A technique is provided the user of an interactive computer graphics application which allows the user to attach lines to objects such that when the objects move, the lines will rubber-band to follow the object. The objects are provided with an attribute that describes their "stickiness". If an object is given the "sticky" attribute, then lines which touch or overlay the object will rubber-band to follow the object's movement. It is also possible to provide end points of lines with the "sticky" attribute so that the end points of lines can be attached to objects. Further, objects of arbitrary geometric shape can be attached to other objects of arbitrary geometric shape using the "sticky" attribute. The feature facilitates editing and revision of graphic files.

The invention concerns a display device for displaying graphical information, in the form of point patterns forming symbols of an arbitrary size and shape, each symbol being allocated a unique symbol code, as well as in the form of non-symbolic information, on a display unit (8) of a raster-scan type and comprising a symbol memory (13) including information on the point pattern of the available symbols, a pixel oriented image memory (2) including all graphical information to be displayed on the display unit, and a processor (1), by which the symbols incorporated in the image are read into the image memory by processing the information that is stored in the symbol memory. What is characteristic of the invention is a pixel oriented auxiliary memory (3) with the same pixel division as the image memory, each symbol in the image memory (2) corresponding to a pixel in the auxiliary memory, which pixel defines the symbol position in the image. The auxiliary memory is also allocated a row memory (4), each pixel row in the auxiliary memory (3) being allocated a memory cell in the row memory. Furthermore, the row memory is allocated a code memory (5) including the symbol codes for the symbols that appear in the image, each memory cell in the row memory including a pointer (ADDRESS 1, ADDRESS 2, ---) to the memory cell in the code memory (5) that includes the symbol code for the first symbol on the corresponding pixel row in the auxiliary memory (3), and the symbol codes for the remaining symbols on this pixel row being allocated the consecutive memory cells of the code memory.

A graphical interactive software training system as disclosed comprising a host computer sub-system operatively connected to a graphic computer sub-system providing training courses for use by a human operator. The training system functions without reference to a control program. It includes a series of stand-alone and isolated pictures having command functions embedded thereon requiring an interactive dialogue with the human operator to progress through the training course.

A display system particularly useful in a computer aided design system. Symbols are formed for the display from a plurality of symbol-fragments stored in memory. By simple code changes, the computer can rotate symbols without, for example, a point-by-point movement of data in memory.

An image processing method for generating a convex hull of a configuration in a digital image. Top points of the convex hull are selected. A reference line connecting the top points is defined. For each area between two adjacent top points, continuous pixels are selected from one top point toward the other so that each pixel is the nearest to the referfence line as well as between the reference line and a contour of the configuration. Distances from pixels to the reference line are calculated from chain codes.