The invention concerns a graphics display system of the kind having a refresh raster display device, a line clipping mechanism for clipping line primitives for viewing in a selected rectangular viewport having horizontal and vertical edges parallel to the X and Y axes respectively of the display area of the display device, and a vector-to-raster converter for converting the clipped lines to patterns of discrete pels in a refresh buffer, the vector-to-raster converter including an incremental line drawing mechanism responsive to line information supplied by the clipping mechanism to draw the line starting at one endpoint and proceeding towards the other. In such a system, for at least some lines of a kind which have a first endpoint in the viewport and the second endpoint outside the viewport, the clipping mechanism is arranged to avoid calculating the intersection point of the line with the viewport edge intersected by the line, and the vector-to-raster converter is arranged to start the line drawing mechanism at the first endpoint and halt the line drawing mechanism when the currently generated coordinate along the display axis normal th the said viewport edge equals the coordinate of the viewport edge along the same axis.

A method for clipping two and three dimensional graphic primitives for use in a computer graphics workstation. During the first clipping stage, the system removes all graphic primitives which lie outside an arbitrary clipping volume or window that is an enlargement of the desired viewing volume or window. This clipping volume (window) is then projected and mapped onto a virtual viewport which is larger than the real viewport. After this mapping occurs, rendering effects (such as line styles, line width, pattern fill, hatch fill, etc.) are applied to the primitive. A subsequent stage of clipping is then applied to clip the virtual viewport to the real viewport which is the user's visible area. Use of the clipping volume (window) during the first stage of clipping makes it simpler to transform geometric primitives which are partially visible to the viewer, and gives visible primitives a richer set of invariant geometric properties so that their rendering effects may be more easily and correctly carried out.

A graphic apparatus of the invention having a clip function includes a clip register storing coordinates of points defining a display designation clip area. The contents of the clip register and an address supplied to a bit map memory are compared by a comparator. Control operation for clipping is performed such that only data falling within the clip area is written in the bit map memory in accordance with the comparison result of the comparator.

A method for identifying a bounded area of a graphic display. The method uses the division of the display screen into nine areas then the central square is the correlation window. This window can be small, two or three pel width, or large if the application is seeking to identify a larger area. The point indicated by the operator is in the center of the correlation window. An object (bounded area) which intersects this square will register a bit. It can be seen that a bounded area will intersect the correlation window if either: (a) any part of the boundary actually passes through the window, or (b) any arbitrary point within the window is inside the area.

A pattern write protect system approximates a memory area for data entry with a multiplicity of rectangles and determines when a point for entry is within the window by establishing a relative position of the address point coordinates to the coordinates of the sides of the rectangles. A less than/greater than signal for each address coordinate relative to the sides of each rectangle is derived. These signals are summed and the resulting sum signals are then multiplied to provide an enable gate signal.