plain wall

Hello everyone

I'd like to ask what the phrase "plain wall" implies:

In digital photography, if the camera is moved while a picture is
being taken, especially in dim light conditions, you can easily get a
double image. For example, suppose you are taking a picture of someone
standing in front of a plain wall, and while doing so you point the
camera at an adjacent wall where there is a doorway. If the shutter
remains open while you point the camera at the second background,
then the camera will record a double image—the original scene with
the person standing against a plain wall superimposed over the image
of the wall with the doorway in it.

Does "plain wall" mean a smooth and flat wall without ledges (prominent parts), or does it mean a wall painted only in one colour, without any decorations, drawings etc?

BOTH of them - a flat, smooth surface, usually of one unobtrusive colour, no pictures or other objects on it, no window, no shelves, no furniture showing.