METHOD AND DEVICE FOR REPRODUCING IMAGES

Abstract: In the case of images comprising an area made up of pixels, the contrast observation in darker parts of the image can be improved by varying, as a function of the initial lightness of a pixel, the brightness and/or colour and/or spatial filtering of said pixel. The variation is smaller as the initial brightness increases.

Description: Title: Method and device for reproducing images The invention relates to a method for reproducing images comprising an area made up of pixels and having a brightness value for each pixel, which method comprises varying the brightness values of the pixels according to a function which is dependent on and not constant with time. The invention also relates to a method for reproducing images comprising an area made up of pixels and having a brightness value for each pixel, wherein each pixel may be composed per se of at least two different colours and wherein the intensity for at least one and at most all with the exception of one of the colours may also permanently be zero. A method in which the brightness value of the pixels is varied is known per se from Tom N. Cornsweet: "Visual Perception", Academic Press, New York and London, for the purpose of improving the contrast sensitivity of an observer in relation to an observed image . The obtaining of medical X-ray images is referred to as radiography. Radiography is nowadays performed both by an analog procedure and by a digital procedure. In the analog radiography procedure, a photographic film is exposed to the X-ray image to be reproduced, developed and subsequently reproduced by placing the developed film in a generally somewhat darkened room in front of a light box and then viewing it. In the digital manner of radiography, the image is obtained electronically. It can then be reproduced on a viewing screen. However, compared with the standard analog method in which the developed film is viewed on a light box, the image on the viewing screen has a poorer contrast reproduction. Specifically, viewing screens have the following contrast-reducing properties: light scattering between the phosphor layer and the glass window; reflection of the ambient light at the outer surface of the glass window. Both phenomena cause a non-imagewise brightness increase which reduces the contrast. As a result, the viewing screen has a relatively poor contrast reproduction, especially in darker image regions. Although the influence of the ambient light can be reduced by working in darkness, it is known that this causes fatigue. Another reason for fatigue resides in the image repetition frequency. The brighter the image, or a part thereof, the more trouble an observer will have with flickering. This phenomenon is also more serious as the extent to which the brighter image parts fall in the peripheral part of the viewing field increases. As the extent to which an image (part) increases in brightness and/or is increasingly observed in the periphery, the higher the image repetition frequency has to be in order to prevent fatigue phenomena. It is furthermore known per se (see: Tom N. Cornsweet: "Visual Perception", Academic Press, New York and London) that the contrast sensitivity of an observer in relation to an observed image depends on the depth of modulation with which said image is presented to an observer image by image. Thus, the contrast sensitivity can be increased by changing the image brightness from image to image, for instance by modulating the image brightness with a frequency between 5 and 50 Hz. In practice, this is not applied because such images cause fatigue. In the case of film and television reproduction, even the image reproduction frequency is therefore chosen high enough for no flickering to be observed even in the peripheral field. In order to improve the abovementioned poor contrast reproduction of images on a viewing screen, the brightness of the reproduced image could also be increased, but that is not possible to an adequate extent due to technical limitations, such as maximum electron current and saturation of the phosphor. The light scattering in the thin film is also less than in the thick glass window of the viewing screen. Compared with the film with its matt black surface in combination with the light box with its high brightness, reproduction of an image on an image screen is therefore disadvantageous. For this reason, the digital image present in electronic form is quite often first printed on film in order to be able to view it subsequently on a light box. It is evident that this method is very time-consuming. The object of the invention is to improve the contrast observation of an image comprising an area made up of pixels, such as, for example, a medical X-ray image on a viewing screen. Other situations in which an image comprising an area made up of pixels is observed and in which the invention can be used to improve the contrast observation are, inter alia: a film for a light box as described in Dutch Patent Application 84.01605, a television image projected on a screen, a video monitor, a film for a screen, on which screen a television image is projected, an LCD monitor. The method according to the invention, which method comprises varying the brightness value of the pixels is characterized in that the function is also dependent on the initial brightness with which at least the respective pixel would be reproduced if the function were not used and in that, for higher initial brightnesses, the ratio between the highest and the lowest value of the function is at least just as close or closer to one than for lower initial brightnesses. The method according to the invention in which each pixel may be made up of at least two different colours is characterized in that the colour coordinates of each pixel are varied according to a function which is not constant with time and which is dependent on the initial brightness with which at least the respective pixel would be reproduced if the function were not used and in that, for higher initial brightnesses, the variation in colour coordinates is at most just as great or is smaller than for lower initial brightnesses. The image comprises an area made up of pixels and having a brightness value for each pixel. In the abovementioned function, both the brightness values of the pixels and those of the surrounding pixels, possibly up to and including all the pixels of the entire image, serve as a variable. If only the magnitude of the instantaneous values is chosen as determining the image-content part of the function, simple implementation of the invention is possible by varying the brightness values. Viewing screen devices to which the invention can be applied often comprise, specifically, a so-called "look-up" table which determines the relationship between a signal presented to the viewing screen device and the signal which controls the image brightness on the screen, for example the intensity of the electron beam. A device in which the invention is used in this simple way is therefore characterized by at least two non-identical tables and means for changing from the one to the other "look-up" table during a picture return. If the mutual spatial relationships are also involved in the invention, a method according to the invention is characterized in that at least two different, spatially filtered variants of the original image are alternated. In this case, the reproduction system should therefore have at its disposal a larger memory from which the various filtered images can be retrieved. It is self-evident that combination with the first simple implementation is consequently possible as a result of, for example, alternating the method with look-up tables changing in turn with the method with spatially filtered variants changing in turn or by choosing at the same time as the choice of the filtered image a look-up table associated therewith. As has already been stated in the introduction, in the event of the choice of the spatial filtering and/or the look-up tables, it must be ensured that the brightest image parts are reproduced at a constant brightness in order to avoid troublesome flicker effects. This will mean in practice that the image filtering will not or virtually not alternate at high brightnesses and will in fact do so at low brightnesses. In other words, the image filtering will not therefore have to be linear in nature. Consequently, the result is achieved that darker image parts are reproduced in a manner in which optimum use is made of the increased contrast sensitivity of an observer at lower brightness of the respective image part and that brighter images are reproduced without trouble being experienced due to flickering. The increase in the contrast sensitivity depends both on the frequency and the depth of modulation of the modulated signal and is associated with the brightness to be reproduced. According to the invention, there are two variables which are adjusted independently of one another. The first is the function which specifies the processing for the signal which determines or reproduces the brightness and/or the colour of the image on the viewing screen. The second is the function which specifies the order in which the images are reproduced which have not been processed or have been processed using the first function. Examples of the second function are: processed - not processed - processed - not processed etc., also to be described as yes/no/yes/no/yes/no... yes/no/no/yes/no/no/yes/no/no... yes/no/no/no/yes/no/no/no processed according to function 1/not processed/not processed according to function 2/not processed..., also to be described as yesl/no/yes2/no/yesl/no/yes2/no... yesl/no/no/yes2/no/no/yesl/no/no/yes2/no/no... The function which specifies the processing can be, according to the above: another brightness, another colour, another spatial filtering. Thus, by way of example, the depth of modulation can be made to increase with decreasing brightness and/or the modulation frequency can be chosen closer to that for optimum contrast observation. If the second function is chosen, it is advantageous to choose the period time as a whole multiple of the image interval, as in the examples reproduced above. In the case of an image repetition frequency of 70 Hz, which is standard per se, 35, 23.3, 17.5, 14, 11.6, 10, 9.1, 8.3 Hz etc. are then obtained as possible frequencies.

Claims: CLAIMS 1. Method for reproducing images comprising an area made up of pixels and having a brightness value for each pixel, which method comprises varying the brightness values of the pixels according to a function which is dependent on and not constant with time, characterized in that the function is also dependent on the initial brightness with which at least the respective pixel would be reproduced if the function were not used and in that, for higher initial brightnesses, the ratio between the highest and the lowest value of the function is at least just as close or closer to one than for lower initial brightnesses. 2. Method according to Claim 1, characterized in that the function is dependent on an average brightness of the entire image. 3. Method according to Claim 1 or 2, characterized in that the function is dependent on the brightness values of the pixels in the local environment of a pixel to which the function is applied. 4. Method for reproducing images comprising an area made up of pixels and having a brightness value for each pixel, wherein each pixel may be composed per se of at least two different colours and wherein the intensity for at least one and at most all with the exception of one of the colours may also permanently be zero, characterized in that the colour coordinates of each pixel are varied according to a function which is not constant with time and which is dependent on the initial brightness with which at least the respective pixel would be reproduced if the function were not used and in that, for higher initial brightnesses, the variation in colour coordinates is at most just as great or is smaller than for lower initial brightnesses. 5. Device for applying the method according to Claim 1, 2, 3 or 4, characterized in that the device comprises a pixelwise electronically drivable viewing screen for reproducing the images. 6. Device for applying the method according to Claim 1, 2, 3 or 4, characterized in that the device comprises a pixelwise electronically drivable viewing screen and in that means are present for mounting an image present on an at least partially transparent carrier at least temporarily in front of the viewing screen. 7. Device for applying the method according to Claim 1, 2, 3 or 4, characterized in that the device comprises a projection screen for projecting a projection image using a projector which is drivable for each pixel of said projection image and in that means are present for mounting an image present on an at least partially transparent carrier at least temporarily in front of the projection screen. 8. Device according to one of Claims 5, 6 or 7, characterized in that the device is provided with at least two look-up tables and with means for changing from the one to the other look-up table during a picture return and in that means are present which drive the means for changing. 9. Device according to Claim 8, characterized in that the means for driving can be operated manually. 10. Device according to Claim 8, characterized in that the means for driving comprise detection means for detecting the initial brightness of a pixel. 11. Device according to Claim 8, characterized in that the means for driving comprise detection means for detecting the brightness of the image. 12. Device according to Claim 5, 6 or 7 in as far as dependent on Claim 4, which device comprises pixels which can deliver a coloured image whose colours can be made up of at least two colours and in which the intensity can also be permanently zero for at least one and at most all except one of the colours, characterized in that means are present for varying as a function of the initial brightness of a pixel the colour coordinates of at least said pixel with respect to the initial colour coordinates . 13. Device according to Claim 5, 6, 7 or 12, characterized in that means are present for filtering the image spatially and that means are present for changing between no spatial filtering and spatial filtering or between a first and a second spatial filtering depending on the initial brightness.

Inventors: GELUK RONALD JAN, GELUK, RONALD, JAN
Applicants: DELFT INSTR INTELLECTUAL PROPE, GELUK RONALD JAN, DELFT INSTRUMENTS INTELLECTUAL PROPERTY B.V., GELUK, RONALD, JAN
Application year: 1998
Main classification: H04N5
IPC codes: H04N5