jit.gl.pix background color

hi all,
i want to build a simple jit.gl.pix quad viewer,
4 ins, 1 out.
the four inputs are seen in the only one output.

Using the "rect" argument, on the jit.gl.pix box,
i suppose to be in the "efficient" world,
so here is my way (see patch).

as you can see on the pwindow at left,
background is grey.
This why , I suppose, the final output content is enlightened.
(max 8.1.6 - Mac OS 10.13.6)

what is the way, to tell the background to be pure black ?
does it have to be told to the gl.pix ?

making a quad-viewer may be more efficient, I'm sure :)
don't hesitate :)....

thanks for your tips :)

Mat