame_end + 1):
        bpy.context.scene.set_frame(frame)
        fobj = obj.create_mesh(bpy.context.scene,True,'PREVIEW')
        fobj.calc_normals()
        nframe = md3Frame()
        nframe.name = str(frame)
        for vi in vertlist:
            vert = fobj.verts[vi]
            nvert = md3Vert()
            nvert.xyz = vert.co * obj.matrix_world
            nvert.xyz[0] = (round(nvert.xyz[0] + obj.matrix_world[3][0],5) * settings.scale) + settings.offsetx
            nvert.xyz[1] = (round(nvert.xyz[1] + obj.matrix_world[3][1],5) * settings.scale) + settings.offsety
            nvert.xyz[2] = (round(nvert.xyz[2] + obj.matrix_world[3][2],5) * settings.scale) + settings.offsetz
            nvert.normal = nvert.Encode(vert.normal)
            for i in range(0,3):
              nframe.mins[i] = min(nframe.mins[i],nvert.xyz[i])
              nframe.maxs[i] = max(nframe.maxs[i],nvert.xyz[i])
            minlength = math.sqrt(math.pow(nframe.mins[0],2) + math.pow(nframe.mins[1],2) + math.pow(nframe.mins[2],2))
            maxlength = math.sqrt(math.pow(nframe.maxs[0],2) + math.pow(nframe.maxs[1],2) + math.pow(nframe.maxs[2],2))
            nframe.radius = round(max(minlength,maxlength),5)
            nsurface.verts.append(nvert)
        md3.frames.append(nframe)
        nsurface.numFrames += 1
        bpy.data.meshes.remove(fobj)
      md3.surfaces.append(nsurface)
      md3.numSurfaces += 1

    elif obj.type == 'EMPTY':
      md3.numTags += 1
      for frame in range(bpy.context.scene.frame_start,bpy.context.scene.frame_end + 1):
        bpy.context.scene.set_frame(frame)
        ntag = md3Tag()
        ntag.origin[0] = (round(obj.matrix_world[3][0] * settings.scale,5)) + settings.offsetx
        ntag.origin[1] = (round(obj.matrix_world[3][1] * settings.scale,5)) + settings.offsety
        ntag.origin[2] = (round(obj.matrix_world[3][2] * settings.scale,5)) + settings.offsetz
        ntag.axis[0] = obj.matrix_world[0][0]
        ntag.axis[1] = obj.matrix_world[0][1]
        ntag.axis[2] = obj.matrix_world[0][2]
        ntag.axis[3] = obj.matrix_world[1][0]
        ntag.axis[4] = obj.matrix_world[1][1]
        ntag.axis[5] = obj.matrix_world[1][2]
        ntag.axis[6] = obj.matrix_world[2][0]
        ntag.axis[7] = obj.matrix_world[2][1]
        ntag.axis[8] = obj.matrix_world[2][2]
        md3.tags.append(ntag)

  if md3.numSurfaces < 1:
    message(log,"Select a mesh to export!")
    if log:
      log.close()
    return

  file = open(settings.savepath, "wb")
  md3.Save(file)
  print_md3(log,md3,settings.dumpall)
  file.close()

  message(log,"MD3: " + settings.name + " saved to " + settings.savepath)
  if log:
    print("Logged to",settings.logpath)
    log.close()

from bpy.props import *
class ExportMD3(bpy.types.Operator):
  """Export to Quake Model 3 (.md3)"""
  bl_idname = "export.md3"
  bl_label = 'Export MD3'

  filepath: StringProperty(subtype = 'FILE_PATH',name="File Path", description="Filepath for exporting", maxlen= 1024, default= "")
  md3name: StringProperty(name="MD3 Name", description="MD3 header name / skin path (64 bytes)",maxlen=64,default="")
  md3log: StringProperty(name="MD3 Log", description="MD3 log file path",maxlen=1024,default="export_md3.log")
  md3overwritelog: BoolProperty(name="Overwrite log", description="Overwrite log (off == append)", default=True)
  md3dumpall: BoolProperty(name="Dump all", description="Dump all data for md3 to log",default=False)
  md3ignoreuvs: BoolProperty(name="Ignore UVs", description="Ignores uv influence on mesh generation. Use if uv map not made.",default=False)
  md3scale: FloatProperty(name="Scale", description="Scale all objects from world origin (0,0,0)",default=1.0,precision=5)
  md3offsetx: FloatProperty(name="Offset X", description="Transition scene along x axis",default=0.0,precision=5)
  md3offsety: FloatProperty(name="Offset Y", description="Transition scene along y axis",default=0.0,precision=5)
  md3offsetz: FloatProperty(name="Offset Z", description="Transition scene along z axis",default=0.0,precision=5)

  def execute(self, context):
   settings = md3Settings(savepath = self.properties.filepath,
                          name = self.properties.md3name,
                          logpath = self.properties.md3log,
                          overwrite = self.properties.md3overwritelog,
                          dumpall = self.properties.md3dumpall,
                          ignoreuvs = self.properties.md3ignoreuvs,
                          scale = self.properties.md3scale,
                          offsetx = self.properties.md3offsetx,
                          offsety = self.properties.md3offsety,
                          offsetz = self.properties.md3offsetz)
   save_md3(settings)
   return {'FINISHED'}

  def invoke(self, context, event):
    wm = context.window_manager
    wm.fileselect_add(self)
    return {'RUNNING_MODAL'}

  @classmethod
  def poll(cls, context):
    return context.active_object is not None

def menu_func(self, context):
  newpath = os.path.splitext(bpy.context.blend_data.filepath)[0] + ".md3"
  self.layout.operator(ExportMD3.bl_idname, text="Quake Model 3 (.md3)").filepath = newpath

def register():
  bpy.types.TOPBAR_MT_file_export.append(menu_func)

def unregister():
  bpy.types.TOPBAR_MT_file_export.remove(menu_func)

if __name__ == "__main__":
  register()