KR100902419B1 - Apparatus and method for image processing in capable of displaying captured image without time delay, and computer readable medium stored thereon computer executable instruction for performing the method - Google Patents

Abstract

An image processing apparatus and method capable of displaying a captured image without time delay are provided. In the present invention, the video signal processing module sequentially outputs not only the image data for storing the captured image, but also the display image data to the multimedia application processing module, and the multimedia application processing module stores the image data for storage in the memory. Display image data is displayed on the display unit. The display image data is already processed in the image signal processing module in accordance with the format of the display unit, and in particular, since no compression coding is performed, the multimedia application processing module can immediately display the captured image on the display unit without time delay.

Digital Image Processing, ISP, MAP, Captured Image Display, Time Delay, Preview Image

Description

Apparatus and method for image processing in capable of displaying captured image without time delay, and computer readable medium stored thereon computer executable instruction for performing the method}

The following drawings attached to this specification are illustrative of preferred embodiments of the present invention, and together with the detailed description of the invention to serve to further understand the technical spirit of the present invention, the present invention is a matter described in such drawings It should not be construed as limited to.

1 is a block diagram of an image processing apparatus according to a preferred embodiment of the present invention.

FIG. 2 is a detailed block diagram of the image signal processing module shown in FIG. 1.

3 is a flowchart illustrating an image processing process according to an exemplary embodiment of the present invention.

4 is a timing diagram illustrating a process of transmitting image data according to a preferred embodiment of the present invention.

5 is a flowchart illustrating an image processing process according to an exemplary embodiment of the present invention in the continuous capture mode.

The present invention relates to image processing, and more particularly, to an image processing apparatus and method capable of displaying a captured image without time delay.

Recently, digital cameras using an image sensor such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) have been widely used. Digital cameras are commercially available as camera-only products, but also in combination with hand-held terminals such as mobile phones and PDAs.

In the handheld terminal, however, the clock speed and the memory capacity of the central processing unit are not superior to those developed for the personal computer. In addition, since the development direction of the terminal is oriented to a thin thickness and a small size, there are a lot of space constraints in mounting an additional device such as a camera. On the other hand, despite these spatial constraints, the number of pixels of a digital camera mounted in a handheld terminal is gradually increasing to 3 million pixels or more, for example. Therefore, the image processing apparatus faces a demand to process a large amount of data in a short time under spatial constraints.

A general image processing apparatus includes an image sensor for capturing an image and an image signal processing module for converting raw image data in analog form received from the image sensor into digital data and converting the image data into image data suitable for a general image format. ), A multimedia application processing module for storing video data received from the video signal processing module in a storage medium, and displaying a video, and for displaying a preview video or a captured video. It has a display unit such as a view finder. The image sensor, the image signal processing module, and the multimedia application processing module are typically implemented in separate chip forms, and are mounted in an image processing device (a digital camera or a handheld terminal) together with, for example, a liquid crystal display module constituting a display unit.

In the operation of the image processing apparatus configured as described above, the analog image raw image data captured by the image sensor is converted into digital data by the image signal processing module, and the general image is processed through color correction, gamma correction, and color coordinate system conversion. It is converted into video data suitable for the format. The digital image data converted by the image signal processing module is transmitted to the multimedia application processing module, and the multimedia application processing module compresses and encodes the received image data according to a predetermined standard such as JPEG encoding and stores it in a memory such as an SDRAM. The image data stored in the memory is decoded and displayed on the display unit.

However, as the number of pixels of the digital camera mounted in the handheld terminal increases, the amount of data to be processed by the multimedia application processing module also increases, and thus the processing speed of the multimedia application processing module does not keep up. For example, when a multimedia application module needs to compress-code, store, and display image data input at a high speed of 10 frames per second or more in a high pixel imaging apparatus of 3 million pixels or more, image data of one frame may be displayed. During encoding, data of the next frame may be input, in which case high-speed data interfacing becomes unstable, such as data collision. To solve this problem, the clock frequency of the multimedia application module must be increased considerably, which is technically impossible to implement. Therefore, in the related art, the clock frequency of the image signal processing module is reduced in accordance with the limit of the clock frequency of the multimedia application processing module. However, this leads to the deterioration of the image quality.

On the other hand, in order to solve such a problem, a method of performing compression encoding in the video signal processing module used in the conventional multimedia application processing module may be used. That is, an encoding unit for compression encoding is added to the image signal processing module in addition to the pre-processing blocks for the conversion or correction performed by the conventional video signal processing module to encode the image captured by the image sensor. At this time, when the image data of the next frame is input during encoding for one frame, it is skipped or the vertical synchronization signal V_sync indicating the start of input of the next frame is delayed to prevent data collision during encoding. Meanwhile, the image data encoded by the image signal processing module is transmitted to the multimedia application processing module, stored in a memory or decoded, and displayed on the display unit.

According to this method, however, when the captured image is displayed on the display unit by the multimedia application processing module, the captured image must be decoded first and generally down-scaled to match the resolution of the display unit lower than the resolution of the stored image. Therefore, considerable time is required until the display of the captured image. Therefore, this method can solve the instability of data interfacing due to high pixelation, but it cannot meet the user's demand for quick confirmation of the captured image and rapid capture of the next image. In particular, the slow display of the captured image is unnaturally apparent as the capture and display of the image are delayed at a time interval in the continuous shooting mode that continuously captures the image within a short time. Commercialization becomes substantially difficult.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems of the prior art, and an object thereof is to provide an image processing apparatus capable of quickly displaying a captured image while resolving instability of high-speed data interfacing due to high pixels.

Another object of the present invention is to provide an image processing method capable of quickly displaying a captured image while resolving instability of high-speed data interfacing due to high pixels.

Still another object of the present invention is to provide a computer-readable recording medium in which an image processing method capable of quickly displaying the captured image is programmed.

In order to achieve the above technical problem, in the present invention, the image signal processing module sequentially outputs not only the image data for storing the captured image, but also the display image data to the multimedia application processing module, so that the multimedia application processing module stores the image data for storage. Is stored in the memory, and the display image data is displayed on the display unit. Since the image data for display is already processed by the video signal processing module according to the format of the display unit, the multimedia application processing module does not need to perform a separate operation for displaying the captured image on the display unit. The display of the captured image is immediately performed without time delay.

Specifically, the image processing apparatus according to an aspect of the present invention, the original image processing unit for processing the captured image captured by the image sensor in accordance with a preset storage image data format; A display image processor configured to process the captured image according to a format of the display unit; And an image output unit configured to sequentially output image data processed by the original image processor and image data processed by the display image processor. And a multimedia application processing module for storing the image data processed by the original image processing unit output from the image output unit in a memory and displaying the image data processed by the display image processing unit on a display unit.

In addition, an image processing method according to another aspect of the present invention is an image processing method in a capture mode in an image processing apparatus including an image sensor, an image signal processing module, a multimedia application processing module, and a display unit. Processing, by the processing module, a captured image captured by the image sensor according to a preset storage image data format; (b) processing the captured image according to the format of the display unit in the image signal processing module; (c) in the image signal processing module, sequentially outputting the image data processed in the step (a) and the image data processed in the step (b) to the multimedia application processing module; And (d) storing, by the multimedia application processing module, the image data processed in the step (a) received from the image signal processing module in a memory, and displaying the image data processed in the step (b) on the display unit; It includes.

Still another technical problem of the present invention can be achieved by a computer-readable recording medium in which the image processing method according to the present invention is programmed and recorded.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms or words used in the specification and claims should not be construed as having a conventional or dictionary meaning, and the inventors should properly explain the concept of terms in order to best explain their own invention. Based on the principle that can be defined, it should be interpreted as meaning and concept corresponding to the technical idea of the present invention. Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

The image processing apparatus according to the present invention is used in combination with various digital image photographing apparatuses. Here, the digital image photographing apparatus includes a digital camera, a digital camcorder, a mobile phone equipped with a digital camera, a PDA, a personal multimedia player, and the like, and acquires an image of a subject according to a user's shutter operation, converts the image into a digital image, and stores the image. Refers to a device that performs a function of storing in a medium.

1 is a block diagram of an image processing apparatus according to a preferred embodiment of the present invention.

Referring to FIG. 1, the image processing apparatus according to the present embodiment includes an image sensor 100, an image signal processing module 200, a multimedia application processing module 300, a storage medium 400, and a display unit 500. do.

The image sensor 100 captures an image of an object and outputs an analog raw image signal to the image signal processing module 200. Preferably, the image sensor 100 is an imaging device configured of a Charge Coupled Device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS). However, the present invention is not limited by the type of the image sensor 100.

The image signal processing module 200 receives an analog raw image signal output from the image sensor 100, converts the analog raw image signal into a digital image signal, and processes the image signal according to the present invention to output to the multimedia application processing module 300. Specifically, the image signal processing module 200 of the present embodiment, as shown in FIG. 2, includes a preprocessor 210, an original image processor 220, a display image processor 230, and an image outputter 240. do.

The preprocessor 210 converts the analog raw video signal into a digital video signal, converts color coordinate systems such as YUV and RGB as necessary, and performs normal video signal processing such as color correction, gamma correction, and noise removal. Here, the preprocessing refers to a process performed before the stored image processing and the display image processing according to the present invention to be described later, and the processing performed by the preprocessing unit 210 is not directly related to the features of the present invention. Since the preprocessing is known to those skilled in the art as a process by a general video signal processing module known as an ISP (Image Signal Processor) in the technical field to which the present invention belongs, the detailed description thereof will be omitted.

The original image processing unit 220 is a general image to be stored in the storage medium 400 by the multimedia application processing module 300, which will be described later by the image sensor 100 and the captured image preprocessed by the preprocessor 210. Function block that processes data according to its format.

In detail, the original image processor 220 includes a stored image scaler 221. The stored image scaler 221 scales the captured image preprocessed by the preprocessor 210 according to a resolution standard (eg, 640 * 480, etc.) preset by the user or defaulted by the image capturing apparatus.

In addition, the original image processing unit 220 may include a JPEG encoder 223 as an encoding unit that compresses and encodes the captured image scaled by the storage image scaler 221. That is, the JPEG encoder 223 is provided inside the video signal processing module 200 without providing the inside of the multimedia application processing module 300, thereby storing image data for storage to be stored in the storage medium 400. By compressing at 200, the data is sent to the multimedia application processing module 300 to reduce the data rate to stabilize data interfacing between the image signal processing module 200 and the multimedia application processing module 300. On the other hand, in the present embodiment, although the encoding unit is shown and described as compression encoding according to the JPEG standard, the present invention is not limited to the JPEG encoding scheme.

The display image processor 230 is configured to display the image data to be displayed on the display unit 500 by the multimedia application processing module 300 which will be described later by the image sensor 100 and the captured image preprocessed by the preprocessor 210. Function block to process according to the format.

In detail, the display image processor 230 includes a display image scaler 231 and a display image buffer 233. The display image scaler 231 scales the captured image preprocessed by the preprocessor 210 according to the size of the display unit (eg, 320 * 240) implemented by the viewfinder of the image capturing apparatus. The display image buffer 233 temporarily stores the image data scaled by the display image scaler 231.

Although not shown, the display image processor 230 may further include an encoder (eg, a JPEG encoder) that compressively encodes the image scaled by the display image scaler 231. In this case, in addition to the above-described storage image data, the display image data is also compressed and sent to the multimedia application processing module 300, thereby further interfacing data between the image signal processing module 200 and the multimedia application processing module 300. It can be stabilized.

The image output unit 240 stores the storage image data compression-coded by the JPEG encoder 223 of the original image processing unit 220 and the scaled (or compressed-coded) display by the display image scaler 231. Image data for output to the multimedia application processing module 300. In this case, the storage image data and the display image data may be output in various ways. In the present embodiment, the storage image data and the display image data are sequentially output.

Referring back to FIG. 1, the multimedia application processing module 300 receives storage image data and display image data from the image signal processing module 200 (actually the image output unit 240), and stores the image for storage. The data is stored in a storage medium 400 such as, for example, SDRAM, and the image data for display is displayed on the display unit 500 made of, for example, an LCD module.

3 is a flowchart illustrating an image processing process according to an exemplary embodiment of the present invention, and FIG. 4 is a timing diagram illustrating a process of transmitting image data according to an exemplary embodiment of the present invention. Referring to these drawings, the image processing method of the present invention will be described in detail as follows.

Commercially available digital video recording devices, unlike traditional cameras, support a preview function that previews objects to be included in the captured video through the viewfinder. In other words, when the user turns on the digital image capturing apparatus (or operates in the camera mode) for capturing, the user enters the preview mode and the image of the subject is displayed through the viewfinder as a moving image that is changed at short frame intervals. Then, when the user operates the shutter when the desired image is captured, the user enters a capture mode to capture a digital still image of the subject. The present invention relates to an operation in a capture mode. In the preview mode, steps S40, S50, S60, and S80 are not executed in FIG. 3, and only the preview image is processed as a display image and displayed on the display unit. In FIG. 4, VSYNC is a vertical synchronization signal indicating the start of each frame, and in the preview mode, the image signal processing module 200 and the multimedia application processing module 300 process the preview image implemented in each frame image in synchronization with the VSYNC. Will be displayed.

In the preview mode, the image captured by the image sensor 100 or the image data processed by the image signal processing module 200 may be an image having the maximum size (resolution) supported by the image sensor 100 or the photographing apparatus. As the photographing apparatus becomes high pixel, a lot of time is required for processing the preview image. In order to solve this problem, there is a method of widening the frame interval, but since it does not become a natural video, it is common to operate at a low resolution even if the image quality is somewhat deteriorated. On the other hand, the image captured in the capture mode is an image of the maximum size supported by the image sensor 100 or the photographing apparatus or an image of a size preset by the user. In addition, in the capture mode, the flash may operate or the exposure time may be different. As a result, the image displayed in the preview mode and the image captured in the capture mode may be different.

When the user enters the capture mode by operating the shutter, the image sensor 100 captures an image of a subject at a predetermined resolution and outputs an analog raw image signal to the image signal processing module 200 (S10). The image signal processing module 200 then processes the analog raw image, in particular a slight time delay in preprocessing and buffering until encoding is started by the JPEG encoder 223 and until the encoded storage image data is output. This inevitably occurs. Accordingly, the multimedia application processing module 300 may not receive the storage image data and the display image data before the next vertical synchronization signal that operates in synchronization with the multimedia application processing module 300, and may discard one frame. Accordingly, when the image is captured, the VSYNC signal is delayed by the delay time d to synchronize the image signal processing module 200 and the multimedia application processing module 300 with the changed VSYNC.

Subsequently, the preprocessor 210 of the image signal processing module 200 receives an analog raw image signal output from the image sensor 100 and performs analog-to-digital conversion, color coordinate conversion, color correction, gamma correction, noise removal, and the like. The series of preprocessing described above is performed (S20).

The image data preprocessed by the preprocessor 210 is input to the stored image scaler 221 of the original image processor 220 and the display image scaler 231 of the display image processor 230. Then, the display image scaler 231 scales the captured image preprocessed by the preprocessor 210 according to the size (eg, 320 * 240) of the display unit of the image capturing apparatus (S30) and the display image buffer 233. The scaled image data is temporarily stored.

Meanwhile, the stored image scaler 221 scales the captured image preprocessed by the preprocessor 210 according to a resolution standard (for example, 640 * 480, etc.) preset by the user or defaulted by the image capturing apparatus (eg, 640 * 480). S40). Subsequently, the captured image scaled by the stored image scaler 221 is compressed and encoded by the JPEG encoder 223 (S50).

Subsequently, the image output unit 240 outputs the storage image data compressed and encoded by the JPEG encoder 223 and the display image data scaled by the display image scaler 231 to the multimedia application processing module 300. . In this case, the storage image data and the display image data may be output in various ways. In the present embodiment, the storage image data and the display image data are sequentially output. That is, the storage image data from the JPEG encoder 223 is first outputted (S60), and when the output of the storage image data is completed, the display image data is read from the display image buffer 233, and then the multimedia application processing module. Output to 300 (S70). Here, the output order of the storage image data and the display image data may be changed. In addition, the length of the storage image data and the display image data may be variable or fixed, respectively. If fixed, dummy data may be added to match the length.

In addition, the video signal processing module 200, when the storage video data and the display video data output by the video output unit 240 exceeds one frame period, the vertical synchronization signal VSYNC _{k +} indicating the start of the next frame. You can also skip ₁ or have a delay such as VSYNC ^' _{k +1} . In this case, dummy data may be added to the next vertical sync signal VSYNC _{k +2} or the delayed vertical sync signal VSYNC ^' _{k +1} from the end of the output image data.

The multimedia application processing module 300 receiving the storage image data and the display image data from the image output unit 240 stores the image data for storage in a storage medium 400 such as SDRAM (S80). The display image data is displayed on the display unit 500 formed of, for example, an LCD module (S90). Although the image data for display is described as not separately stored, the image data for display may also be stored in a predetermined storage area. Here, the storage area of the display image data may be provided inside the multimedia application processing module 300 or may be provided inside the storage medium 400. Such storing of display image data is particularly useful in the continuous capture mode described later.

Meanwhile, as described above, the display image processor 230 may further include an encoder (eg, a JPEG encoder) that compressively encodes the image scaled by the display image scaler 231, or the JPEG encoder of the original image processor 220. The display image data may also be compressed and encoded using 223. In this case, data interfacing between the image signal processing module 200 and the multimedia application processing module 300 may be further stabilized. However, since the multimedia application processing module 300 needs to decode and display the compressed and encoded display image data on the display unit 500, it takes more time than the display of the uncompressed display image data. Since the size is much smaller than the normal storage image, the decoding time is short, so the time delay felt by the user is not so much a problem. Rather, the compression-coded data of the small-sized display image can be usefully used as a thumbnail image.

The above description is about a process of capturing and displaying one still image, but the present invention can be very usefully applied to a continuous capture mode, that is, a mode of capturing a plurality of images continuously at short time intervals. This will be described in detail with reference to FIG. 5 as follows.

First, the capture from one capture image to the display on the display unit is the same as described with reference to steps S10 to S90 of FIG. 3. In the continuous capture mode, in addition to this, it further includes the following steps S100 to S140.

The display image data is displayed on the display unit (S90) and stored in the display image storage area described above (S100).

Subsequently, it is determined whether the continuous capturing has been completed, that is, whether a predetermined number of image capturings have been performed (S110). If not, the storage capturing step (S100) of display image data is repeated in the image capturing step (S10). That is, by immediately displaying the captured image on the display unit 500 for every captured image, the user can immediately check the captured images, and the display image data is stored in preparation for the user's final selection.

On the other hand, when all the continuous capture is finished, all display image data of the continuous captured image stored in the display image storage area is read (S120).

The display image data thus read is first down scaled to an appropriate size to display a plurality of images on one screen, and then displayed on one screen of the display unit 500 for user selection (S130).

Then, the user selects the capture image that he / she likes most, and the storage image data corresponding thereto is finally stored in the storage medium (S140).

<Simulation example>

Hereinafter, in order to confirm the effects of the present invention, an example of simulating a time taken until the display of the captured image is simulated when only the image data obtained by compressing and encoding the captured image in the image signal processing module according to the conventional method is sent to the multimedia application processing module. do.

In this example, the image sensor uses a 3 million pixel scale and simulates the time until the captured image is decoded and encoded according to the JPEG standard under the following conditions and then the captured image is output to the display unit.

Advanced RISC Machine (ARM) speed: 200 MHz

Cache size: 16KB each of data and code cache RAM size

Bus speed: 100 MHz

Raw file size before 3 million pixels compression: 3M pixels x 2 = 6MBytes (YCbCr 4: 2: 2 requires 2 bytes per pixel)

File size after 3 million pixels compression: Compression rate varies from image to image but typically between 1/4 and 1/8 compression rate, approximately 0.75 Mbytes to 1.5 MBytes.

In these systems, the JPEG file decoding time was found to be at least 600ms. That is, in the conventional method, it takes more than 600 ms to restore a 3 million pixel compressed image to the display unit, and thus, the total capture time felt by the user is very unsatisfactory. On the other hand, in the present invention, since the display image data processed according to the format of the display unit by the display image processing unit is displayed on the display unit as it is, no additional time is required for displaying the captured image, thereby enabling rapid display.

The image processing method according to the present invention described above may be embodied as computer readable codes on a computer readable recording medium. The computer-readable recording medium includes all kinds of recording devices in which data that can be read by a computer system is stored. Examples of computer-readable recording media include ROM, RAM, CD-ROM, magnetic tape, floppy disk, optical data storage, and the like, and may also be implemented in the form of a carrier wave (for example, transmission over the Internet). Include. The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. In addition, functional programs, codes, and code segments for implementing the image processing method may be easily inferred by programmers in the art.

As described above, although the present invention has been described by way of limited embodiments and drawings, the present invention is not limited thereto and is intended by those skilled in the art to which the present invention pertains. Of course, various modifications and variations are possible within the scope of equivalents of the claims to be described.

According to the present invention, the video signal processing module sequentially outputs not only the image data for storing the captured image but also the display image data to the multimedia application processing module, and the multimedia application processing module stores the image data for storage in the memory. Display image data for display. Since the image data for display is already processed by the video signal processing module in accordance with the format of the display unit, and especially its size is very small even if no separate compression encoding is performed, the multimedia application processing module is adapted to display the captured image on the display unit. If no separate decoding is required or necessary, it can be decoded in a short time. Therefore, the display of the captured image is performed immediately without time delay. In addition, according to the present invention, in particular, in the continuous capture mode, since the captured images are displayed immediately, it is possible to quickly check and select the image by the user.

Claims (17)

An image signal processing module for generating the raw image received from the image sensor into image data for storage that has been subjected to compression encoding and image data for display that has not been subjected to compression encoding; And And a multimedia application processing module for controlling the received display image data to be displayed without decoding processing, and the received storage image data to be stored in a memory. And the image signal processing module sequentially outputs the generated storage image data and display image data to a multimedia application processing module that is physically divided through a single interface. delete The image signal processing module of claim 1, And a storage image scaler for scaling the raw image to a size of a preset storage image data. The image signal processing module of claim 1, And a display image scaler for scaling the raw image to a size of a display unit on which the display image data is displayed. The method of claim 1, The memory includes a storage image storage area and a display image storage area, The multimedia application processing module, And the storage image data is stored in the storage image storage area, and the display image data is stored in the display image storage area. The method of claim 5, wherein the multimedia application processing module, In the continuous capture mode in which the capturing operation by the image sensor is performed continuously, the storage image data is continuously stored in the storage image storage region, and the display image data is continuously stored in the display image storage region. And an image processing apparatus for successively displaying on the display unit. The method of claim 6, wherein the multimedia application processing module, And when the continuous capture in the continuous capture mode is completed, all images continuously stored in the display image storage area are read out, downscaled, and displayed on one screen of the display unit. The video signal processing module of claim 1, And if the output image data exceeds one frame period, skip or delay the vertical synchronization signal indicating the start of the next frame. An image processing method in an image processing apparatus including an image sensor, an image signal processing module, a multimedia application processing module, and a display unit, An image data generation step of generating, by the image signal processing module, the raw image received by the image sensor into storage image data that has been subjected to compression encoding processing and display image data that has not been compression encoding processing; An image data output step of the image signal processing module sequentially outputting the generated storage image data and the display image data to the multimedia application processing module physically separated through one interface; And And controlling, by the multimedia application processing module, the received display image data to be displayed without decoding processing, and the received storage image data to be stored in a memory. delete The method of claim 9, wherein the image data generating step, And scaling the raw image to a size of preset image data for storage. The method of claim 9, wherein the image data generating step, And scaling the raw image according to the size of the display unit. The method of claim 9, The memory includes a storage image storage area and a display image storage area, The control step, And the multimedia application processing module stores the storage image data in the storage image storage area, displays the display image data on the display unit, and stores the display image data in the display image storage area. The method of claim 13, wherein the multimedia application processing module, In the continuous capture mode in which the capturing operation by the image sensor is performed continuously, the storage image data continuously received from the image signal processing module is continuously stored in the storage image storage area, and the display image data is stored in the storage image storage area. And storing the data in the display image storage area continuously. The method of claim 14, wherein the multimedia application processing module, When the continuous capture by the continuous capture mode is completed, all images stored in the display image storage area is read out, downscaled and displayed on one screen of the display unit. The method of claim 9, wherein the image data output step, And when the output image data exceeds one frame period, skipping or delaying the vertical synchronization signal indicating the start of the next frame. A computer-readable recording medium having recorded thereon a program for causing a computer to execute the method according to any one of claims 9 or 11 to 16.

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