Well you might surprise to what a convex window? In common, on the basis of the shapes the windows are categorized into two classes:
i) A Convex shaped windows: Windows of a shape so if we acquire any two points within the window then the line joining them will never cross the window boundary; that shaped windows are considered as convex shaped windows.
Figure: Types of Windows
ii) A Concave or non convex shaped windows: Windows of a shape hence if we can decide a pair of points within the window hence the line joining them may cross the window boundary or several section of the line segment might lie outside the window. Such shaped windows are considered to as concave or non-convex shaped windows.