WO2005106788A1

WO2005106788A1 - Method of and system for changing the projection behavior of image projection means

Info

Publication number: WO2005106788A1
Application number: PCT/IB2005/051273
Authority: WO
Inventors: Paul Ullmann
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-04-29
Filing date: 2005-04-19
Publication date: 2005-11-10

Abstract

To calibrate an image projection arrangement (3) by means of which images (P) are projected onto a projection surface (2), wherein respective projected images (P) comprising the projected pixels (P(x, y)) are obtained, at least one projected image (P) is recorded by means of a image recording arrangement (4), and the image projection arrangement (3) is adjusted by means of a control unit (5) on the basis of information derived from the recorded image (C); here, the image recording arrangement (4) is a digital recording unit, which acquires the projected image in a plurality of pixels, and an assignment of pixels (C(x, y)) of the digital recording unit (4) to pixels (P(x, y)) of the image projection arrangement (3) is carried out, wherein the image projection arrangement (3) for the projection of individual pixels (P(x, y)) is correctively adjusted by means of the control unit (5) on the basis of analysis information that is derived from associated pixels (C(x, y)) of the digital image recording arrangement (4) in relation to the pixels (P(x, y)) of the image projection arrangement (3).

Description

Method of and system for changing the projection behavior of image projection means

FIELD OF THE INVENTION The invention relates to a method of and a system for changing the projection behavior of image projection means, by means of which images are projected onto a projection surface. The invention relates furthermore to a projection device.

BACKGROUND OF THE INVENTION Apart from professional applications, the projection of images with image projection means is being employed with increasing frequency in the private sector. One example of this is the so-called home cinema, in which video information originating from a video signal source, such as a video recorder, DVD player or a TV station, is projected by means of the image projection means onto a projection surface and in this way is reproduced for visual perception. Other frequently used devices are projectors that are connected to a data processor or computer, for example, a laptop, in order to project images containing data or graphics. It is also possible to project individual images originating from a photo CD via the data processor and the image projection means. In the private sector, however, the ambient conditions when projecting images as described above are seldom ideal: mostly, either it is impossible to position the image projection means ideally or there is no ideal projection surface available. One therefore has to tolerate impairment of the quality of the projected images, which is attributable to various causes, for example to the quality of the projection surface, to scattered light or ambient light, as well as to the image projection means themselves or to artifacts introduced by the image projection means. To remedy this situation, various compensation or correction techniques have been proposed in the past, for example, digital and optical keystone correction, lens correction and color correction. Color correction corrects the color temperature of the projected image, and attempts to optimize the color reproduction. In this connection, in relation to the color of the projection surface there is an attempt to optimize the color setting as a whole. Thus, the document US 2002/0153472 already describes a technique for correcting images projected using liquid crystal projection means, in which a light sensor configuration is mounted laterally, at the rear side or at the front side on the projector housing in order to detect the brightness and color of ambient light and subsequently to change the projection behavior of the liquid projection means. Here, the light sensor configuration consists of a red sensor, a blue sensor and a green sensor, the total incident light being detected. Similar to the other correction techniques described before, a global adjustment in respect of light conditions generally prevailing in the room is accordingly performed also with this technique for correcting brightness and color of the projected images. Here, a dynamic correction is provided insofar as the brightness of the projected images is intensified when a specific background brightness at the projection surface (or in the room generally) is ascertained by means of the light sensor configuration, and moreover a general color balancing is performed, for example, by a reduction in the red color component in the projected image and an enhancement of the green and blue components if the projection surface has an overall reddish color. This correction of brightness and color is carried out across the entire image area. The document US 6 483 537 B describes a correction method, in which the image area occupied by a projected image on the projection surface is subdivided into sub-areas, either the entire image area being illuminated by the projector means and the sub-areas being detected individually one after another with an image sensor, or the sub-areas being individually illuminated in succession and the entire image area being detected by means of an image sensor, for example, a CCD camera. This subdivision of the image area into sub- areas enables compensation signals or correction signals for correcting distortions to be obtained only for the individual sub-areas, so that correction of distortions is only possible area by area. It should be mentioned here that, color artifacts often occur in the case of projectors at an advanced stage of service life, for instance, yellow spots in the case of LCD projectors, which cannot be corrected by the known techniques. Moreover, in the case of projectors, dust deposits on all of the optical components of the projector can occur in the course of operation; these dust deposits interfere considerably with the quality of the image, which is a drawback.

OBJECT AND SUMMARY OF THE INVENTION In the above-mentioned context, it is an object of the invention to produce a remedy and to propose a method as well as a system for changing the projection behavior of image projection means, by means of which local imperfections, such as local scattered light or local color spots on the projection surface can also be corrected. Furthermore, it is an object of the invention to allow improved color dynamics and brightness dynamics, in order thus to make allowances for the actual conditions when projecting images in conventional living rooms and office rooms not ideal for projection of images. Apart from local imperfections or color spots on the projection surface, it shall also be possible to take defects and color artifacts in the optical systems of the image projection means into consideration. To achieve the above-described object, inventive features are provided in a method according to the invention, so that a method according to the invention can be characterized in the manner specified below, namely: Method for changing the projection behavior of image projection means, which image projection means are designed to generate pixels and which image projection means contain reproduction information that represents specific reproduction values of pixel parameters, in which method the method steps cited below are carried out, namely: projection of images onto a projection surface by means of the image projection means on the basis of the reproduction information, during which projection there are obtained, firstly, projected pixels associated with the reproduction information and, secondly, respective projected images comprising the projected pixels, and recording by means of image recording means of at least a part of each projected image provided for recording, during which recording there are obtained, firstly, recorded pixels having specific recording values of the pixel parameters and, secondly, for each projected image provided for recording, a recorded image comprising the recorded pixels and, thirdly, recording information associated with the recorded pixels and representing the specific recording values of the recorded pixels, and assigning of projected pixels and recorded pixels to one another on the basis of a geometrical relation between on the one hand the projected pixels of each projected image provided for recording and on the other hand the recorded pixels of each recorded image, and derivation of analysis information on the basis of, firstly, reproduction information associated with the projected pixels and, secondly, recording information associated with the recorded pixels obtained by recording the projected pixels, and corrective adjustment of the reproduction information contained in the image projection means on the basis of the analysis information. To achieve the object described above, inventive features are provided in a system according to the invention, so that a system according to the invention can be characterized in the manner specified below, namely: System for changing the projection behavior of image projection means, which system contains the means specified below, namely: image projection means, which image projection means are designed to generate pixels and in which image projection means there is contained reproduction information that represents specific reproduction values of pixel parameters and with which image projection means projection of images onto a projection surface is possible, during which projection there are obtained, firstly, projected pixels associated with the reproduction information and, secondly, respective projected images comprising the projected pixels, and image recording means for recording at least a part of each projected image provided for recording, during which recording there are obtained, firstly, recorded pixels having specific recording values of the pixel parameters and, secondly, for each projected image provided for recording, a recorded image comprising the recorded pixels and, thirdly, recording information associated with the recorded pixels and representing the specific recording values of the recorded pixels, and assignment means for assigning projected pixels and recorded pixels to one another on the basis of a geometrical relationship between on the one hand the projected pixels of each projected image provided for recording and on the other hand the recorded pixels of each recorded image, and analyzing means for deriving analysis information on the basis of, firstly, reproduction information associated with the projected pixels and, secondly, recording information associated with the recorded pixels obtained by recording the projected pixels, and calibration means for corrective adjustment of the reproduction information contained in the image projection means on the basis of the analysis information. With the solutions according to the invention, for each pixel to be projected, that is, also for groups of pixels or the totality of pixels, of a projected image, a correction or compensation or adjustment of undesirable and disruptive influences on pixel parameters, such as color, brightness, definition and the like, is rendered possible, so that local correction of color defects or brightness defects or variations in definition ensures a substantially higher image quality compared with the prior art. In this connection, it is also not a problem if the image projection means and the image recording means, for example, an LCD panel in the projection means and a CCD camera or a CMOS image sensor in the image recording means, have different resolution values. A further advantage is that it is not essential to have especially high resolution values for good quality image projections and image recordings, because the quality is already very good even at comparatively low resolution values if the color resolution or rather the color tone resolution is correspondingly good. Normally, therefore, for example for home cinema applications, for PowerPoint projections or photo projections, resolution values of the order of magnitude of one (1) megapixel will be quite sufficient, reasonably priced CCD cameras having such resolution values already being available. Mostly, the resolution values in the case of the image projection means compared with the resolution values of the image recording means will be somewhat greater, a ratio of, for example, 1.2 : 1 up to a ratio of 1.5 : 1 being possible in practice. This means that the projected pixels in the projected image cannot be allocated on a "one to one" basis to the recorded pixels in the recorded image, so that a corresponding conversion via a transformation matrix is necessary for allocation. This conversion does not, however, demand a particularly high arithmetic complexity and can be accomplished on the basis that those area portions of the recorded pixels in the recorded image that are material to the respective projected pixel in the projected image are take into account appropriately weighted. The result of this may be, for example, that a 60% proportion of a recorded pixel in the recorded image and a 5% proportion of an adjacent recorded pixel in the recorded image and a further 5% or 10% proportion of a further adjacent recorded pixel in the recorded image is material to or significant to a projected pixel in the projected image, a standardization to one (1) expediently being carried out. Preferably, for all pixels in a projected image a correction calculation and a correction adjustment in respect of brightness and color as well as in respect of distortion are carried out, so that a restitution as well as especially optimized color dynamics and brightness dynamics are achievable. This is explained in even more detail in the following. Let it merely be mentioned here that the brightness to be generated with the image projection means per pixel and its color (this brightness will be different for different pixels and different colors) is limited by two factors, namely, on the one hand by the ambient brightness that is normally present, namely in contrast to an absolutely dark room, and on the other hand by the maximum admissible brightness that may be set in order still to be able to create a completely homogeneous and orthochromatic image. Between these two limits, in the solutions according to the invention the respective color is adjusted in its brightness for each pixel; in the simplest case, a linear interpolation can be performed for the resulting "compression" (compared with the limits under ideal conditions). Control of the brightness per pixel and color in the image projection means in accordance with a non-linear compression curve is also conceivable, however. Altogether, a multiple, flexible correction per pixel in the projected image is therefore possible, wherein brightness and color tone respectively contrast as well as distortions can be corrected ("compensated"), even if this is necessary only in very small local areas within the projected image, for instance because of local color spots or scattered light spots on the projection surface of because of defects or anomalies in the projection means themselves. Another advantage is that because of the analysis information obtained after the image recording, focusing of the image projection means can be performed automatically, so that manual focusing is unnecessary. Normally, a projection device for projecting images has a control unit in order to influence the image projection and color reproduction by manually performed inputs. In the case of the solutions according to the invention, a control unit is associated with the image projection means, which control unit furthermore includes the calibration means. The calibration means facilitates a pixel-by-pixel control of the image projection means, in order to achieve a corrective adjustment of or influence on the reproduction information contained in the image projection means at least during a projection. These calibration means are connected to analyzing means, which analyzing means are associated with the image recording means in order to allow a pixel-by-pixel acquisition of analysis information. As already mentioned, when effecting assignment of the projected pixels of the projected image to recorded pixels of the recorded image, the relationships can actually be arbitrary, and therefore, apart from different resolution values, also different optical systems for the image projection means on the one hand and for the image recording means on the other hand can be provided, so that these two means can be operated completely independently of one another. With regard to simple operation of the image projection means and the image recording means, these two means are accommodated in a common housing. It is conceivable or thinkable to connect the two optical systems of the image projection means and the image recording means fixedly with one another. The image projection means and the image recording means can even have a combined optical system, although a beam splitter in the beam path is then necessary, which allows through the light used for image projection onto the projection surface but diverts the light coming back from the projection surface to the image recording means and the image sensor thereof. If in the case of the combined optical system additionally the resolution values of the actual image generating means, for example, an LCD panel or a DLP chip, and the actual image recording means, for example a CCD camera, a CMOS chip etc., are of the same magnitude, a direct assignment of the projected pixels of the projected image to recorded pixels of the recorded image is possible, that is, a "one-to-one assignment". As has already been mentioned above, the conversion involved in the assignment is not very complex, however, and consequently in most cases of application the advantage of independence from image projection means and image recording means with separate optical systems outweighs the advantage of a combined optical system for image projection means and image recording means. For an especially simple and rapid conversion during the pixel-by-pixel assignment, it is an advantage if the image recording means and the image projection means have approximately the same resolution values, such as, for instance, 1.2 to 1.5 megapixels respectively 1 to 1.2 megapixels. One solution according to the invention can be designed so that the image projection means are automatically activated in a test run such that successive test images are projected in the primary colors red, blue and green (with maximum brightness).

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention are apparent from and will be lucidated, by way of non-limitative example, with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows schematically the arrangement of image projection means and image recording means of a system according to the invention in front of a projection surface. Fig. 2 illustrates schematically, in a view from the front, an image projected by image projection means onto a projection surface and in correlation therewith the image recorded by the image recording means. Fig. 3 shows schematically, in the form of a block diagram, an exemplary embodiment of a system according to the invention. Fig. 4 shows, to an enlarged scale, a cut-out from the projected image and from the recorded image shown in Fig. 2, with the respective pixels being illustrated schematically. Fig. 5 shows, in a different cut-out, a group of adjacent pixels of the recorded image, wherein in addition to the particular pixel under consideration also adjacent pixels are taken into account in a calibration, in fact preferably in accordance with a local Gaussian curve illustrated schematically in Fig. 5 in relation to this cut-out. Fig. 6 illustrates schematically in a graph the adaptation of the brightness per pixel and primary color. Fig. 7 shows a modified embodiment, along the lines of Fig. 3, of a system according to the invention. DESCRIPTION OF EMBODIMENTS Fig. 1 shows schematically a system 1 for changing the projection behavior of image projection means 3. The image projection means 3 are designed to generate pixels. The image projection means 3 contain reproduction information WI, which reproduction information WI represents specific reproduction values of pixel parameters. The reproduction information WI is formed by data blocks, each data block represents a reproduction value, such as a color value, a brightness value, a focus value and the like, of a pixel parameter, such as a color, the brightness, the image definition and the like. The image projection means 3 are provided and designed for projecting images P onto a projection surface 2, during which projection projected pixels (P_(X, _y)) associated with the reproduction information WI are obtained. In the present case, the reproduction information WI is supplied to the image projection means 3 from an external data source, and is contained at least during projection in the image projection means 3, further details of this being given below. Let it be mentioned here that the image projection means 3 can already contain the reproduction information WI from the outset. As shown in Fig. 1, the system 1 contains image projection means 3 projecting in the form of pixels and image recording means 4 recording in the form; of pixels and adjusting means 5 in a controlled system 6 between the image recording means 4 and the image projection means 3. These pixels have a square form with a specific side length of the square. Such pixels may have a different form, for example, a circular form of a specific diameter or a rectangular form with a first rectangle side length that is the same as the side length of the square, and a second rectangle side length that is longer than the side length of the square. The image projection means 3 and the image recording means 4 each have their own optical system 7 respectively 8, in order to project a particular image P onto the projection surface 2 - as per first light rays 9 - respectively to record an associated image C on the projection surface 2, as is illustrated with second light rays 10 in Fig. 1. In the plan view of Fig. 1, the projected image P has upper edge points PI and PH. Correspondingly, the recorded image C has upper edge points CI and CH. The image projection means 3 and the image recording means 4 are in this case housed jointly in a projection device 11, which is indicated in Fig. 1 by broken lines. The adjusting means 5 in the controlled system 6 are also provided inside the projection device 11. It should be mentioned that the system 1 may comprise image projection means 3 and image recording means 4 that are separately constructed, which projection means 3 and recording 4 are then connected with one another over a section of the controlled system 6, the adjusting means 5 preferably being located in the image projection means 3. The image P projected by the image projection means 3 onto the projection surface 2 and the image C recorded with the image recording means 4 are illustrated in Fig. 2 in a view from the front, wherein it is apparent that the projected image P can have local imperfections 12; these imperfections 12 can be caused, for example, by light spots or color spots on the projection surface 2 or by uneven points 12' (cf. also the plan view illustration in Fig. 1) or for example by patterns, such as patterns of the room decor, on a wall surface. Especially when the projection surface 2 is constituted by a wallpapered wall, such imperfections 12 caused by the wallpaper pattern are present locally in the projected image P. Without further measures, the local imperfections 12 impair the quality of the projected images P, which can be very vexing for a user of the system 1 wishing to view such projected images P. The corner pixels Pi, Pn, Pm and Piv of the projected image P are furthermore shown in Fig. 2, and it is moreover apparent that the projected image P is distorted, that is, is not exactly rectangular corresponding to the dotted lines 13 and 14 in Fig. 2, these dotted lines together with the two lines between the corner pixels Pi, Pπ, Pm and Piv producing a rectangle. Coordinates xι,yι and xn,yπ, in each case in brackets, associated with the corner pixels Pi and Pπ respectively, are indicated in Fig. 2. Each pixel P(_{Xj y}) has coordinates x, y in the main plane of the projection surface 2. With regard to the image recording means 4, it should be noted that the image recording means 4 are provided and designed to record at least part of each projected image P provided for recording. In such a recording process by means of the image recording means 4, recorded pixels _{Xj y}) are obtained with specific recording values of the pixel parameters. Furthermore, for each projected image P provided for recording, a recorded image C comprising the recorded pixels C_{(Xι y}) is obtained. Furthermore, recording information Al is obtained, which represents the specific recording values of the recorded pixels C_(X, _y) and is associated with the recorded pixels C(_X, _y The relationship between the pixels P(_{Xι y}) projected with the image projection means 3 and the pixels C(_{Xj y)} recorded with the image recording means 4 is explained in detail below. Before this, with reference to Fig. 3 a preferred embodiment of a system 1 according to the invention with image projection means 3 and image recording means 4 and adjusting means 5 is explained in detail. According to Fig. 3, the image projection means 3 and the image recording means 4, as also illustrated in Fig. 1, have separate optical systems 7 and 8, which are illustrated in Fig. 3 in each case simply by a lens element with associated ray path, that is, with the light rays 9 and 10 respectively. Separate optical systems 7 and 8, as shown in Figs 1 and 3, are advantageous compared with a construction with a shared optical system, as will be explained in detail hereinafter with reference to Fig. 7, insofar as the optical path in the image projection means 3 is not adversely affected by other optical elements otherwise necessary for beam splitting. On the other hand, to realize the image recording means 4 there now exist economically and technically extremely advantageous devices, for instance a CCD camera or a CMOS chip, which are normally also already equipped with their own optical system. These known image recording devices have, for example, a resolution of about one (1) megapixel, which is wholly satisfactory for the present application. The possible disadvantage that separate optical systems 7 and 8 cannot be coordinated with one another sufficiently well can also be compensated for during assignment of the pixels _X, _y) recorded with the image recording means 4 to the pixels P(_{x> y}) projected with the image projection means 3. It is moreover conceivable that that even if separate optical systems 7 and 8 are present, these need not be absolutely separate but can be kept mechanically or electromechanically synchronous: this applies in particular to the image focusing and/or to the focal width setting and/or to any correction of parallax. A parallax correction can also be approximately achieved by inclining the image recording means 4 in dependence on the image focus, distance information being obtained by the image focus setting. A mechanical or electromechanical coupling of the separate optical systems 7 and 8 is not absolutely necessary, however, rather, a separate adjustment of image focus, focal width etc can be effected; these adjustments can be evaluated mathematically and can be taken into consideration during assignment of the recorded pixels C_{(Xj y}) to the projected pixels P(_X,_y>. Compared with this, a mechanical or electromechanical coupling of the optical systems is technically relatively complicated and expensive. According to Fig. 3, the image-projection means 3 comprise in conventional manner an LCD panel (a liquid crystal display panel) 15 and a light source 16. A digital signal, that is, the reproduction information WT, is fed to the LCD panel 15 from an LCD driver circuit 17, which LCD driver circuit 17 in its turn receives the image signal from a scaling device 18. The scaling device 18 serves to scale or process a digital or digitized image signal, for which purpose it is connected with a control input to a control unit 19. The control unit 19 can be formed, for example, by a microcontroller, which comprises memory means (not shown more specifically in Fig. 3), such as RAM memory means and ROM memory means. Control of the scaling unit 18 (also known as a "pixel machine") by the control unit 19 is effected by way of a UART interface connection 20 (UART - Universal Asynchronous Receiver-Transmitter), the corresponding interfaces SCI (Serial Communication Interface) also being indicated in Fig. 3. The control unit 19 controls the image projection means 3 by way of a first BUS system 21. An input stage 22 is activated by the control unit 19 by way of a second BUS system 23. It should be mentioned that the first BUS system 21 and the second BUS system 23 can be different from one another or can be coupled with the one another. The input stage 22 can be, for example, a video signal (VS) input stage, which is designed to receive an analog video signal VS from a video signal source and generates a digitized video signal that is supplied to the scaling unit 18. In the exemplary embodiment according to Fig. 3, the image projection means 3 are accordingly provided and constructed for the projection of video images, wherein these video images can be components of a video film or can be individual images. It goes without saying that it is possible to project different kinds of images P onto the projection surface 2 by means of the image projection means 3, for instance, digital- form tables, documents or photographs. Input means in the form of a keypad input unit 24 and an infrared receiver 25 are furthermore connected to the microcontroller forming the control unit 19. The infrared receiver 25 is part of a remote control device, which is not illustrated more specifically. The keypad 24 and the infrared receiver 25 can be used to supply control and navigation signals. By means of the microcontroller forming the control unit 19, additional means for generating control information for control or adjustment of image parameters, such as brightness, color tone and color dynamics are formed. For that purpose, calibration means 26, which at the same time can be regarded as part of the adjusting means 5 and details of which are given hereinafter, are formed in the control unit 19. In the present case, the calibration means 26 contain assignment means 26a, which assignment means 26a are constructed to assign the individually projected pixels P(_X, _y) of the projected image P and the recorded pixels C(_{X> y}) of the recorded image C to another mathematically, wherein for this purpose a transformation matrix is determined by geometrical calculation which also takes account of what projected pixels P_{(X; y)} are masked by what recorded pixels C_(X, _y) and to what extent. This is explained in detail in the following with reference to Fig. 4. The image recording means 4 comprise a two-dimensional image sensor 27, which is formed by a CCD camera. It should be mentioned that the image sensor 27 can likewise be formed by a CMOS chip (CMOS - Complementary Metal Oxide Semiconductor) or by a DLP chip (DLP - Digital Light Processing), especially also by a so-called Foveon sensor. Connected to this image sensor 27 are evaluating and analyzing means 28, which are likewise part of the control means 5, for which the control system 6 from the image recording means 4 to the image projection means 3 is realized. In the case of the construction according to Fig. 3 (unlike that according to Fig. 7), the mutual assignment of the projected pixels P(_X, _y) and the recorded pixels C(_{Xj y)} need not be effected directly, since the image recording means 4 are realized independently of the image projection means 3 and their separate optical systems 7 and 8 are not coupled with one another, apart from the fact that the resolution values of the image projection means 3 (namely, the LCD panel 15) and the image recording means 4 (namely, the image sensor 27) are also not perforce identical. As a rule, the two resolution values can correspond in magnitude and amount to approximately one (1) megapixel, but the resolution value of the image projection means 3 will usually be somewhat higher than the resolution value of the image recording means 4, in fact approximately in a ratio of 1.2:1 to 1.5:1. Added to this is the fact that - as is also apparent from Fig. 2 - the projected image P and the recorded image C are not necessarily the same size, but the image C recorded by the image sensor 27 is somewhat larger than the image P projected by the LCD panel 15. It follows from this, however, that even with identical resolutions a "one to one" assignment of projected pixels P_(X, _y) to recorded pixels C(_X, _y) and vice versa is not possible. In this case too, a conversion is required for mutual assignment of the projected pixels P(_x, _y) and the recorded pixels C_(X, _y). The transformation matrix used for this conversion is determined afresh each time, preferably in separate calibration processes that precede the desired image projection processes. This pixel conversion for mutual assignment of the respective pixels has the advantage that the image projection means 3, specifically in the exemplary embodiment shown the LCD panel 15, and the image recording means 4, specifically the image sensor 27 formed by a CCD camera etc., can easily be chosen with resolutions independent of one another. Furthermore, there is no need to couple the optical systems 7 and 8 with one another. Finally, there is also no need to arrange the optical systems 7 and 8 locally close together, for instance, to minimize parallax errors. For mutual assignment of the projected pixels P_{(x> y)} and the recorded pixels C(_{Xj y}), in the course of a calibration process to begin with an image P is projected by the image projection means 3 on the basis of the reproduction information WI onto the projection surface 2, during which projection firstly projected pixels P(_X)3,) associated with the reproduction information WI and secondly respective projected images P comprising the projected pixels P_(X, _y) are obtained. By means of the image recording means 4, this projected image P provided for recording is recorded and determined in respect of magnitude, position and form, during which recording there are obtained, firstly, recorded pixels C_{(Xj y)} having specific recording values of the pixel parameters and, secondly, for the projected image P provided for recording, a recorded image C comprising recorded pixels C(_{X; y)} and, thirdly, recording information (Al) associated with the recorded pixels (C(_{X) y})) and representing the specific recording values of the recorded pixels (C(_X, _y)). For calibration purposes, the projected image P is generated such that all projected pixels P_(X, _y) have their maximum brightness value. Preferably, three projected images P are generated in succession, in a respective of the primary colors red, green and blue. Fig. 4 illustrates schematically and by way of example, how the individually projected pixels P(_x, _y), for instance the projected pixels Pi, ι, Pi, ₂ etc., are to be assigned to the recorded pixels C(_X,_y), that is, Ci, i, Cι,₂ etc. As is evident, the image projection means 3 have a higher resolution, since the projected pixels P(_X, _y) indicated essentially by squares are smaller than the recorded pixels _x, _y), likewise indicated essentially by squares. It is also evident that the projected pixels P(_{Xj y}) and the recorded pixels _x, _y) are present in columns and rows, corresponding to the pixels of the LCD panel 15 and image sensor 27 organized in columns and rows. It is important in the present case — as already mentioned above with reference to Fig. 2 - that the image P projected onto the projection surface 2 always lies entirely within the sensor area, that is, within the image C recorded by means of the image recording means 4 and the image sensor 27, in fact at every zoom setting of the image projection means 3 and independently of the projection distance, that is, of the spacing of the projection surface 2 from the image recording means 3. Once the coordinates of the image incident upon the image sensor 27, that is, the coordinates of the recorded image C, have been acquired or "measured", the assignment to one another or the mutual assignment of the projected pixels P(_x,_y) and the recorded pixels C_{(X, y)} is effected mathematically on the basis of a geometrical relationship between on the one hand the projected pixels (P(_{X> y})) of each projected image (P) provided for recording and on the other hand the recorded pixels (C(_X, _y)) of each recorded image (C). For that purpose, a transformation matrix is determined by geometrical calculations, wherein in particular it is determined how each projected pixel P(_X, _y) is overlapped by several recorded pixels C_{(X> y)}, that is to say how it is "seen" or "recorded", the extent to which each projected pixel P_{(X, y)} is overlapped by each overlapping recorded pixel C_{(x> y}> being determined. If the example according to Fig. 4 is taken as a basis, it is apparent that the projected pixel Pi, i is to be assigned only to the recorded pixel Ci, ι, the overlap being given by about 0.6 (that is to say 60%) area sections of the recorded pixel Ci, i. The recorded pixels Ci, i and , ₂ are assigned to the projected pixel Pι_{j 2}, the overlap being given by about 0.05 area sections of the recorded pixel Pi, and about 0.55 area sections of the recorded pixel C_2> i. The assignments to the recorded pixels _{x> y)} are determined in this form for all projected pixels P(_X, _y), each projected pixel P_{(X; y}) being assigned in the transformation matrix an area- weighted proportion of each recorded pixel C(_x, _y) overlapping the relevant projected pixel P(_x, _y>. In the course of this assignment on the basis of the transformation matrix thus created, allowances can be made for nonlinear distortions as well, for example cushion- form and barrel-form distortions. Furthermore, the technology according to the invention with the pixel-by-pixel assignment and hence pixel-by-pixel correction allows prevention or at least reduction of a wide variety of image errors in the projected images P, it being possible in particular to prevent local imperfections 12 in projected images by correction. For example, if the projection surface 2 in the case of a home cinema application is a wall papered with patterned wallpaper, the disruptive influence of this wallpaper pattern on projected images can be prevented or reduced, that is, "suppressed". In this way, the quality of the projected image P for the user or viewer can be considerably improved, wherein color reproduction, brightness dynamics and contrast can be optimized. For that purpose, in the course of a static preliminary calibration, provision is preferably made to record, that is, to acquire, by means of the image recording means 4 in each case first a projected image P with a maximum brightness value (light output of 100%) in the three primary colors red, blue and green and then a projected image P with a minimum brightness value (light output 0%, that is, no projection or projected image P). On the basis of the resulting data sets, - which data sets are obtained by means of the analyzing means 28 from analysis information on the basis of reproduction information WI associated with the projected pixels P(_X, _y) and on the basis of recording information Al associated with recorded pixels C(_{X> y}> obtained by recording the projected pixels P_{(Xj y}), brightness and color tone being used as pixel parameters in the present case, and therefore the data sets, that is to say the reproduction information WI and recording information Al, in the present case corresponding to the limits of brightness, that is a maximum brightness value and a minimum brightness value - a corresponding adjustment of the brightness and color tone in the range defined by the limit values is achieved for each projected pixel P(_{Xj y}) in the projected image P. In this connection, according to Fig. 3 a corresponding conversion stage 29 is provided in the region of the scaling unit 18, which conversion stage 29 is controlled by the control unit 19 and in particular by the calibration means 26 thereof. Here, in the range between the upper and lower brightness limit values (per primary color) it is possible to carry out a linear interpolation, but as and when required a non- linear compression curve can be provided for that purpose. In particular, several different compression curves can be predefined, and by switching by way of the input means 24 or 25 the user can select one of these compression curves in the particular case. Such a non-linear compression curve 30 is illustrated in Fig. 6. If, on the other hand, a linear interpolation is sought, then this can be carried out in accordance with the linear curve 31 in Fig. 6. For the rest, the minimum brightness value A_mi_n and the maximum brightness value A_max (per primary color), as these values are detected by the image recording means 4, are plotted in Fig. 6. Furthermore, for the theoretical limits there are illustrated the values zero (0) (for absolute darkness) and 255 (for a theoretically maximum brightness lying above Amax, which would only be possible, however, under ideal conditions, that is, in an absolutely dark room having an absolutely neutral, white and uniform projection surface 2 with no imperfections, with no color artifacts in the optical system 7 of the image projection means 3 and with no vignetting). In that case, an unshortened curve (straight line) 32 would also be effective as the ideal curve. Owing to the ever-present ambient brightness and also owing to different local imperfections on the projection surface, lower and upper limits are to be set for the brightness, that is to say the control range is to be compressed, in order thus to be able to create a completely homogeneous and orthochromatic image (on the basis of the control of the individual pixels P(_X, _y)). The result is the "compression" apparent from Fig. 6 between the brightness values A_mj_n and A_max instead of between the values zero (0) and 255 (these values zero (0) to 255 having been selected as an example of an 8 bit representation of the brightness control, that is to say, the minimum value of the brightness control corresponds to the value zero (0), corresponding to 2°, and the ideal maximum value of the brightness corresponds to the value 255 (corresponding to 2⁸ - 1). For the rest, in Fig. 6 the desired brightness value is plotted on the x-axis and the brightness value actually delivered as a result of the appropriate correction (in each case per primary color and per pixel) is plotted on the y-axis. In other words, the x-axis represents the desired reproduction information WI relating to the particular projected pixel P(_{Xj y}) in one of the particular primary colors, that is to say that particular pixel-brightness- value in the respective primary color that would be assigned to the image projection means 3 without the correction described here, although then, owing to the ambient conditions and defects etc., without additional measures this desired pixel brightness (reference pixel brightness) would not actually be achieved in the projected image. To optimize presentation of the image, namely allowing for the local imperfections at the projection surface 2, an adjustment is therefore made, corresponding to the curve 30 or the curve 31, to the brightness values actually to be delivered, namely by corrective adjustment of the reproduction information WI. For example, for each pixel P(_X, _y) to be projected, six (6) calibration variables can be determined as follows: R_mi_n - the minimum brightness value to be set in (primary) red; G_mi_n - the minimum brightness value to be set in (primary) green; B_mi_n - the minimum brightness value to be set in (primary) blue; R_max - the maximum brightness value to be set in (primary) red; G_max - the maximum brightness value to be set in (primary) green; B_max - the maximum brightness value to be set in (primary) blue. Altogether, a three-dimensional matrix is therefore produced, in which the elements x and y specify the position of the respective projected pixel P(_x, _y) and an element z = one (1) to six (6) specifies the preceding calibration variables and adjustment variables. In other words, for each pixel six values are involved for definition of the dynamic range, namely, the minimum brightness value and the maximum brightness value for each of the three primary colors. This can also be interpreted as writing down six two-dimensional matrices for specifying the minimum and maximum brightness values per primary color for the particular pixels in order to obtain the basis for the calibration or adjustment to be performed. In order to determine the individual matrix elements, advantageously in a static preliminary calibration for each individual primary color (red; blue; green) in succession firstly the maximum brightness value is established such that the zone (seen by the image recording means 4) appearing darkest in the projected image P is used as a reference. Then the brightness of the pixels of the image projection means 3, with the exception of those in this darkest zone, that is the reference zone, is adjusted so that the reference level is reached. For that purpose, a relaxation algorithm can be used, which keeps the adjustment time (calculation time) of the adjustment as brief as possible, the brightness values of the remaining pixels (reference zone excepted) being adapted in an iterative process. This adaptation is effected first of all, for example, in one step from 100% to 50% of the brightness value, a new image then being recorded by means of the image recording means 4. Thereupon, depending on whether the remaining pixels appear darker or lighter than the reference zones, the brightness value is set to a value of 75% (for "too dark") or 25% (for "too light") of the original value (the maximum value), and again an image is recorded with the image recording means 4, after which a comparison with the reference zone is made again. Depending on whether the remaining pixels still appear lighter or darker than the reference zone, an adjustment to 12.5%, 37.5%, 62.5 or 87.5 % of the original value (the maximum value) is carried out. After six such iterations, the brightness values can be set with an accuracy of better than 0.8%, which is satisfactory. In this way, for each primary color the actual maximum brightness value A_max is obtained for each pixel. Allowances can consequently be made for local imperfections, spots, etc. in the projection surface 2, so that after correction has been made, in a subsequent actual image projection a high-contrast image that is homogeneous in respect of brightness and color tone is obtained. For a "white balance", homogeneous test images are then projected in succession in the primary colors red, green and blue with maximum brightness values onto the projection surface 2, and each time the brightness values are determined per pixel by means of the image recording means 4. This may optionally also occur generally over the entire image area, that is, a pixel-by-pixel coverage is not absolutely necessary, in fact, for example, when merely ambient light influences have to be corrected or balanced. Also, if the image sensor 27 has good color insensitivity, a single measurement at this maximum brightness can be carried out, for example, in one primary color or with a mixture of all primary colors. If specific brightness values or brightness value distributions are obtained for the three primary colors, the weakest brightness value, for example for the color red, is used as reference, and the brightness values for the two other colors, for example, blue and green, are correspondingly reduced, in order to adapt these more strongly projected colors to the weakest projected color as reference color. The factors thus obtained are used for fine adjustment of the matrix elements of the above-mentioned three-dimensional transformation matrix. Thirdly, to determine the minimum brightness value A_min, a "black" test image is produced by means of the image projection means 3, wherein the image recording means 4 do not really "see" a completely black image, which is attributable to scattered light, ambient light etc., that is to say "background light". In particular, this "background light" can also be inhomogeneous, that is, there may be a pattern of brighter and less bright zones in the projection surface 2. In the recorded image, the brightest zone is now established as reference zone by analysis in the analyzing means 28, and, in a comparable relaxation algorithm, as explained above for determining the maximum brightness value A_max, the minimum brightness value A_mi_n is now established for all three primary colors red, green and blue. In other words, here the brightest region is taken as reference zone, and the other pixels are correspondingly brightened in order to obtain a uniform minimum brightness value in the primary colors red green and blue for all pixels. Instead of using the described relaxation algorithm, which after seven iterations leads to a uniformly bright image accurate to ± 0.8%, so-called over-relaxation algorithms can be used. Whereas in the first case upon each iteration only the conclusion as to whether a pixel is lighter or darker than a reference value is reached, in the latter case how much of the respective pixel is (probably) too light or too dark is taken into account, and a corresponding - approximated - adaptation is carried out. This process achieves its aim usually after basically few iterations. Also possible is a so-called "dynamic" calibration, which is carried out during (instead or in place of before) the actual image or video projection. However, the calibration variables are then determined not on the basis of ideal test images, but on the basis of a comparison of actual and desired values, which is performed on the basis of a comparison of images occurring in practice, that is, on the basis of a comparison of the "real" projection material (film, still images or data), as the image recording means 4 see it, with the desired values, that is to say, desired reproduction information, of the respective images, present in the scaling unit 18 or the image projection means 3. In this connection, possibly not all imperfections in the projection surface 2 will be determined and corrected or adapted within a given time frame, since not all regions of the projection surface 2 are illuminated with sufficiently different colors. Accordingly, an accumulation of measurement data, that is, the data recorded by the image recording means 4, over a relatively long period of time is also necessary, and this involves a relatively slow determination of the coefficients (calibration variables). It should be mentioned that for this dynamic calibration a relatively high computer power and a relatively large memory requirement might be needed, compared with the static preliminary calibration. From Fig. 5 it is apparent that during the mutual pixel assignment it is possible to take into consideration not only in each case the individual pixel P(_{X) y}) actually in question, according to the multiple overlap evident from Fig. 4, but also to draw on other, adjacent pixels. These adjacent pixels are, however, included to a lesser extent in the calibration, this additional influencing factor due to the adjacent pixels being added to the previously determined value, namely, on the basis of the assigned pixels as shown in Fig. 4. The influencing factors (area assignments) as a whole per pixel P(_{x> y}) then have to be standardized to "1" again, of course. This additional influence can preferably be determined from a local Gaussian distribution, a Gaussian bell curve, as indicated schematically in the lower half of Fig. 5. This inclusion of the adjacent pixels enables the accuracy of the correction achievable by the adjustment (feedback) to be increased, if scattering and blurred effects occur in the never quite perfect optical systems 7 and 8. Fig. 7 illustrates a modified embodiment of the system 1 according to the invention, which corresponds largely to the embodiment according to Fig. 3, so that a repeated description of the identical parts can be omitted and the description of the modified embodiment according to Fig. 7 is limited to the differences from the construction according to Fig. 3. The same reference numerals are used for identical units and modules. In the embodiment according to Fig. 7, only one optical system 7 is provided for both the image projection means 3 and the image recording means 4. The mathematical assignment of the recorded pixels _x, _y> to the projected pixels P(_X, _y) on the basis of a transformation matrix explained in conjunction with the system according to Fig. 3 and with reference to Figures 2 and 4 to 6 can therefore be dispensed with, since the projection by the image projection means 3 and the recording by the image recording means 4, conditional on the system, relates always exactly to the same region (in respect of zoom and sharpness, that is to say focus). Here, however, an additional optical element in the form of a beam splitter 33 has to be used in the optical path, preferably between the LCD panel 15 and the optical system 7; this additional optical element allows the projection light coming from the light source 16 and controlled via the LCD panel 15 to pass through to the optical system 7 and hence to the projection surface, not shown in Fig. 7, yet guides the light coming from the projection surface and likewise passing through the optical system 7 to the image sensor 27, as is indicated in Fig. 7 by the arrow 34. It is understood that the image sensor 27 is to be aligned accordingly. For the rest, the system 1 according to Fig 7 is of the same construction as that according to Fig. 3. Provided that the resolution values are identical, in the system 1 according to Fig. 7 the assignment of the projected pixels P(_x, _y) and the recorded pixels _x, _y) is advantageously effected virtually directly, without conversions being required. In the case of the system 1 according to Fig. 7 care should be taken, however, that the additional optical element, that is to say, the beam splitter 33, causes as far as possible no light loss and artifacts in the optical path of the image projection means 3, which can be ensured by means of a very high quality additional optical element. The invention is explained above with reference to especially preferred exemplary embodiments, but further variations and modifications are of course possible within the scope of the invention. Thus, as the information source for the image projection, instead of a video signal VS supplied by a video recorder or a TV antenna circuit, it is possible to use a computer, for example a personal computer, in order for data already present in digital form to be projected in the form of characters or images. Instead of a video film, individual images can also be projected, such as may originate from a photo CD for instance. Another possibility for using the control path 6 in indicated in Fig. 3: focus adjusting means 35 can be associated with the image projection means 3 or rather more accurately with the optical system 7 of the same, in order to move the optical system 8 by motor for the focusing. These focus adjustment means 35 are preferably controlled by the control unit 19 and generally by the adjustment means 5, in order to effect automatic focusing of the optical system 7 on the basis of the analysis information obtained by means of the image recording means 4. One advantage of the described calibration technique is also that the service life of the image projection means 3 is extended, since impairment of quality (for instance as a consequence of heating up, light-stress etc.) in the case of the optical elements (for example polarizers, filters, analyzers) are automatically compensated. For example, LCD projectors have a tendency to produce local yellow spots, in fact usually initially in the middle of the image, which is attributable to the relatively rapid and inhomogeneous ageing of the blue system. In the case of the above-described solutions, in each case an entire projected image (P) provided for recording is actually recorded by means of the image recording means. It should be expressly pointed out that it is possible actually to record by means of the image recording means just a part, for example only the central part, of a projected image (P) provided for recording, thus advantageously enabling shorter adjustment processes and correction processes to be achieved. As a parameter value of the pixels - as described above - the brightness value of the pixels and the color value or the three color values in accordance with the three primary colors red, blue and green can be used. It is possible to utilize the area value of the pixels, which area value is dependent on the focusing. The parameter value utilized in each case is pixel-dependent, that is to say of different magnitude in the case of different pixels.

Claims

CLAIMS:

1. A method of changing the projection behavior of image projection means (3), which image projection means (3) are designed to generate pixels and which image projection means (3) contain reproduction information (WI), which represents specific reproduction values of pixel parameters, in which method the method steps cited below are carried out, namely: projection of images (P) onto a projection surface (2) by means of the image projection means (3) on the basis of the reproduction information (WI), during which projection there are obtained, firstly, projected pixels (P(_X,_y>) associated with the reproduction information (WI) and, secondly, respective projected images (P) comprising the projected pixels (P(_X,_y)), and recording by means of image recording means (4) of at least a part of each projected image (P) provided for recording, during which recording there are obtained, firstly, recorded pixels ( _x,y)) having specific recording values of the pixel parameters and, secondly, for each projected image (P) provided for recording, a recorded image (C) comprising the recorded pixels ( _x, y>) and, thirdly, recording information (Al) associated with the recorded pixels (C(χ,y)) and representing the specific recording values of the recorded pixels (C(_x,_y)), and assignment of projected pixels (P_{(X; y)}) and recorded pixels (C_{(Xj y})) to one another on the basis of a geometrical relationship between on the one hand the projected pixels (P(_X,_y)) of each projected image (P) provided for recording and on the other hand the recorded pixels (C(_X, _y)) of each recorded image (C), and derivation of analysis information on the basis of, firstly, reproduction information (WI) associated with the projected pixels (P(_{x> y})) and, secondly, recording information (Al) associated with the recorded pixels (Q_{X) y})) obtained by recording the projected pixels (P(_X, _y)), and corrective adjustment of the reproduction information (WI) contained in the image projection means (3) on the basis of the analysis information.

2. A method as claimed in claim 1, wherein a corrective adjustment of the reproduction information (WI) that represents the reproduction value of the brightness for the pixels selected as pixel parameter is carried out.

3. A method as claimed in claim 2, wherein a corrective adjustment of the reproduction information (WI) that represents the reproduction value of the brightness for each of the primary colors red, blue and green is carried out.

4. A method as claimed in claim 3, wherein a maximum brightness value (A_max) to be set and a minimum brightness value (A_ram) to be set for each projected pixel (P(_x, _y)) of each projected image (P) provided for recording are determined as reference values on the basis of the darkest recorded pixels (C(_X, _y)) respectively the lightest recorded pixels (C(_{X> y}>) in each recorded image (C).

5. A method as claimed in claim 4, wherein a respective brightness value of the individual projected pixels (P(_X, _y)) of each projected image (P) provided for recording is set in accordance with a compression curve (30; 31) running between the minimum brightness value (A_mi_n) and the maximum brightness value (A_max).

6. A method as claimed in claim 1, wherein during the derivation - carried out on the basis of, firstly, reproduction information (WI) associated with the projected pixels (P(_X,_y>) and, secondly, recording information (Al) associated with the recorded pixels (C(_x, _y)) obtained by recording the projected pixels (P(_x,_y)) - of the analysis information for a specific projected pixel (P_{(X] y)}), in addition to the reproduction information (WI) associated with the specific projected pixel (P(_x,_y>) also the reproduction information (WI) associated with adjacent projected pixels (P(_x, _y)) is included.

7. A method as claimed in claim 6, wherein the reproduction information (WI) associated with the adjacent projected pixels (P(_X, _y)) is included in accordance with a local Gaussian distribution.

8. A method as claimed in claim 1, wherein the images projected during a normal projection of images onto the projection surface are used as projected images (P) provided for recording.

9. A method as claimed in any one of the preceding claims, wherein the assignment to one another of the projected pixels (P(_X, _y)) and the recorded pixels (C(_X, _y)) is additionally used for correction of distortions in the projected images (P).

10. A system (1) for changing the projection behavior of image projection means (3), which system contains the means specified below, namely: image projection means (3), which image projection means (3) are designed to generate pixels and in which image projection means (3) there is contained reproduction information (WI) that represents specific reproduction values of pixel parameters and with which image projection means (3) a projection of images (P) onto a projection surface (2) is possible, during which projection there are obtained, firstly, projected pixels (P(_x, _y)) associated with the reproduction information (WI) and, secondly, respective projected images (P) comprising the projected pixels (P(_X, _y)), and image recording means (4) for recording at least a part of each projected image (P) provided for recording, during which recording there are obtained, firstly, recorded pixels (C_{(X; y)}) having specific recording values of the pixel parameters and, secondly, for each projected image (P) provided for recording, a recorded image (C) comprising the recorded pixels (C(_Xj _y)) and, thirdly, recording information (Al) associated with the recorded pixels (C(_X, _y)) and representing the specific recording values of the recorded pixels (C(_X, _yχ), and assignment means (26a) for assigning projected pixels (P(_x, _y)) and recorded pixels (C(_X, _y)) to one another on the basis of a geometrical relationship between on the one hand the projected pixels (P(_X, _y)) of each projected image (P) provided for recording and on the other hand the recorded pixels (C_(X,_y)) of each recorded image (C), and analyzing means (28) for deriving analysis information on the basis of, firstly, reproduction information (WI) associated with the projected pixels (P(_{x> y})) and, secondly, recording information (Al) associated with the recorded pixels (C(_{X; y)}) obtained by recording the projected pixels (P_{(X> y)}), and calibration means (26) for corrective adjustment of the reproduction information (WI) contained in the image projection means (3) on the basis of the analysis information.

11. A system (1) as claimed in claim 10, wherein a control unit (19) is associated with the image projection means (3), which control unit (19) furthermore includes the calibration means (26).

12. A system (1) as claimed in claim 10, wherein the image projection means (3) and the image recording means (4) comprise optical systems (7, 8) separate from one another.

13. A system (1) as claimed in claim 10, wherein the image projection means (3) and the image recording means (4) comprise a combined optical system (7').

14. A system (1) as claimed in claim 10 or 12 or 13, wherein the image projection means (3) and the image recording means (4) are accommodated in a common housing (11).

15. A system (1) as claimed in claim 10, wherein the image recording means (4) comprise a CCD camera as image sensor (27).

16. A system (1) as claimed in claim 10, wherein the image recording means (4) and the image projection means (3) each have predetermined resolution values and the two resolution values are at least approximately similar.

17. A system (1) as claimed in claim 11, wherein the control unit (19) is designed to control the image projection means (3) automatically, such that the image projection means (3) project successive calibration images in the primary colors red, blue and green onto the projection surface (2).

18. A system (1) as claimed in claim 11, wherein the image projection means (3) comprise focus adjusting means (35) and the control unit (19) is designed to activate the focus adjusting means (35) on the basis of the analysis information for automatic focus adjustment.

19. A projection device (11), wherein the projection device (11) is provided with a system (1) as claimed in any one of claims 10 to 18.