Access violation when adding a scene to viewport items

1

I have a simple procedure that looks like this, with some WriteLnLog calls to help with debugging:

procedure TViewMain.AddCar;
begin
  WriteLnLog('start');
  Car := TCastleScene.Create(FreeAtStop);
  WriteLnLog('after create');
  Car.Load('castle-data:/car.gltf');
  WriteLnLog('after load');
  Car.Translation := Vector3(Car.Translation.X, 4, Car.Translation.Z);
  WriteLnLog('after translate');
  MainViewport.Items.Add(Car);
  WriteLnLog('after add');
end;

“Car” is defined as a private field on TViewMain.
I also call this procedure like this:

if Event.IsKey(keyP) then
begin
  AddCar;
  Exit(true);
end;

When I run the game and press the P key, this is what appears in the console:

start
after create
after load
after translate
An unhandled exception occurred at $00000001014A10B5:
EAccessViolation: Access violation
  $00000001014A10B5
  $00000001014A11DD
  $0000000100A31150
  $0000000100A30ECE
  $00000001007E1E70
  $00000001007DCC26
  $00007FF811425764
  $00007FF81142548F
  $00000001007DF5DA
  $00000001007DF696

Exception "Exception":
Process "/Volumes/Data/dev/pascal/viewport-3d-tut/castle-engine-output/macos/Viewport 3D.app/Contents/MacOS/viewport-3d-tut" (absolute path "/Volumes/Data/dev/pascal/viewport-3d-tut/castle-engine-output/macos/Viewport 3D.app/Contents/MacOS/viewport-3d-tut") failed with exit status 217

I’m confused as to why this is happening, since there is a demo (in examples/viewports_and_scenes/cars_demo) with seemingly the exact same scene creation code. Any ideas?

2

The Car seems to be initialized OK in the code you show.

Are you sure the MainViewport is initialized OK?

The cars_demo doesn’t initialize it explicitly, but it gets initialized by Castle Game Engine because:

  • MainViewport: TCastleViewport is in the published section of the view
  • MainViewport was created using CGE editor, it is loaded from the castle-data:/gameviewmain.castle-user-interface.

See Writing code to modify scenes and transformations | Manual | Castle Game Engine , in particular the section “Refer to the designed components”.

You can add more debug to test whether my guess is correct:

WritelnLog('Car assigned? %s', [BoolToStr(Car <> nil, true)]);
WritelnLog('MainViewport assigned? %s', [BoolToStr(MainViewport <> nil, true)]);
MainViewport.Items.Add(Car);

If my guess is right, you will see Car assigned? true but then MainViewport assigned? false.

It this doesn’t help, then please submit the testcase – I can investigate and help better :slight_smile:

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3

Turns out the component that I thought was called “MainViewport” was actually called “Viewport1”…

I changed the name to “MainViewport” and everything works correctly.

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