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9.1 Programming with libplot: An overview

GNU libplot 3.0 is a free function library for drawing two-dimensional vector graphics. It can produce smooth, double-buffered animations for the X Window System, and can export graphics files in many file formats. It is `device-independent' in the sense that its API (application programming interface) is to a large extent independent of the display type or output file format. The API is thread-safe, so it may be used in multithreaded programs.

There are bindings for C, C++, and other languages. The C binding, which is the most frequently used, is also called libplot, and the C++ binding, when it needs to be distinguished, is called libplotter. In this section we use libplot to refer to the library itself, irrespective of binding.

The graphical objects that libplot can draw include paths, circles and ellipses, points (i.e., pixels), markers, and `adjusted labels' (justified text strings). A path is a sequence of line segments, circular arcs, elliptic arcs, quadratic Bezier curves, and/or cubic Bezier curves. Paths may be open or closed. User-specified filling of paths, circles, and ellipses is supported (fill rule and fill color, as well as pen color, may be specified). There is support for maintaining a Postscript-style stack of graphics contexts, i.e., a stack of drawing attribute sets. Path-related attributes include line thickness, line type, cap type, and join type, and text-related attributes include font name, font size, and text angle.

The fundamental abstraction provided by libplot is that of a Plotter. A Plotter is an object with an interface for the drawing of vector graphics which is similar to the interface provided by a traditional pen plotter. There are many types of Plotter, which differ in the output format they produce. Any number of Plotters, of the same or different types, may exist simultaneously in an application.

The drawing operations supported by Plotters of different types are identical, in agreement with the principle of device independence. So a graphics application that is linked with libplot may easily be written so as to produce output in any or all of the supported output formats.

The following are the currently supported types of Plotter.

A distinction among these types of Plotter is that all except X and X Drawable Plotters write graphics to a file or other output stream. An X Plotter pops up its own windows, and an X Drawable Plotter draws graphics in one or two X drawables.

Another distinction is that the first four types of Plotter (X, X Drawable, PNM, and GIF) produce bitmap output, while the remaining types produce output in a vector graphics format. In bitmap output the structure of the graphical objects is lost, but in a vector format it is retained.

An additional distinction is that X, X Drawable, Tektronix and Metafile Plotters are real-time. This means that they draw graphics or write to an output stream as the drawing operations are invoked on them. The remaining types of Plotter are not real-time, since their output streams can only be emitted after all functions have been called. For PNM and GIF Plotters, this is because the bitmap must be constructed before it is written out. For Illustrator and Postscript Plotters, it is because a `bounding box' line must be placed at the head of the output file. For a Fig Plotter, it is because color definitions must be placed at the head of the output file.

The most important operations supported by any Plotter are openpl and closepl, which open and close it. Graphics may be drawn, and drawing attributes set, only within an openpl...closepl pair. The graphics produced within each openpl...closepl pair constitute a `page'. In principle, any Plotter may be opened and closed arbitrarily many times. An X Plotter displays each page in a separate X window, and Postscript, PCL, and HP-GL Plotters render each page as a separate physical page. X Drawable Plotters and Tektronix Plotters manipulate a single drawable or display, on which pages are displayed in succession. Plotters that do not draw in real time (PNM, GIF, Illustrator, Postscript, CGM, Fig, PCL, and HP-GL Plotters) may wait until their existence comes to an end (i.e., until they are deleted) before outputting their pages of graphics.

In the current release of libplot, Postscript and CGM Plotters delay outputting graphics in this way, but PCL and HP-GL Plotters output each page of graphics individually, i.e., when closepl is invoked. PNM, GIF, Illustrator and Fig Plotters are similar, but output only the first page. That is because PNM, GIF, Illustrator and Fig formats support only a single page of graphics.

The graphics display, or `viewport', that is drawn in by a Plotter is normally a square or rectangular region on its output device. But when using any Plotter to draw graphics, a user will specify the coordinates of graphical objects in device-independent `user' coordinates, not in device coordinates. A Plotter transforms user coordinates to device coordinates by performing an affine transformation.

Immediately after invoking openpl to open a Plotter, an application should invoke space. space specifies a rectangular `window' in the user coordinate system that will be mapped affinely to the viewport on the output device. The transformation from user coordinates to device coordinates may be updated at any later time by reinvoking space, or by invoking fconcat. The fconcat operation will concatenate (i.e., compose) the current affine transformation with any specified affine transformation. This sort of concatenation is a capability familiar from, e.g., Postscript.

Each Plotter maintains a Postscript-style stack of graphics contexts. This makes possible the rapid, efficient drawing of complicated pages of graphics. A graphics context includes the current affine transformation from user coordinates to device coordinates. It also includes such modal drawing attributes as graphics cursor position, linemode, line thickness, pen and fill colors, and the font used for drawing text. The state of any uncompleted path (if any) is included as well, since paths may be drawn incrementally, one portion (line segment, arc, or Bezier curve) at a time. The current graphics context is pushed onto the stack by calling savestate, and popped off by calling restorestate.

To permit vector graphics animation, any page of graphics may be split into `frames'. A frame is ended, and a new frame is begun, by invoking the erase operation. On a Plotter that does real-time plotting (i.e., an X, X Drawable, Tektronix, or Metafile Plotter), this erases all plotted objects from the graphics display, allowing a new frame to be drawn. Displaying a sequence of frames in succession creates the illusion of smooth animation.

On most Plotters that do not do real-time plotting (i.e., PNM, Illustrator, Postscript, CGM, Fig, PCL, or HP-GL Plotters), invoking erase deletes all plotted objects from an internal buffer. For this reason, most Plotters that do not do real-time plotting will display only the final frame of any multiframe page.

GIF Plotters are in a class by themselves. Even though they do not do real time plotting, a GIF Plotter can produce multi-image output, i.e., an animated pseudo-GIF file, from a multiframe page. As noted above, the pseudo-GIF file produced by a GIF Plotter will contain only the first page of graphics. But if this page consists of multiple frames, then each invocation of erase after the first will be treated, by default, as a separator between successive images.

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