fig1 illustrates the architecture of a general purpose computer system 2 . a main bus 4 interconnects a central processor unit 6 , a main memory 8 , a storage controller 10 , a user input device interface 12 and a display driver 14 . user input devices , such as a mouse 16 and a keyboard 18 , are connected to the user input device interface 12 . a computer monitor 20 is connected to the display device driver 14 . a permanent storage device , such as a hard disc 22 , is connected to the storage device controller 10 . the general purpose computer 2 is controlled to perform the desired operations by computer program . the computer program can be written in different computer languages and can have various different forms all producing the same external operation . the sample image data upon which the simulation is to be performed is stored on the hard disc drive 22 . the processing operations being simulated by the software may be compression and decompression or perhaps a filtering operation involved in a domain transformation . fig2 illustrates the display device driver 14 in more detail . data , address and control signals are passed to a graphics controller 24 via the main bus 4 . the data representing the pixel values to be displayed is written from the graphics controller 24 into the image memory 26 . pixel values ( pv ) are read out of the image memory 26 via a multiplexer 28 to a display palette 30 . the display palette 30 maps the 8 bit pixel values into three 8 bit numbers each representing the magnitude of a respective red , green and blue component for that pixel value . the three 8 bit numbers from the display palette 30 are fed to a digital - to - analog converter 32 where they are converted into analog red , green and blue component values for driving the monitor 20 . the data stored within the display palette 30 defining the mapping of pixel values to components is loaded into the display palette 30 via buffers 34 under control of address signals input via the multiplexer 28 during a palette loading mode . it will be seen from this arrangement that the display device driver 14 supports 256 different colours that may be simultaneously displayed . it is possible to vary the colours loaded into the display palette 30 over a much wider range , but only 256 different colours can be used any one time . the display palette 30 contains 3 * 256 bytes , which is a relatively small amount of data that can be rapidly modified ( totally or in part ) via the main bus 4 . fig3 illustrates the mapping of pixel values to directly displayable appearance values that are stored within the display palette 30 when the enhancement feature is not being used . it will be observed from fig3 that each of the three components remains in balance throughout the range of pixel values and so the image that is produced will be a monochrome ( black and white ) image . an increase in pixel value corresponds to an increase in luminance . table 1 below illustrates the data that would be stored within the display palette 30 to perform such a mapping . table 1______________________________________pixel value r component g component b component______________________________________ 0 0 0 0 1 1 1 1 2 2 2 2 : : : :: : : : n - 2 n - 2 n - 2 n - 2n - 1 n - 1 n - 1 n - 1n n n nn + 1 n + 1 n + 1 n + 1n + 2 n + 2 n + 2 n + 2n + 3 n + 3 n + 3 n + 3n + 4 n + 4 n + 4 n + 4n + 5 n + 5 n + 5 n + 5n + 6 n + 6 n + 6 n + 6n + 7 n + 7 n + 7 n + 7n + 8 n + 8 n + 8 n + 8n + 9 n + 9 n + 9 n + 9 n + 10 n + 10 n + 10 n + 10 n + 11 n + 11 n + 11 n + 11 n + 12 n + 12 n + 12 n + 12 n + 13 n + 13 n + 13 n + 13 n + 14 n + 14 n + 14 n + 14 n + 15 n + 15 n + 15 n + 15 n + 16 n + 16 n + 16 n + 16 n + 17 n + 17 n + 17 n + 17 : : : :: : : : 254 254 254 254255 255 255 255______________________________________ fig4 illustrates the mapping performed by the display palette 30 in accordance with one embodiment of the invention . in contrast with fig3 it will be observed that between the pixel values n and ( n + 15 ) the mapping does not follow the monotonic increase that produced the previous monochrome image for that portion of pixel values . more particularly , the balance between the respective components is widely varied over this range and the complete dynamic range of the component values spanned . in this way , enhanced displayable appearances ape generated in which adjacent pixel values map to more readily visually distinguishable displayable appearances . an example of the data that would be stored in the display palette 30 is given in table 2 below . table 2______________________________________pixel value r component g component b component______________________________________ 0 0 0 0 1 1 1 1 2 2 2 2 : : : :: : : : n - 2 n - 2 n - 2 n - 2n - 1 n - 1 n - 1 n - 1n 255 0 0n + 1 223 32 0n + 2 191 64 0n + 3 159 96 0n + 4 127 128 0n + 5 95 160 0n + 6 63 192 0n + 7 31 224 0n + 8 0 255 0n + 9 0 223 32 n + 10 0 191 64 n + 11 0 159 96 n + 12 0 127 128 n + 13 0 95 160 n + 14 0 63 192 n + 15 0 32 223 n + 16 n + 16 n + 16 n + 16 n + 17 n + 17 n + 17 n + 17 : : : :: : : : 254 254 254 254255 255 255 255______________________________________ fig5 is a colour chart illustrating the variation of colour within the range of enhanced displayable appearances given by the mapping of fig4 and table 2 . each of the vertexes of the chart represent 100 % of a particular colour component . a point at the centre of the chart ( w ) represents a balance between the three components that gives a white output . it will be seen that the enhanced range starts with pure red then progresses via a mix of red and green to pure green . from pure green the enhanced range progresses via a mix of green and blue to pure blue . fig6 illustrates an alternative modification to the data within the display palette 30 that yields an enhanced range . in this case all the data is modified to give a monochrome display in which the variation in luminance with pixel value is increased over the enhanced range n to ( n + 15 ). fig7 is a flow diagram illustrating how the data within the display palette 30 is modified in the case of the mapping of fig4 . at step 36 the computer receives a user input , such as a mouse movement or keyboard stroke , that signifies a change in the position of the range of enhanced displayable appearances . if it is determined at step 38 that the new appearance is to be fully monochrome , then step 40 returns the data in the palette to that shown in table 1 and the procedure ends . if the new appearance is not be fully monochrome , then the process passes from step 38 to step 42 where a test is made as to whether or not enhanced displayable appearances are already in use . if such enhanced displayable appearances are already in use , then step 44 returns any pixel values that are currently within the enhanced range but that will be outside this range when it is moved to its new position to their directly displayable values . at step 46 the data for the enhanced range at its new position is written into the display palette 30 via the main bus 4 and buffers 34 under the control of address data supplied via multiplexer 28 . fig8 illustrates an example image 48 comprised of a horizontal block 50 and a vertical block 52 together with the user interface for controlling the analysis . the horizontal block 50 and the vertical block 52 are of uniform appearance . a bar of monotonically varying pixel values 54 comprising the palette scale representation is displayed beneath the image 48 . at the left hand end of the bar 54 is a black and white portion 56 where the range cursor is positioned should it be desired not to use the enhanced displayable appearances feature . to use the enhanced displayable appearances feature , the user would manipulate the mouse to drag the range cursor controlling the enhanced displayable appearances out of the black and white region 56 . this is illustrated in fig9 . the range cursor 58 is dragged to a user selected position within the palette scale illustrated by the bar 54 . whilst the enhanced range is being so dragged , the modified mapping data is dynamically written into the display palette 30 so that the effect upon the image 48 can be observed and a suitable position for highlighting a distortion can be readily found . in the example image shown , the distortion takes the form of a horizontal ringing in the pixel value that occurs after a sudden change in pixel value . such a ringing could be introduced by inappropriate horizontal filtering . this ringing is of an amplitude such that it would not be readily detectable from the directly representative displayable appearances . however , by mapping the range of pixel values adjacent to the sudden transition to the enhanced range , the slight variation in these pixel values produced by the ringing becomes visible . an additional effect that increases the usefulness of this technique is that if an area contains pixel values that are not within the enhanced range whilst those surrounding it are , then these pixels will stand out as not being coloured . in fig9 the position of the enhanced range 58 is such that the ringing in the background is highlighted . fig9 also illustrates the feature of the invention whereby a graph 60 of the numeric pixel value along a selectable analysis line 62 is displayed adjacent the image . this graph 60 allows an identified problem to be quantified . in the case shown in fig9 the graph 60 is taken along the analysis line 62 intersecting the vertical block 52 . fig1 is similar to fig9 except that the position of the range cursor 58 has been altered so as to highlight the slight variations within the uniform appearances of the horizontal block 50 and the vertical block 52 . the position of the analysis line 62 has also been moved so as to intersect the horizontal block 50 . once again , the uses of the enhanced range makes the slight variation in pixel values at the horizontal edges of the blocks more visible . although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings , it is to be understood that the invention is not limited to those precise embodiments , and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims .