Rotating cube getting cutoff

[Knocker]

Hi everyone.

I managed to code something I never could do back in the day - a rotating vector cube, see attachment (only rotates around one axis for brevity). Unfortunately lines in the top ~1/8 of the screen are cutoff. I've done quite a bit of troubleshooting:

Tried two different start-up routines (see source and a VB IRQ one)
Tried two different drawline routines (see source and the one from Amiga System Prog's Guide)

Meaning the problem should be in my RotateCube routine.

Tried two ways of erasing previous cube: 1. Blit zeroes to screen (see source) 2. Redraw cube with black lines ($dff072=0).

Using A500/ROM1.3/A501, and tested both WinUAE (PAL & NTSC) and real HW (NTSC).

Tried to rotate the code every other VB, and then it works fine.

Maybe the code is simply too slow?!

Thoughts anyone? Help much appreciated.

[Apollo]

Had just a quick view on the code: you don't double-buffer, don't you? My guess is that the raster-beam is already some lines down the screen before the lines are drawn. So you only see the cleared bitmap, because you clear the bitmap first after reaching the first rasterline.
I would create a second bitmap. Then you have to change the bitmap pointers every frame. One bitmap is shown, the other bitmap is getting worked on.

best regards,
Stefan

P.S.
To keep the things simple you could try to start clearing, rotating and drawing the stuff not at line 1 but much lower before you reach the vertical blank. This could be just enough time for clearing the bitmap and drawing stuff. But anyway, if you're going to add things you have to double-buffer to avoid chasing-the-beam stuff

[Knocker]

Thanks Stefan - I tried double buffering and then it renders properly. A little surprised it is needed for something as relatively simple as this, although I haven't tried your other suggestion.