Thank you Martin for this answer,
I knew about the SetFader API function, and already used it; but I don't think I can really use it for a T-bar (or master video fader, which is in fact what I really need, but no matter). But maybe I am wrong ? Please tell me : here is the point : when I set value=255, the sources (I mean preview and active) switch in vMix, and the "crossfader" in vMix interface comes back to 0. This behaviour is really smart and useful in vMix interface... but problematic when dealing with external controlling, because the "physical" fader position is still "255". Do you understand what I mean ?
In other words, here is the process :
* at the beginning my "physical" fader is at the lowest level (="0"), and the crossfader in vMix interface is on the left.
* I use the external fader, increassing the value => no problem, the physical fader and the interface crossfader behave the same; when my fader is nearly at the maximum level, the vMix interface crossfader is nearly at the right side of its curse - OK !
* when I reach the maximum level for my physical fader (which means I want to see only the new image, "B", the one from the "preview window"), vMix interface crossfader comes back to 0 (ie left side), the views are switched, but my physical fader is still at its top level - could still be OK, let's say this position means "only B image on the screen";
* but of course when I slightly decrease the position of the physical fader (from "255" to "254"), in order to achieve a "reverse crossfade", the vMix interface crossfader "jumps" from 0 to 254, which makes suddenly A image "cut" B image.
And I have no way to reflect the behavior of the vMix interface crossfader in my physical fader, because I can't physically jump from max position to lowest position (when the vMix interface crossfader does so) without coming across intermediate levels.
Sorry for these too long explanations... I hope you better understand what I mean by now ?
So, that's why I was hoping for a real "T-bar behaviour" in the vMix interface, that could be controlled via an API function; or, if easier to implement for you, a "video master" or "fade to black fader", or "alpha level fader" (let's call it as you want !) that could be controlled via an API function - since what I really need is a manual controlled fade-to-black (using the crossfader (with a black image in the preview window) was just one of the solutions).
Finally, here are my questions :
* is there a way to control the fade-to-black "level", using "SetFader" another way, or any other function ? No matter how. I suggested using the "alpha level" of the inputs because the functionnality is already available in vMix, so only the corresponding API function needs to be implemented... but any other way would be OK too;
* if not, is it planned for a future release ?
* if not : could an enthusiastic developper (like me ;-)) participate in vMix development :-) ?!
* if not... too bad for me ! But still vMix is a really good software. Keep up :-) !
Many thanks for your answers,