In the processing book there are examples where a ball moves and a transparant  rect gets drawn every frame so you get a motion blur effect.
I want to do this in 3d but the rect only has a x and y parameter, no z.
And if I use (not, no transparanty)

 fill(204, 0, 0);
 rect(0, 0, width, height);

then for some reason(s) I still see what is drawn before.
What is a good way to solve this for a 3d space?

I don't have the solution, but I can explain what happens now. The 3D object you draw is drawn around a center point, which is on the same z-plane as your rect. Half of it sticking in front, half behind. P3D can't render this very well.

As far as I know, no shape drawing function incorporates the z-axis in it's parameters. Instead, you use translate(x,y,z) to move up and down through the z-axis.

But the "trick" described in the book effectively uses layering of semi-transparant layers to achieve the progressively more opaque look. In 3D-case, this would meen that you'd work on different z-levels, but this works differently. As you'll find out.

void setup() {
 size(400, 400, P3D);smooth();
}
     

void draw() {

hint(DISABLE_DEPTH_TEST);
fill(0,50);
rect(0,0,width,height);
hint(ENABLE_DEPTH_TEST);
fill(255);


translate(mouseX,mouseY);
sphere(50);
}