Video underlays are working as intended (more or less).

I split the MPD buffers into two, symbologyBuffer and compositeBuffer. The symbology buffer is texturestage2 and rendered by the shader at full brightness. If the underlay is active the TADS/PNVS buffer is copied to outputbuffer which is texturestage0 (the simple diffuse buffer). The two are mixed by the material shader which is applied to each MPD screen.

The mix works well, providing enough contrast between video and symbology even when the video is quite bright.

I did a version that mixed symbology and the video buffer into a single composite via a shader but the mix was often overbright and hard to read. A good case for adding a working video level control? Maybe in version 1.5.

The final modification is to use the TADS/PNVS postbuffer instead of the raw camera render.

Source