Patent Publication Number: US-8126286-B2

Title: Method for correcting distortion of image projected by projector, and projector

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a method for correcting distortion of an image projected by a projector, and a projector. 
     2. Related Art 
     Currently, a projector which modulates light emitted from a light source according to inputted image signals to form projection images is known, and used for presentation and home theater. 
     Images projected by the projector are generally formed on a screen. This screen looks flat, but is not completely flat but actually has concaves and convexes. A certain portable winding-type screen or the like has large concaves and convexes. When images are projected on this type of screen, distortion is caused on the images in some cases. 
     For example, when the projector is disposed immediately below the screen for image projection, images are projected on the screen from below toward above. Thus, projection images are considerably distorted in the up-down direction. 
     For eliminating this distortion, JP-A-11-146307 discloses a technology for converting distorted images on a screen into appropriate images by correcting picture images displayed on a display panel. 
     Also, JP-A-2001-83949 discloses a technology for capturing images of projection images formed on a screen having freely curved surface by using a camera disposed at a viewpoint, producing correction data for giving opposite distortion beforehand, and correcting picture images desired to be projected based on the correction data. 
     According to the technology shown in JP-A-11-146307, distortion of relative positions between the screen and the projector can be corrected. However, distortion produced on the screen due to the concaves and convexes of the screen cannot be corrected. 
     According to the technology shown in JP-A-2001-83949, in case of images having partial large distortion, correction patterns for obtaining detailed information about the entire areas of the images are required for correcting images with high accuracy. In this case, processing load increases. 
     SUMMARY 
     It is an advantage of some aspects of the invention to provide a method for correcting distortion of an image projected by a projector, capable of correcting distortion of a projection image highly accurately without increasing processing load, and to provide a projector. 
     According to a first aspect of the invention, there is provided a method for correcting distortion of an image projected by a projector including: a first detection image data producing step which produces first detection image data containing marker images whose flat surface positions in the projection image can be calculated; a first detection image displaying step which displays a first detection image based on the first detection image data; a first image comparing step which compares the first detection image with the first detection image data; an area selecting step which selects an area having larger distortion of the first detection image than distortion in other area based on the comparison; a second detection image data producing step which increases positioning density of the marker images in the selected area to produce second detection image data; a second detection image displaying step which displays a second detection image based on the second detection image data; a second image comparing step which compares the second detection image with the second detection image data; and an image correcting step which corrects the projection image. 
     The detection image data may be various types of data which contains marker images whose positions on a flat surface in the projection image can be calculated. For example, image data containing a plurality of marker images discretely disposed on a flat surface, image data containing grating pattern on the entire image, and image data containing checkered pattern on the entire image may be employed. 
     The image correcting step may be any step as long as it can finally correct distortion of the projection image. The image correction may control concaves and convexes on the screen to correct the distortion of the projection image, or correct image signals inputted to the projector. 
     According to this aspect of the invention, the area having large distortion of the detection image is selected by the area selecting step, and the positioning density of the marker images in the selected area is increased by the detection image data correcting step to obtain more detailed information about the condition of the distortion. Thus, the condition of the concaves and convexes of the screen is recognized with high accuracy, and the degree of correction is easily calculated by recognizing the distortion with high accuracy. Accordingly, correction can be performed by the image correcting step without increasing processing load. 
     It is preferable that the first detection image data is image data containing a plurality of marker images discretely arranged on a flat surface. In this case, the first image comparing step captures an image of the first detection image using an image capturing unit, and calculates distortion between the first detection image data and captured data of the first detection image using statistical method. The second image comparing step captures an image of the second detection image using the image capturing unit, and calculates distortion between the second detection image data and captured data of the second detection image using statistical method. The area selecting step selects an area having distortion equal to or larger than a predetermined threshold. 
     The statistical method may be various types of analysis method such as main component analysis, regression analysis, and multiple regression analysis. 
     In this case, it is preferable that the image correcting step corrects an inputted image signal. 
     According to this structure, an image of the detection image is captured by the image capturing unit, and the detection image data is automatically corrected by an information processing device such as computer or an image processing unit included in the projector. Thus, distortion on the projection image caused by concaves and convexes of the screen can be automatically detected without requiring operation by the observer or the like. 
     Since the inputted image signal is corrected by the image correcting step, the correction process is performed within the information processing device or the image processing unit included in the projector. Thus, a series of processes containing those from detection to correction can be automatically executed. 
     It is preferable that the statistical method employed in the first image comparing step and the second image comparing step is least squares method. 
     According to this structure, distortion is calculated using least squares method generally used in wide application fields. Thus, processing load given to the information processing device or the image processing unit of the projector can be further reduced. 
     It is preferable that a relative distortion calculating step which calculates relative distortion between adjoining marker images is performed between the first image comparing step and the area selecting step, and that the area selecting step also selects an area having relative distortion calculated by the relative distortion calculating step equal to or larger than the predetermined threshold. 
     According to this structure, an area having small distortion with respect to the detection image data but large distortion with respect to an adjoining marker image in the distortion produced in the detection image can be detected. Thus, the accuracy of the distortion correction of the projection image can be further increased. 
     The invention is applicable to a projector, and operations and advantages similar to those in the distortion correction method described above are provided by the projector. 
     According to a second aspect of the invention, there is provided a projector which forms a projection image including: a first detection image data producing unit which produces first detection image data containing marker images whose flat surface positions in the projection image can be calculated; a second detection image data producing unit which increases positioning density of the marker images in selected area to produce second detection image data; a detection image displaying unit which displays a first detection image based on the first detection image data, and displays a second detection image based on the second detection image data; an image capturing unit which captures an image of the first detection image displayed, and captures an image of the second detection image displayed; an image comparing unit which calculates first distortion between captured data of the first detection image captured by the image capturing unit and the first detection image data, and calculates second distortion between captured data of the second detection image captured by the image capturing unit and the second detection image data; an area selecting unit which selects an area having the calculated distortion equal to or larger than the predetermined threshold; and an image signal correcting unit which corrects the inputted image signal based on the correction result of the second detection image data correcting unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a block diagram showing a structure of a projection system according to a first embodiment of the invention. 
         FIG. 2  schematically illustrates a structure of detection image data according to the first embodiment. 
         FIG. 3  is a flowchart showing processes for setting coordinate positions performed by an image comparing unit in the first embodiment. 
         FIG. 4  schematically illustrates a method for calculating distortion by the image comparing unit in the first embodiment. 
         FIG. 5  schematically illustrates a data structure in a memory storing distortion calculated by the image comparing unit in the first embodiment. 
         FIG. 6  schematically illustrates relative distortion in the first embodiment. 
         FIG. 7  schematically illustrates condition where positioning density of marker images is increased by a detection image data correction unit in the first embodiment. 
         FIG. 8  schematically illustrates a correction method for an image signal using an image signal correction unit in the first embodiment. 
         FIG. 9  is a flowchart showing a method for correcting distortion of an image projected by a projector in the first embodiment. 
         FIG. 10  is a block diagram showing a structure of a projector according to a second embodiment of the invention. 
         FIG. 11  is a flowchart showing a method for correcting distortion of an image projected by the projector in the second embodiment. 
         FIG. 12  schematically illustrates a structure of detection image data according to a modified example of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments according to the invention are hereinafter described with reference to the drawings. 
     First Embodiment 
       FIG. 1  illustrates a projection system  1  which executes a method for correcting distortion of images projected by a projector according to a first embodiment of the invention. The projection system  1  includes a computer  2 , a projector  3  connected with an input/output port of the computer  2  such as USB via a cable or the like, and a CCD camera  4 . 
     The computer  2  is basically constituted by a general-purpose computer, including a CPU (central processing unit)  6 , a memory  7 , a user interface  8  such as keyboard and mouse, and a hard disk  9  capable of communicating with one another via a bus  5 . An image processor  10  is connected with the bus  5  such that the image processor  10  can communicate via the bus  5 . 
     The projector  3  is an optical device which modulates image information outputted from the computer  2  according to respective images to form optical images, and projects the optical images on a screen SC. The projector  3  includes a projection control circuit  31 , a liquid crystal panel  32 , a light source device  33 , and a projection lens  34 . 
     The projection control circuit  31  is roughly divided into a block for processing inputted image information, a block for controlling overall inputs and outputs of the projector  3 , a block for controlling drive of the liquid crystal panel  32 , and a block for controlling drive of the light source device  33 . 
     The liquid crystal panel  32  has an image forming area containing a plurality of pixels disposed in matrix to produce gradation display using respective pixels according to the inputted image information. 
     The liquid crystal panel  32  has a structure produced by sealing liquid crystals between a pair of transparent substrates for driving the liquid crystals by TFTs (thin film transistors) provided on one of the substrates. Not-shown entrance side polarization plate and exit side polarization plate are disposed on the light entrance side and light exit side of the liquid crystal panel  32 , respectively. The liquid crystal panel  32  controls orientation of the liquid crystals by the function of the TFTs as switching elements, and produces gradation display corresponding to the image information by controlling the quantity of light released from the exit side polarization plate. Though not shown in  FIG. 1 , the liquid crystal panel  32  has three panels each of which modulates corresponding color light of red, blue, and green lights. 
     The light source device  33  has a light source lamp constituted by a discharge arc tube and a reflector. An integrator lens as equalizing illumination system, and a dichroic mirror as color separating system are disposed on the optical path of light emitted from the light source device  33 . In this structure, the in-plate illuminance of the light emitted from the light source device  33  is equalized, and the equalized light enters the corresponding panels of the liquid crystal panel  32  after separation into red light, green light, and blue light. 
     The CCD camera  4  as image capturing unit captures projection images projected on the screen SC from the projector  3 , and outputs image data of the obtained images to the image processor  10  of the computer  2 . The CCD camera  4  has an image capturing processing circuit  41 , a CCD (charge coupled device)  42 , and an image capturing lens  43  to capture images in response to control commands issued from the computer  2 . 
     The image capturing processing circuit  41  as a section for processing images obtained by the CCD  42  converts the captured data after photoelectric conversion process by the CCD  42  into image data having preferable color reproducibility by performing correcting process such as gamma correcting process and other correcting process for color unevenness, luminance unevenness, and other conditions based on LUT. 
     The CCD  42  is a section which detects light quantity of optical images received via the image capturing lens  43  and performs photoelectric conversion to produce image signals. The CCD  42  contains a plurality of detection pixels arranged flat. 
     The image processor  10  has a detection image data production unit  11  as a program operating on a calculating device, a detection image display unit  12 , an image comparing unit  13 , an area selection unit  14 , a detection image data correction unit  15 , and an image signal correction unit  16 . 
     The detection image data production unit  11  is a section which produces detection image data used for correcting distortion of projection images of the projector  3  caused by the distortion of the screen SC. 
     The detection image data production unit  11  produces detection image data G 1  shown in  FIG. 2 , for example. In this embodiment, the detection image data G 1  has a structure containing marker images p 1 , p 2 , p 3 , and others discretely arranged on a black image, and sets coordinates p 1  (x 1 , y 1 ), p 2  (x 2 , y 2 ), p 3  (x 3 , y 3 ), and others as the positions of the marker images p 1 , p 2 , p 3 , and others with the origin O put at the left upper end of the detection image data G 1 . 
     The detection image display unit  12  is a section for displaying the detection image data G 1  produced by the detection image data production unit  11  as a projection image of the projector  3 . The detection image display unit  12  outputs the detection image data G 1  to the projector  3 , and the projector  3  displays the detection image data G 1  as a projection image. An image of the projection image projected by the projector  3  is captured by the CCD camera  4  discussed above, and given to the image processor  10  as captured data. 
     The image comparing unit  13  is a section for comparing the captured data with the detection image data G 1  to calculate distortion of the projection image. The image comparing unit  13  which initially needs to obtain the coordinates of the captured data performs the process shown in the flowchart in  FIG. 3 . 
     Initially, the image comparing unit  13  obtains pixels having the maximum luminance and the minimum luminance in the captured data, and sets the intermediate luminance of the maximum and minimum as threshold to provide binary values for all the pixels in the captured data based on the threshold (process ST 1 ). 
     Then, the image comparing unit  13  obtains areas where the maximum luminance continues in the binary image, and labels each of the areas (process ST 2 ). 
     Finally, the image comparing unit  13  determines the maximum luminance as coordinates of the area for each of the areas of the captured data sectioned by the labeling process (process ST 3 ). In setting the coordinates, the geometrical center position or the center of gravity may be used as coordinates based on the contours of the respective areas labeled using the binary image. 
     After the coordinate positions of the marker images in the captured data are established, the image comparing unit  13  compares the marker images of the captured data with those of the detection image data G 1  using projective transformation. According to an example shown in  FIG. 4 , a point pn of the detection image data G 1  has coordinates (xn, yn), and a point Pn in the captured data corresponding to the point pn has coordinates Pn (Xn, Yn). When projective transformation is completely performed on the screen SC having no concave and convex, the correspondence between the points pn and Pn is expressed in the following equation (1) using projective transformation matrix A. 
     
       
         
           
             
               
                 
                   
                     
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     When the screen SC has concaves and convexes or the like, a part of the positions of the marker images are shifted to distort the image as indicated by an area A 1  shown in  FIG. 4 . 
     In this case, the image comparing unit  13  calculates difference between the coordinates of the respective marker images in the captured data and projective transformation coordinates P′n (X′n, Y′n) obtained using the above equation (1) to calculate distortion DXn (horizontal direction) and DYn (vertical direction). More specifically, the image comparing unit  13  calculates the relationship between the projective transformation coordinates and the coordinates of the captured data using least squares method. 
     When all the distortions DXn, DYn between the captured data of the marker images P 1 , P 2 , P 3 , and others and the projective transformation coordinates are calculated, the image comparing unit  13  stores the distortions DXnr DYn of the respective marker images P 1 , P 2 , P 3 , until Pn in a table T 1  shown in  FIG. 5  in the memory area of the memory  7 . 
     Then, the image comparing unit  13  produces relative distortion as the sum of the distortion DYn of the received DXn, DYn and distortion DYr of the adjoining marker image on the right side of the DYn for each of the marker images P 1 , P 2 , P 3 , until Pn, and produces relative distortion as the sum of the distortion DXn and distortion Dxu of the adjoining marker image positioned below to store the relative distortion in the table T 1 . 
     The distortion DYr of the adjoining marker image positioned on the right side and the distortion DXu of the adjoining marker image positioned below are added to correct large distortion ΔD produced between the relative positions of the marker image Pn and the adjoining marker image Pr due to waves of the screen SC or the like even when the distortion between the marker image Pn and the marker image P′n after projective transformation is small as illustrated in  FIG. 6 . 
     The area selection unit  14  is a section for selecting parts having larger distortion in the captured data based on the distortion DXn, DYn and the relative distortion DYn+DYr, DXn+DXu calculated by the image comparing unit  13 . 
     According to the selection method of the area selection unit  14 , a screen requiring selection by an operator of the computer  2  is displayed for the operator. 
     The detection image data correction unit  15  adds marker images P+ in the area A 1  selected by the area selection unit  14  to increase the positioning density of the marker images for correction as a detection image data G 3  shown in  FIG. 7 . In this embodiment, the density of the marker images P+ in the selected area A 1  is doubled, but the density may be higher or arbitrarily set by the operator of the computer  2 . 
     The image signal correction unit  16  is a section for correcting the inputted image signal based on the distortion DXn, DYn and the relative distortion DYn+DYr and DXn+DXu stored in the table T 1  retained in the memory  7 . More specifically, when distortion obtained from the captured data is in the condition of image data G 4 , the image signal is corrected using data containing reversely deformed distortion as in image correction data G 5  illustrated in  FIG. 8 . The specific correction can be easily calculated from the distortion DXn, DYn and the relative distortion DYn+DYr and DXn+DXu stored in the table T 1  retained in the memory. 
     The method for correcting distortion of images projected by the projector according to this embodiment is now described with reference to a flowchart shown in  FIG. 9 . 
     Initially, the detection image data production unit  11  produces the detection image data G 1  and outputs the detection image data G 1  to the projector  3  (step S 1 ). 
     The projector  3  displays a detection image on the screen SC based on the inputted detection image data G 1  (step S 2 ). 
     The CCD camera  4  captures an image of the detection image projected on the screen SC in response to a control command issued from the computer  2 , and outputs the captured data G 2  to the image processor  10  (step S 3 ). 
     The image comparing unit  13  displays the detection image data G 1  and the captured data G 2  on the display of the computer  2 , and requires the operator to compare the detection image data G 1  with the captured data G 2  (step S 4 ). 
     The operator judges whether the distortion correction is necessary or not by observing the image data G 1  and G 2  on the display (step S 5 ), and ends the process when the distortion correction is judged unnecessary. 
     When the operator judges that the distortion correction is necessary, the operator selects an area having large distortion (step S 6 ). 
     When the operator determines the area containing large distortion, the detection image data correction unit  15  corrects the detection image data G 3  by adding the marker images P+ as illustrated in  FIG. 7 . 
     The detection image display unit  12  projects and displays the corrected detection image on the screen SC (step S 8 ). 
     The CCD camera  4  captures an image of the corrected detection image projected on the screen SC, and outputs the captures data G 2  to the image processor  10  according to the control command from the computer  2  (step S 9 ). In this case, the range of the corrected detection image data G 3  captured by the CCD camera  4  lies only on the selected area A 1 , and distortion of only the corresponding area is calculated by the image comparing unit  13 . 
     The image comparing unit  13  displays the corrected detection image data G 3  and the image pickup data corresponding to the corrected detection image on the display of the computer  2 , and requires the operator to compare these image data (step S 10 ). 
     When the operator determines that further correction is necessary based on the result of the comparison of these image data (step S 11 ), the process returns to step S 7  to repeat steps from correction of the detection image data. 
     When the operator determines that correction is unnecessary, the image comparing unit  13  sets coordinate positions of the marker images P 1 , P 2 , P 3  until Pn based on the captured data G 2  (step S 12 ). 
     Subsequently, the image comparing unit  13  calculates distortions DX 1 , DX 2 , DX 3  until DXn, and DY 1 , DY 2 , DY 3  until DYn for the marker images P 1 , P 2 , P 3  until Pn, and also calculates the relative distortions DY 1 +DYr, DY 2 +DYr, DY 3 +DYr until DYn+DYr, and DX 1 +DXu, DX 2 +DXu, DX 3 +DXu until DXn+DXu (step S 13 ) to store these distortions in the table T 1  of the memory  7 . 
     The image signal correction unit  16  corrects the image signal based on the distortions DX 1 , DX 2 , DX 3  until DXn, and DY 1 , DY 2 , DY 3  until DYn, and the relative distortions DY 1 +DYr, DY 2 +DYr, DY 3 +DYr until DYn+DYr, and DX 1 +DXu, DX 2 +DXu, DX 3 +DXu until DXn+DXu stored in the memory  7  (step S 14 ), and ends the process. 
     According to this embodiment, the area having large distortion is selected by comparison between the image data G 2  corresponding to the image obtained from the detection image displayed on the screen SC and the detection image data G 1  for correction of the detection image data. By this method, more detailed information about the condition of the distortion is obtained by increasing the density of the marker images P+, and thus the condition of the concaves and convexes of the screen SC is recognized with high accuracy. Accordingly, image signal can be corrected highly accurately, and the processing load can be decreased to the minimum. 
     Moreover, the distortions DX 1 , DX 2 , DX 3  until DXn, and DY 1 , DY 2 , DY 3  until DYn are calculated using least squares method. Thus, the processing load given to the computer  2  can be reduced. 
     Furthermore, the relative distortions DY 1 +DYr, DY 2 +DYr, DY 3 +DYr until DYn+DYr, and DX 1 +DXu, DX 2 +DXu, DX 3 +DXu until DXn+DXu as well as the distortions DX 1 , DX 2 , DX 3  until DXn, and DY 1 , DY 2 , DY 3  until DYn are calculated. Since not only the large distortions in the detection image data G 1  but also the relative distortions relatively large with respect to the adjoining marker images are calculated, distortion correction of the projection image can be achieved with further accuracy. 
     Second Embodiment 
     A second embodiment according to the invention is now described. Similar reference numbers are given to parts similar to those in the above embodiment, and the same explanation is not repeated. 
     According to the first embodiment described above, the image processor  10  which performs the distortion correction method for images projected by the projector of the invention is included in the computer  2  connected with the projector  3 , and outputs the detection image data G 1  and the corrected detection image data G 3  from the computer  2  to the projector. 
     However, the distortion correction method for images projected by the projector in the second embodiment is different from the distortion correction method in the first embodiment in that an image processor  53  and the CCD camera  4  are included in a projector  50  to perform shape distortion assist method for projection images within the projector  50  as illustrated in  FIG. 10 . 
     According to the first embodiment described above, the area to which the marker images P+ are added is selected by displaying the screen which requires the operator of the computer  2  to select by the function of the area selection unit  14  such that the operator can select appropriate area. 
     However, the distortion correction method for images projected by the projector in the second embodiment is different from the distortion correction method in the first embodiment in that an area selection unit  58  judges whether distortion is equal to a certain threshold or smaller to automatically select the area and correct the detection image data as shown in  FIG. 11 . The method according to the second embodiment is now discussed in more detail. 
     As illustrated in  FIG. 10 , the projector  50  includes a light source control circuit  51  constituting a projection control circuit, an image processing circuit  52 , the CCD camera  4 , and the image processor  53 . 
     According to the projector  50  having this structure, frequency of RGB signals inputted through an image input pin  54  such as USB is converted by a converter  55 , for example. The converted RGB signals are accumulated in a frame buffer  56  as image signals by frames, and outputted to an image corrector  57 . The image corrector  57  performs gradation correction, luminance unevenness correction, color unevenness correction, and other correction according to the liquid crystal panel  32  to control drive of the liquid crystal panel  32 . 
     The CCD camera  4  has the CCD  42  and the image capturing processing circuit  41  similarly to the first embodiment. A sensing image processed by the image capturing processing circuit  41  is outputted to the image processor  53 . 
     The image processor  53  has the detection image data production unit  11 , the detection image display unit  12 , the image comparing unit  13 , the area selection unit  58 , the detection image data correction unit  15 , and the image signal correction unit  16  similarly to the first embodiment. 
     The area selection unit  58  is different from the area selection unit  14  in the first embodiment in that the area selection unit  58  automatically selects an area containing distortion larger than a predetermined threshold based on the distortion and relative distortion calculated by the image comparing unit  13 . Then, the detection image data correction unit  15  increases the positioning density of the marker images in the selected area. Whether the distortion is smaller than the threshold is judged based on the judgment whether the distortion between the coordinates (X′n, Y′n) converted by the projective transformation matrix A in the equation (1) in the first embodiment and the coordinates (Xn, Yn) in the captured data satisfies the following equation (2):
 
(threshold)&gt;( Xn−X′n ) 2 +( Yn−Y′n ) 2   (2)
 
     When the distortion correction method for images projected by the projector is performed by using the image processor  53 , the detection image data production unit  11  initially produces detection image data (step S 1 ), and writes the produced detection image data to the frame buffer  56  as shown in  FIG. 11 . Then, the detection image display unit  12  displays a detection image on the screen SC based on the detection image data written to the frame buffer  56  (step S 2 ). 
     Then, the CCD camera  4  captures an image of the projected detection image, and outputs captured data to the image processor  53  (step S 3 ). 
     The image comparing unit  13  sets the coordinate positions of marker images based on the received captured data (step S 12 ), and calculates distortion and relative distortion of the marker images similarly to the first embodiment (step S 13 ). 
     The area selection unit  58  judges whether distortion correction is necessary based on the calculated distortion, relative distortion and the thresholds set for them, respectively (step S 15 ). When it is judged that distortion correction is unnecessary, the process ends. 
     When it is judged that distortion correction is necessary, the area selection unit  58  judges whether distortion and relative distortion are smaller than the predetermined threshold (step S 16 ), and corrects the detection image data for the area having distortion and relative distortion equal to or larger than the threshold (step S 17 ). 
     When the area selection unit  58  determines that distortion and relative distortion in all areas are smaller than the threshold after repeating the above steps, the image signal is corrected to finish the process similarly to the first embodiment (step S 14 ). 
     According to this embodiment, the area having large distortion can be automatically selected by the area selection unit  58  based on the distortion and relative distortion calculated by the image comparing unit  13  in addition to the advantages provided in the first embodiment. Thus, correction of the detection image data can be automatically performed, and distortion correction for the projection image can be achieved without requiring operation by the observer. 
     Moreover, the distortion correction method for the projection image is performed within the image processor  53  included in the projector  50 . Thus, distortion correction for the projection image can be automatically executed by the single body of the projector  50 . 
     Modified Example 
     The invention is not limited to the embodiments described herein, but may be practiced otherwise as in the following modified examples. 
     According to the first embodiment, the detection image data G 1  contains the white marker images p 1 , p 2 , p 3  and others discretely arranged on the black screen. However, a detection image data G 6  having radial grating pattern shown in  FIG. 12  may be employed. In the detection image data G 6 , the coordinate positions need to be specified only at grating points. Moreover, distortion correction for the projection image considering upward projection of the projector can be performed by radially expanding the grating pattern from approximately the center of the lower end of the image data. 
     While the projector  3  having the liquid crystal panel  32  executes the distortion correction method for the projection image of the projector according to the invention in the first embodiment, a projector including a device using micromirror may be employed for performing the distortion correction method. 
     Other structure or the like may be included in place of the specific components and processes discussed herein as long as the advantages of the invention can be provided. 
     The entire disclosure of Japanese Patent Application No. 2008-083394, filed Mar. 27, 2008 is expressly incorporated by reference herein.