Apparatus and method for processing graphical information to minimize page crossings and eliminate processing of information outside a predetermined clip window

DETAILED DESCRIPTION The present invention is directed to an apparatus and method for processing graphical information in a manner such that 1) accesses to a frame buffer memory device in which the graphical information is to be stored result in a minimum of page crossings in the frame buffer; and 2) time spent processing graphical information outside a predetermined clip window is minimized.
These two goals are sometimes conflicting and the present invention determines how the graphical information should be processed so that frame buffer accesses are always performed with the least overhead.
In a dynamic RAM or frame buffer memory device (video RAM in the preferred embodiment), pixels of information to be displayed are arranged in rows and columns.
When addressing the memory, a memory addressing scheme internal to the RAM must be utilized to address individual pixels.
In this connection, a commonly employed scheme is to define a row of pixels as a page which can be accessed by use of an internal row address register.
That is, by providing a row address register with the base address of a page, pixels within that page can be accessed by a relative offset from the row address, the offset being the column address.
For this reason, it is desirable to minimize page crossings whenever possible to reduce the overhead associated with reloading the row address register used by the RAM with the base address of a new page.
For a given object which is to be displayed within a defined window, by determining the portions of the object which are inside the window and the portions which are outside the window, it frequently is possible to determine whether the object should be drawn from top to bottom, bottom to top, left to right or right to left so as to minimize page crossings and minimize the time spent processing portions of the object outside a predetermined clip window.
The present invention will be described with reference to a graphics subsystem in which clipping is performed in hardware.
In this connection, the hardware of the present invention makes available certain information which is needed to determine the manner in which graphical operations should be processed so as to minimize page crossings and eliminate time spent processing clipped or obscured pixels