Distribution: COMPANY CONFIDENTIAL
Project: NC Model 1
Issue: 0.01
Author(s): Steve Cormie
Date: 12-Mar-96
Last Issue: None
0.01 SMC 12-Mar-96 Created from Tim Dobson's documentation.
There are no outstanding issues.
This document specifies a software module named NHTwitter which provides interfaces which are used to "anti-twitter" areas of the display (see below for a description of "twittering").
The NCD is capable of outputting a screen image to either a monitor (at resolutions up to 1024x768) or to a PAL or NTSC television. When a screen image is displayed on a television there can be problems with interference between highly contrasting horizontal lines which are next to each other eg. a line of white followed by a line of black (quite common in desktop displays). This can result in a shimmering effect which we have named "twittering". This effect can also be caused by a dense contrasting pattern eg. a black and white dither pattern to approximate grey.
The algorithm used works by averaging horizontal lines of the screen area being processed and is destructive of screen contents. This means that an area of the screen must not be anti-twittered more than once.
The NHTwitter module provides the following * command for anti-twittering a rectangular area of the display.
Where (x,y) are the coordinates of the bottom left of the rectangle in OS coordinates and w, h are the width and height respectively of the rectangle, again in OS coordinates.
The module provides a SWI interface for anti-twittering a rectangular area of the display.
On entry: R0 = x R1 = y R2 = w R3 = h On exit: All registers preserved.
Where (x,y) are the coordinates of the bottom left of the rectangle in OS coordinates and w, h are the width and height respectively of the rectangle, again in OS coordinates.