A display unit includes a central processing unit (CPU) for outputting display data, a frame buffer having first and second ports and temporarily storing the display data received from the CPU via the first port, a display device having a display screen for displaying the display data read from the frame buffer means and received via the second port and having a display controller for generating and outputting image generating signals, a first control register for storing the display data output from the CPU and having a specific address, a second control register for reading the display data from the frame buffer and storing the display data in response to a register set signal, where the second control register supplies the display data stored therein to the CPU, and a comparator circuit for comparing the image generating signals output from the display controller and the display data stored in the first control register and for supplying the register set signal to the second control register when the image generating signals output from the display controller corresponds to the display data stored in the first control register. The CPU has a mechanism for comparing and checking the display data stored in the first control register and the display data stored in the second control register.

A method of converting image outlline data representative of segments defining an outline of an image area to be reproduced, into dot data representative of image dots which collectively represent the image area. In the method, the outline of the image area is superimposed on a coordinated pixel screen wherein a plurality of vertical pixel lines and a plurality of horizontal pixel lines cooperate to provide intersections defining a matrix of dot positions which are selectively used as the image dots. The vertical and horizontal pixel lines form a plurality of sections each having four corners defined by the respective four intersections of the pixel lines. Each one of the plurality of sections is determined as a relevant section if any one of the segments of the outline of the image area passes through the relevant section. The dot data are prepared such that the dot data represents the presence of the image dot at a predetermined one of four corners of each relevant section.

In first and second modes, successive pairs of bytes, each with a suitable number (e.g. 8) of binary indications, are respectively processed in each clock cycle or clock half cycle to provide a true color. In these modes, the successive pairs of bytes may be processed in a 5,5,5, or a 5,6,5 pattern representing the primary colors for a pixel. In a third mode, the bytes may be introduced to a memory having a plurality of positions for storing individual binary combinations, which may be updated by a microprocessor, representing pseudo colors. In the third mode, a particular position in the memory is selected in accordance with the indications in each byte in each clock cycle or half cycle. In an additional mode, three successive bytes in a group may indicate the primary colors defining a true color when the fourth byte in the group provides a particular indication (e.g. 0 for all 8 binary bits). In this mode, indications in the fourth byte other than the particular indication select a particular position in the memory to represent a pseudo color. In this mode, the successive bytes may be respectively provided either in each clock cycle or clock half cycle. The binary indications representing the true color or the pseudo color in the different modes are converted to analog signals for introduction to a video monitor. In the additional mode, the indications in particular positions in the memory may be blocked from conversion to analog signals.

Color information lost in a computer graphics system as a result of encoding by dithering is recovered by passing the encoded data through a filter that has the same effective shape and size as the dither matrix used to encode the data. In a system that employs a dither matrix having 2.sup.N cells, up to N bits of color information, per color component, is recoverable.

A display apparatus consists of a display memory (VRAM), a data modifier circuit and a display unit at least. The display unit, such as a CRT display unit, has a plurality of display dots which include red, green and blue color dots. The display memory must be cleaned at a primary stage and thereafter a plurality of display codes corresponding to only display dots on the outlines of the color image must be written into the display memory. The display codes are converted into display data which indicate red, green and blue color components. The display codes are successively read out from the display memory and supplied to the data modifier circuit wherein the display data are modified so as to compensate the display data which are not written into the display memory. The display unit displays the image corresponding to the modified display data on the screen thereof.