GB2204468A - Display systems - Google Patents

Abstract

A display system displays a portion of an arbitrarily large stored image identified by the position and orientation of a display window (10). The pixel data relating to the arbitrarily large image is divided into tiles ("1"to "16") and the data relating to each one of the tiles is stored in a respective DRAM chip. Whenever the display window (10) together with a sufficient border to accommodate translational movement of the display window is accommodated within a portion of the image covered exclusively by a rectangular sub array of tiles, the storage devices relating to the tiles not being addressed ("4", "8", "12" and "16") are re-written with pixel data relating to the tiles most likely to be next required in dependence upon the direction of movement of the display window (10). After re-writing, the memory addresses are bank switched to indicate the new tiles of the map stored in the re-written DRAMs. <IMAGE>

Description

DISPLAY SYSTEMS The present invention relates to systems for displaying a selected portion of an arbitrarily larger image in a required rotational orientation.

Such a display system may typically be used in navigational systems for vehicles where a small portion of a larger stored map is displayed. The portion of the map selected for display may be considered as a display window of fixed size and shape which is superimposed on a larger map in any orientation. The display window relates to the immediate area of the vehicle and is displayed in an orientation which corresponds to the heading of that vehicle. The present invention is concerned with displays produced with a raster scan format. The basic data of an arbitrarily large map is stored digitally, in any convenient means of mass storage, as data relating to pixels covering the entire area of the map.For display purposes a portion of the arbitarily large map is transferred to an intermediate storage means, which is sized so as to be large enough to accommodate the display window in any possible rotational orientation together with a sufficient boundary to allow the window to be moved in a direction corresponding to any movement made by the vehicle.

In order to achieve a continuous updating of the intermediate memory by writing in of fresh data from the mass storage means, it has been proposed to use an intermediate storage means which is addressable toroidally.

That is, if the memory is regarded as a two-dimensional array of pixel data, with each address location uniquely identifiable by a row and column address, the bottom row of memory is regarded as being just above the top row of memory, and the two side columns of the array are also regarded as being adjacent. Thus, the address locations can be considered as lying over the surface of a toroid. As the display window moves over the 'surface' of the toroid, new data from a mass storage means is written into address locations which correspond to a border area surrounding the display window at a fixed distance from the edge of the window. Of course, the locations of this border area move with the display window and are not fixed locations in memory.Therefore, the display window can move at a rate which is determined by the size of the memory and the speed or writing, without any apparent interruption in the display, despite the limited size of the intermediate storage means, provided that there is sufficient map data stored in a related mass storage means. GB-A-2 070 399 describes such a toroidally addressable intermediate storage means.

In the display system described in the above-mentioned specification, the size of the border is always constant and therefore it is not possible to take advantage of any knowledge of the patterns of movement of the display window.

The toroidal memory is also described solely for use with a display window that moves with its edges parallel to the row and address locations. Since the toroidal memory requires that a processor programme used to select pixels from the intermediate memory means in order to derive the required raster scanning lines is able to take into account the toroidal form of the address locations, rotation of the display window gives rise to complex data processing.

In order to provide a display window image of significant resolution, it is necessary that the intermediate store should contain a large amount of pixel data. For this reason, it is normally necessary for the intermediate storage means to be made up of more than one storage device1 such as a RAM chip. This provides further problems in addressing the required memory locations for the purposes of a toroidal memory.

The present invention is concerned with the technical problems of providing a raster display system which enables a display window in any orientation to be moved in any direction within a larger image area stored in an intermediate storage means, which for reasons of required resolution or improved access time, comprises more than one independently addressable storage device.

The present invention solves this technical problem by using the known technique of memory address bank switching to the storage devices which make up the intermediate storage means in order to allow a continuously moving display to be produced.

Where the intermediate storage means is made up of a number of separate random access memories (RAMs) the data stored in each of the RAMs can relate to a particular "tile" of image area. The term "tile" is used herein to refer to one portion of an image, which is, for example rectangular in shape. The tiles together cover the image without gaps. As a display window moves across the image portion contained in the intermediate storage means, selected RAMs containing tiles which are no longer being addressed for display purposes are re-written. The memory addresses for the bank of RAMs that have been re-written are then switched in accordance with the data that these RAMs now contain.

Display systems embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a block diagram of a display system; Figures 2A - 2C are diagrams for illustrating the underlying principle of the present invention; Figure 3 is a block diagram of an implementation of the display system; and Figure 4 is a diagram showing part of the arbitrarily large sized image to illustrate how the pixel data of said image is stored in the intermediate storage means.

The display system of the invention will be described for use in a navigation system. However, it will be understood that it is not limited solely to this application.

A display system as illustrated in Figure 1 has a mass storage device 2 which contains pixel data for an arbitrarily large map area. The mass storage device 2 is connected to an intermediate storage means 4 which is made up of a number of individual RAMs1 each of which contains the data for a "tiles of the arbitrarily large map. A portion of the map area which is stored in the intermediate storage means 4 is to be displayed on a CRT 6. The displayed portion is defined by a display window 10. The CRT receives data via a display control 8 which selects the required pixel data from the intermediate storage means in order to produce the raster scan lines for a display window given its position and orientation within the area of the map storing the intermediate storage device.Figures 2A 2C show a display window 10 relative to the tiles representing data stored in 16 separate RAMs of the intermediate storage device.

Each of the tiles in Figure 2 is given a reference from "1" to "16" to identify the RAM in which the data relating to that tile is stored. Let us suppose that the display window 10 is required to move continuously in the direction of the arrow X. Figure 2A shows its initial position and Figure 2B shows its position after a finite time interval. In the position shown in Figure 2B, the display window is approaching the edge of the image area stored in the intermediate storage means. It is therefore necessary to read further data from the mass storage device into the intermediate storage means in order to allow the display window 10 to carry on moving continuously in the direction indicated. Since the RAM chips "4", "8", "12" and "16" are no longer being addressed to derive the nececessary raster scan lines for the display, the control means 12 which controls the operation of the whole system re-writes the data stored in these RAMs so that they now contain data relating to the map area to the left, as seen in Figure 2, of the image area previously covered by the tiles. Once the tiles have been re-written the memory addresses held by the control means 12 for these tiles are bank switched in order to represent the new organisation as. depicted in Figure 2C.

The window 10 can therefore continue to move in the direction of the arrow X provided the described re-writing of the RAMs containing the tiles behind the display window with data relating to tiles in front of is continually repeated.

An implementation of a display system using the principle described above will now be described with reference to Figures 3 and 4.

In the display system of Figure 3, the intermediate storage means 4 is shown as containing only nine RAMs instead of the sixteen discussed above. It will be appreciated that the number of RAMs and, therefore, the number of tiles can be varied in accordance with the application of the display sy stem.

In the embodiment of Figures 3 and 4, each tile is an n by m array of pixels. For the purposes of the present example, n = m = 1024. Therefore each tile contains the data for 1048576 pixels. The data for each tile is stored in a 1 Megabit dynamic random access memory (DRAM) chip. It will be appreciated that the size of a tile may be adjusted to any convenient value depending upon the application of the display system. As shown in Figure 4, each tile is designated with a co-ordinate reference (i,j) relative to an arbitrary reference. In this way each tile can be uniquely identified.

The mass storage device 2 is connected to a main processor 16 which controls the writing of data from the mass storage device into an intermediate storage means 4, which comprises nine separate storage devices 20 which are each a 1 Megabit DRAM chip. Therefore, each storage device 20 can store the pixel data relating to one tile. The complete intermediate storage means can therefore store the data relating to nine tiles which are normally arranged in a three by three array, for example as outlined in bold at 13, or in dotted bold lines at 15 in Figure 2. For reasons which will become apparent later, the area of the main map covered by the storage device may temporarily cover a portion of the map which is not a square 3 by 3 array as the portion of the map stored in the intermediate storage means 4 traverses across the larger map.However, the stored portion will continually revert to a 3 by 3 array.

The main processor 16 has an address bus which is connected to an address multiplexer 18 and a line 21 which carries data to a data multiplexer 22. The address multiplexer 18 is connected to an address processor 24. The address processor includes first and second translation means. The first translation means produces an output on one only of nine chip select lines 28, in dependence on the input address. The second translation means outputs address data on an address bus 26 which is connected to each of the storage devices 20 of the intermediate storage means 4. The main processor 6 produces pixel data addresses, which will be referred to herein as first addresses which are row and column addresses of the required pixel data relative to the arbitrary origin of the map.

The first translation means of the address processor 24 processes this first address in order to derive which of the tiles the address relates to and outputs a chip select signal on the appropriate one of the nine chip select lines 28. The first translation means makes use of information contained in the address processor relating to the tile coordinates of the data stored in each of the separate storage devices 20. This tile co-ordinate data is maintained, for example as a look-up table, by the main processor 16, via a control line 30. The look-up table may also contain a bit which can be set to prevent read access to any particular storage device which is currently being re-written under the control of the main processor 16.

The second translation means of the address processor 24 converts the first address into a second address which specifies the location of the required pixel data within the particular storage device. The pixel data for each tile is stored in the same manner in each of the storage devices so that the address bus 26 is connected to the address inputs Ao-A9 of each of the DRAM chips 20. Only the DRAM chip whose chip select input CS has been activated by the first translation means of the address processor over one of the lines 28 will input or output data from the respective one of the two data bus lines 32 which are connected respectively to the data in D and data out Q pins of each of the DRAM chips 20. The data bus lines 32 are connected to the data multiplexer 22.

The main processor 16 is used to write data into respective ones of the DRAM chips 20 making up the intermediate storage means 4. In order to display a second smaller portion of the image stored in the intermediate storage means, a display processor 34 is used. The function of this display processor corresponds to the display control 8 of Figure 1.

The display processor 34 receives inputs on lines 36 from another portion of the navigation system (not described herein) relating to the position and orientation of the display window 10 which may be regarded as superimposed on the portion of the map stored in the intermediate storage means as illustrated in Figure 4. The display processor 34 uses the input data relating to the required position and orientation of the display window 40 in order to derive the first addresses, that is addresses relative to the main map co-ordinates, of the successive pixels required to generate the raster lines of the display. The necessary algorithm for generating the required pixel addresses is known and will not be described in more detail herein. Reference may be made, for example, to GB-A-2 145 309 for a discussion of the techniques involved. These first addresses are output by the display processor 34 on an address bus to the address processor 24 via the address multiplexer 18. These addresses are dealt with as previously described so that the required pixel data is fed out on the data out line of the data bus 32 to the data multiplexer 22 which generates a required video signal from the successive pixel data received. This video signal is then fed to the display 6, for example a CRT display.

The main processor 16 controls the re-writing of tiles in accordance with a strategy which will now be described with reference to Figure 4.

The main processor receives data from the display processor on line 44 relating to the changing position of the display window 40. As can be seen by the dotted circular line 46 in Figure 4, the position of the display window 10 defines a circular envelope area over the main map image which encompasses all the possible rotational orientations of the display window at that position. In order to be able to carry out the continuous moving display of the window, this circular area together with a sufficient border to accomodate translational movements of the display window in the time taken to re-write a maximum of 5 of the DRAM chips 20 representing tiles, must be capable of being stored within a rectangular sub-array of the tiles.Re-writing of tiles can take place whenever the display window envelope plus border is wholly within any 2 by 3 array of tiles or, as shown in Figure 4, any 2 by 2 array of tiles. The main processor operates in accordance with a stored programme to re-write the tiles outside the rectangular sub-array. For example, when the display window 40 has reached the position as indicated in Figure 4 and is moving in the direction shown by the arrow 48, the tiles identified by the coordinates (Li, 3), (4, 4,), (4, 5), (3, 5) and (2, 5) are not being addressed.These tiles can therefore be re-written with the data relating to tiles (3, 2), (2, 2), (1, 2), (1, 3) and (1, 4) from the data contained within the mass storage means 2 so that the intermediate storage means then contains the pixel data for a portion of the image identified by the dotted bold line 15 in Figure 4. It is not material which tile data is written into which storage device provided only that new tile data is written into a storage device which is not being addressed by the display processor during re-writing, and the tile co-ordinates of the new tile are placed into the look-up table of the first translation means at the entry for that storage device once re-writing is complete. Where the look-up table contains a bit which is set to prevent read access this is reset once re-writing is complete.The order of re-writing of the storage devices is determined by the past history of the movement of the display window.

The example illustrated in Figure 4 illustrates the worst possible case in which it is necessary to re-write five of the tiles. It will be appreciated that if the display window is moving parallel to one of the reference coordinates it will generally only be necessary to re-write the tile data for three tiles. For example suppose that the display window 10 is moving in the direction of the dotted line arrow 50 of Figure 4. In that case the storage devices containing the tile data relating to tiles (3, 5) and (2, 5) would be re-written with the data relating to tiles (3, 2) and (2, 2). Depending on whether or not the position of the display window continued to move in the direction of the arrow 50, the storage device containing the data relating to tile (4, 5) could be re-written with the data relating to tile (4, 2).

It will be seen from the above discussion that the portion of the image stored in the intermediate storage means 6 will continually resolve itself into a 3 by 3 array, even though an instantaneous "snapshot" of the portion of the image contained within the intermediate storage means 4 may sometimes be of an irregular shape.

Claims (8)

1. A display system for displaying as a raster scan, a portion of an image defined by a display window which can overlie the image area at any position and in any orientation, the system comprising mass storage means for storing data relating to the pixels of an image of arbitrarily large size, intermediate storage means for storing data relating to part of said image, and comprising a plurality of independently, addressable storage devices, each storing data relating to a tile of said image, display means for addressing the storage devices to display a raster scan image of the image area within the display window on a display device, and control means for controlling the re-writing of predetermined ones of said storage devices from data in the mass storage means in dependence on the location of the display window and for controlling the addressing function of the display means to switch the addresses to identify the new tile after re-writing of a storage device.

2. A display system according to claim 1, wherein the each storage device is a random access memory chip, each of which stores pixel data of a tile comprising an n by m array of pixels within said image.

3. A display system according to claim 1 or 2, wherein the display means comprises a display processor for generating a first address of each pixel data required to produce the display in dependence on the position and orientation of the display window, said first addresses being relative to said image, and an address processor for receiving said first addresses and comprising first translation means for deriving from each first address a signal for selecting one of said storage devices, and second translating means for deriving from each first address a second address relative to said selected storage device.

4. A display system according to any one of the preceding claims, wherein the control means switches the addresses by controlling the first translation means.

5. A display system according to any one of the preceding claims, wherein the control means inhibits addressing of a storage device during re-writing.

6. A display system according to any one of the preceding claims, wherein the control means comprises a main processor for controlling the re-writing of storage devices, said main processor being connected to receive data from the main store at locations identified by first addresses of said data relative to the image.

7. A display system according to claim 6 when dependent on claim 3, wherein the main processor outputs said first addresses via said address processor to determine the second addresses relative to the storage devices at which the associated date is to be written.

8. A display system substantially as herein described with reference to the accompanying drawings.