[Myrsloik]

It's the code review patrol!

This is the only definitively wrong part:

if (!isConstantFormat(d.vi) || d.vi->format->colorFamily == cmCompat || d.vi->format->sampleType != stInteger || d.vi->format->bytesPerSample > 2) {
       vsapi->setError(out, "Deblock: only constant format 8-16 bits integer input supported");
        vsapi->freeNode(d.node);
        return;
}

You probably want this instead:

  if (!isConstantFormat(d.vi) || d.vi->format->sampleType != stInteger || (d.vi->format->bitsPerSample != 8 && d.vi->format->bitsPerSample != 16)) {
        vsapi->setError(out, "Deblock: only constant format 8 or 16 bits integer input supported");
        vsapi->freeNode(d.node);
        return;
    }

The motivation:
1. Normal filters will never get cmCompat formats as input so no need to check for it
2. You actual processing code actually only works for 8 and 16 bit. For 9-15 bits you don't clamp the output range properly so the output will technically be invalid. You could of course fix the clamping too for the 9-16 bit case which is probably more elegant.