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Accelerated Colormap Exchanger for Graphics Rasterizer Adapters

IP.com Disclosure Number: IPCOM000112319D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Erb, DJ: AUTHOR [+2]

Abstract

This method greatly accelerates the task of changing the hardware colormap. It reduces the waiting time spent by the cpu for the the graphics hardware to reach a state that permits the cpu to change the colormap without causing flashes on the screen.

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This is the abbreviated version, containing approximately 97% of the total text.

Accelerated Colormap Exchanger for Graphics Rasterizer Adapters

      This method greatly accelerates the task of changing the
hardware colormap.  It reduces the waiting time spent by the cpu for
the the graphics hardware to reach a state that permits the cpu to
change the colormap without causing flashes on the screen.

      Ordinarily, if the hardware colormap is changed while the
screen refresh is in progress, flashes (dots, lines, or general
fireworks) can appear momentarily on the screen; thus, it would be
necessary to wait for vertical retrace before changing the colormap.
But because there are only 60 to 77 vertical retraces each second,
the cpu may be idle for millions of instruction cycles.

      Instead of waiting for a vertical retrace, it was found that it
is only necessary to wait until the rasterizer becomes idle before
changing the colormap.  In that state, the contents of the frame
buffer is unchanging, which can prevent any sudden flashes of
extraneous color.

      This also assures that any particular object is completely
drawn in its specified color before the colormap is changed.  This is
important to animation packages that draw a frame, change the
colormap, and then draw the next frame; otherwise, some objects may
become invisible.

      This method works best if the ramdac sets a complete colormap
entry (the red, green, and blue components) before refreshing the
screen.  Otherwise, colors may be temporarily incorrect, givin...