(In reply to Peter Frühberger from comment #13)
> So @Christian happy for pointers on how to get it going.

Yeah, completely agree with you. Software workarounds are very common in OpenGL
and nothing unusual.

Here are a few pointers into the code:
1. vlVdpVideoSurfaceGallium() in src/gallium/state_trackers/vdpau/surface.c.
That one is called to "export" video surfaces to OpenGL and the right place to
convert from frame to field.

2. vlVdpDecoderRender() in src/gallium/state_trackers/vdpau/decode.c. Here we
reallocate the buffer backing the surface before the decoding if the
frame/field flag doesn't match what the decoder expects.

3. vlVdpVideoMixerRender() in src/gallium/state_trackers/vdpau/mixer.c. Here we
use a vl_compositor_render() to compose the mixed output picture.

Essentially you only need to test if the surface is field based and if not
reallocate it and either use vl_compositor_render() or pipe->blit() to copy the
frame into fields.