In the X Window System, the X Nonrectangular Window Shape Extension allows windows to be given arbitrary, non-rectangular shapes.
The X Window System is a windowing system for bitmap displays, common on Unix-like operating systems.
In computing, a window is a graphical control element. It consists of a visual area containing some of the graphical user interface of the program it belongs to and is framed by a window decoration. It usually has a rectangular shape that can overlap with the area of other windows. It displays the output of and may allow input to one or more processes.
Two well-known applets that use the shape extension are oclock, which is a simple round analog clock, and xeyes, which shows two googly eyes that follow the cursor on the screen as if they were watching it. Most X systems have one or the other (or both) in their standard installations. In addition, some window managers use the shape extension to draw non-rectangular icons.
Googly eyes, or jiggly eyes, are small plastic craft supplies used to imitate eyeballs. Googly eyes traditionally are composed of a white plastic or card backing covered by a clear, hard-plastic shell, encapsulating a black plastic disk. The combination of a black circle over a white disk mimics the appearance of the sclera and pupil of the eye to humorous effect. The inner black disk is allowed to move freely within the larger clear plastic shell, which makes the eyes appear to move when the googly eyes are tilted or shaken.
An X window manager is a window manager which runs on top of the X Window System, a windowing system mainly used on Unix-like systems.
Implementation
Applications can only create rectangular windows, but can then change their shape to be an arbitrary part of the original rectangle. The remaining area of the rectangle is not only transparent (shows what is below the window); rather, it is not part of the window at all. For example, if a window is shaped with a hole in the middle, not only the hole shows what is below the window, but a click in the hole is considered to be a click in what is below the window.
A window having a rectangular outline but a hole in the middle.
Applications create windows as usual (specifying width and height), but can then change the shape of a window to be a subset of the original area. by sending the X display server either a list of rectangles or a pixmap of color depth one (a black-and-white pixmap). More precisely, a client can request the rectangles or the pixmap to be set as the new shape, but also to be combined in various ways (e.g., intersected or added) with the old shape. A third way for changing the shape of a window is by using the current shape of a window or a part of it.
A display server or window server is a program whose primary task is to coordinate the input and output of its clients to and from the rest of the operating system, the hardware, and each other. The display server communicates with its clients over the display server protocol, a communications protocol, which can be network-transparent or simply network-capable.
Color depth or colour depth, also known as bit depth, is either the number of bits used to indicate the color of a single pixel, in a bitmapped image or video framebuffer, or the number of bits used for each color component of a single pixel. For consumer video standards, such as High Efficiency Video Coding (H.265), the bit depth specifies the number of bits used for each color component. When referring to a pixel, the concept can be defined as bits per pixel (bpp), which specifies the number of bits used. When referring to a color component, the concept can be defined as bits per component, bits per channel, bits per color, and also bits per pixel component, bits per color channel or bits per sample (bps). Color depth is only one aspect of color representation, expressing the precision with which colors can be expressed; the other aspect is how broad a range of colors can be expressed. The definition of both color precision and gamut is accomplished with a color encoding specification which assigns a digital code value to a location in a color space.
The extension allows defining the shape of two separate regions: the clipping and the bounding regions. These two areas are defined even for windows not using the shape extension: the clipping region is the area that can be used for drawing, the bounding region is the total area covered by the window (that is, the clipping region plus the border). The shape extension allows defining two independent shapes for these two regions.
Both the clipping and bounding are associated, in the shape extension, two areas: a default rectangular region and a possibly non-rectangular client region, defined and changed by the client as specified above. The effective region, which is the actual shape of the window, is the intersection of these two areas. This intersection is performed every time either region is changed: for example, if the client regions contains parts that are outside the default region but the default region is then enlarged, the effective region will include these parts.
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