The Computer Graphics Interface

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Even those with few computer skills can now, through the use of GUI, learn how to use computer applications for word processing, finances, inventory, design, artwork or hobbies.

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What is a Graphical User Interface (GUI)? - Definition from Techopedia

Logical environment. The viewing environment produces a picture that is an ideal view of the scene without consideration of device-dependent characteristics of the environment where it will be displayed. Binding of attributes that describe rendering is performed in the logical environment. Examples of such attributes are the nominal width of a line which might depend on the pen size of a plotter or the pixel size of a CRT display , the linestyle used, and the availability of colour.


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  • Just as output devices have differing physical capabilities that must be accommodated, so do input devices. For example, device dependent relative positions from a mouse might be transformed into absolute 2D locations in a device independent coordinate system. Realization environment.

    The realization environment finishes the processing of graphical output by completely defining the image to be displayed. By the logical environment, all graphical output is complete in the sense that all attributes needed to view it have been bound. However, more information may need to be added and compromises must be made if the device is not perfect.

    For example, outputting straight lines on a pixel device is always a compromise. Similarly, input is received from the operator and transformed to the form required by the logical environment.

    The CGRM conveniently sidesteps the issue of whether physical graphical devices are contained within this environment by taking an abstract view of what can be considered to be an operator. If the operator is a traditional human being, then it is convenient to think of the physical environment as encompassing graphical hardware.

    In this case the operator interface is in reality a physical one based on light emitted from displays, ink on paper, and the transduction of physical motion. On the other hand, the operator might be another computer system-such as a virtual terminal, window management system or machine vision system-or even another computer graphics system.

    Although figure 2 shows only single instances of each environment, the CGRM does allow multiple instances of any environment. Thus fan-in many to one mappings and fan-out one to many mappings are allowed only at the interfaces between environments. They may occur between any pair of adjacent environments and both fan-in and fan-out may occur at the same time in a graphics system. This is illustrated in figure 3.

    There is a corresponding example of fan-in in the description of the flow of input data in GKS-3D between the environments modelling workstations and the virtual environment. The CGRM provides a third level of description that defines the internal structure of each environment. This detailed environment model is illustrated in figure 4. The CGRM uses a specific set of data elements round objects in figure 4 and processing elements rectangular objects in figure 4 to describe computer graphics.

    An environment thus consists of a set of such data elements and a set of such processing elements. The number of data and processing elements were chosen based on the level of description felt necessary to characterize the work performed in each environment at this level of abstraction. Their arrangement is based on an abstract view of the common arrangement of data and processing steps found in many graphics standards and systems. Abstract names were chosen for each of them so as not to bias the reader's perception of their purposes based on preconceived notions.

    For example, the word collection store is used rather than segment store or structure store since the later two terms have specific meanings in the GKS and PHIGS standards, respectively. The notion of a collection store neatly abstracts the essential common properties of both segments and structures in a way that is completely general and applicable at each environment. In the view of the model, the most important graphical processing occurs as output primitives-atomic units of graphical output like lines, polygons, or text strings-flow through the various environments from the application to the operator or as input tokens-atomic units of graphical input like locations or text strings-flow through the various environments from the operator to the application.

    At places in the processing chain output primitives or input tokens might be stored or converted from one form to another. For example, a 3 dimensional line might be projected to make a 2 dimensional line. You will notice when you read the CGRM that the wording for input and output is not completely symmetrical.

    This is the perceived difference between human input and human output. Humans produce quite complex, highly-structured graphical output in drawing or painting, but we receive input in quite low-level chunks. Consequently, the imbalance in wording is deliberate and not accidental! The composition provides the abstract notion of capture of the graphical output working set of each environment. This is motivated by considering that the application is striving to create a picture to be observed by the operator.

    The composition captures this idea of picture at each of level of abstraction recognized in the CGRM. The notion agrees so well with our intuition that well-known and instantly recognizable names can be assigned to the composition in each environment. For example, in the construction environment this output set is called the model , while in the viewing environment it is called the picture.

    The composition is constructed from output primitives in a well defined, spatially structured order and there is only a single composition at any one time within each environment. Collection store. A collection is simply a named, structured set of graphical objects. These pieces are the building blocks from which compositions-like models and pictures-are produced.

    It is common for more than one such collection to exist simultaneously at a given level of abstraction, and for collections to have a degree of permanence spanning at least the duration of a typical interactive graphical session. This allows collections to be manipulated edited to eventually produce graphical output. The collection store-as one of the abstract storage mechanisms internal to an environment-provides a mechanism for storing such graphical information.

    The composition and collection store are the only places where you would find graphical output within an environment. Token store. The token store is the analogue in input processing of the composition in output processing. It represents the input working set of the environment.

    Introduction to the Computer Graphics Reference Model

    Aggregation store. The aggregation store is the input analogue of the collection store. It provides a working space that may be used by input processing in constructing input tokens in the form required by the token store. Environment state.

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    The environment state data element represents other state information, for example current values of properties such as "modal" settings of graphical attributes which may be shared between processing elements. The processing elements also have precise purposes in the model. Of the five processes, four are dedicated to transforming input and output data at the four main interfaces to each environment. The fifth performs processing internal to the environment. A more detailed description of each follows. Note that the interfaces at the top of the construction environment and the bottom of the physical environment are special since they are external interfaces rather than interfaces to other graphical environments.

    It is most often appropriate to think of the transformation of graphical data from one form to another as happening as the data is absorbed into a lower environment.