Digital Image Processing

Transcription

1 1 Introduction in IMAQ Vision. Structure of Digital Image Processing Applications Using IMAQ Vision 1.1 Introduction A grey level image is a function expressing the spatial variation of the light intensity f(x,y), where x and y represent the spatial coordinates of a point in an image, and f is the brightness of the point (x,y). Each discrete spatial position in a digital image corresponds to a pixel. The number of bits used to represent the brightness of each pixel/the intensity of each color component corresponding to a pixel and the number of planes (i.e., color components) in an image determine the image type. IMAQ Vision supports multiple image types. In the case of grey scale images, one encodes only one plane the brightness, and the brightness of each pixel can be represented on a different number of bits. IMAQ Vision typically encodes each grey level as an 8 bit integer, 16 bit integer or floating point value. Whether one encodes a grey scale image in 8 bits, 16 bits, or in a floating-point value is influenced by several factors: The nature of the image The type of image processing needed The type of analysis to be performed For example, 8-bit encoding is sufficient if one plans to perform a morphology analysis (for example, surface or elongation factor). On the other hand, if the goal is to obtain a highly precise quantification of the light intensity from an image or a region of image, one must use 16- bit or floating point encoding. VIs that perform frequency-domain operations can be applied to images that are Fourier transformed. Each pixel in a Fourier-transformed image, called a complex image, is encoded by a pair of two single-precision floating-point values. One can also process color image. Color images can be stored in IMAQ Vision in either RGB or HSL formats. RGB images store color information using 8 bits each for the red, green and for blue planes. HSL images store color information using 8 bits each for hue, saturation and luminance. Both formats also have an additional unused 8 bits per pixel, yielding for a total of 32 bits per pixel. IMAQ Vision does not directly support other image types. Table 1.1 shows the icons used to describe the image types that work with the VIs in IMAQ Vision: Table 1.1 IMAQ Vision Image Type Icons Icon Type Description 8 0 Image with 8 bits per pixel bits per pixel F 2 32 bits per pixel C 3 2x32 bits per pixel complex values for FFT coefficients (Fast Fourier Transform) RGB 4 32 bits per pixel RGB, standard HSL 5 32 bits per pixel HSL 1

2 1. Introduction to IMAQ Vision. Structure of Digital Image Processing Applications Using IMAQ Vision 1.2 Manipulation of images by IMAQ Vision An 8-bit encoded image with a 512 x 512 resolution occupies 262,144 bytes or 256 KB of memory. IMAQ Vision is responsible for managing this memory space used for the representation of the digital image. The IMAQ Create VI creates the memory structure needed to represent the digital image. The image structure can contain additional data except for those strictly needed to represent the scene. The exact format of the image structure created by IMAQ Create depends on the type of the image representation. The image structure is represented in the form of a cluster; this is a specific data type for LabView applications. Each image structure is identified by a unique name. One can create multiple images by executing IMAQ Create as many times as needed, but each image structure created by an IMAQ Create instance must be assigned a unique name. Depending on the type of function a VI performs, different combinations of input and output are possible. One can use this flexibility to decide which image to process and where to store the resulting image. If no destination image is wired, the source image is used to store the processing result as well (thus, the output image is stored in the same image structure as the input image in that case the contents of the input image is lost). The following graphics show several connection types used in IMAQ Vision. This connection pane applies only to VIs that analyze an image and therefore does not modify neither the size nor the content of the image. The only pixels in Image that are processed are those whose corresponding pixels in Image Mask are non-zero. The input image Image is processed, and the result is passed at the Image Out output. The Image Dst output can receive either another image or the original, depending on your goals. If the Image Dst and Image Src inputs receive the same image, or if nothing is connected to Image Dst, the processed image is placed into the original image, and the original image data is lost. The above connection pane applies to VIs that perform arithmetic or logical operations between two images. 2

3 a b c In the pane above, the three images are all different. Image Src A and Image Src B are intact after processing and the results from this operation are stored in Image Dst. 1.3 Image structure management functions IMAQ Create Creates an image buffer that can be input into any of the acquisition functions of your IMAQ device. (see Table 1.1) Border Size determines the width in pixels of the border created around an image. These pixels are required for certain image processing functions. Image name is the name that will be associated with the created image. error in is a cluster that describes the error status before this VI executes. error out is a cluster that describes the error status after this VI executes. Image type specifies the type of image that is being created. (see Table 1.1) New image is the image structure that will be supplied as an input to all subsequent functions. IMAQ Dispose Disposes an image and frees the memory allocated for the image. Call IMAQ Dispose only when the image is no longer required for the remainder of the processing. All images? (No) determines whether you want to dispose only the image input into this function or dispose all images. The default value is FALSE. Image is the image to be disposed. ATTENTION If a LabView or BridgeView session is closed before calling IMAQ Dispose, the memory space created by IMAQ Create is not released. IMAQ Status Lists information about all the image structures created by IMAQ Create and about the memory space they occupy. 3

4 1. Introduction to IMAQ Vision. Structure of Digital Image Processing Applications Using IMAQ Vision 1.4 File management functions IMAQ GetFileInfo It obtains information regarding the contents of the file. This information is supplied for standard file formats only: BMP, TIFF, JPEG, PNG, or AIPD. File Path is the complete pathname, including drive, directory, and filename, for the file to be examined. Calibration is a cluster which contains specific elements File Type indicates the file type that is read: BMP, TIFF, JPEG, PNG, or AIPD (internal file format). X Resolution indicates the horizontal resolution, in pixels, of the image file. Y Resolution indicates the vertical resolution, in pixels, of the image file. File Data Type indicates the pixel size defined in the header of the file. IMAQ ReadFile It reads an image file. The file format can be a standard format (BMP, TIFF, JPEG, PNG, and AIPD) or a non-standard format known to the user. In all cases, the read pixels are converted automatically into the image type passed by Image. File Path is the complete pathname, including drive, directory, and filename, for the file to be loaded. Image is the reference to the image structure to which the data from the image file is applied. Load Color Palette? (No) determines whether to load the color table present in the file (if it exists). If loaded, this table is read and made available to the output Color Palette. The default is FALSE. File Options is a cluster of user-optional values that you can use to read non-standard file formats. Color Palette Out contains the RGB color table (if the file has one) read from the file when the user passes the value TRUE for the input Load Color Palette? (No). Image Out is the reference to the image structure containing the data read from the image file. File Type Out indicates the file type that is read. This string returns an identifier of the file format, which can be BMP, TIFF, JPEG, PNG, or AIPD (internal file format). File Type returns xxx if the file format is unknown. File Data Type out indicates the pixel size defined in the header for standard image file types. 4

5 IMAQ WriteFile It writes an image to a file. The default file type is BMP. Other file types supported are AIPD, TIFF, JPEG, and PNG. Color Palette is used to apply colors to a monochrome image. Color Palette is an array of clusters constructed by the user or supplied by IMAQ GetPalette. This palette is composed of 256 elements for each of the three color planes. A specific color is the result of applying a value between 0 and 255 for each of the three color planes (red, green, and blue). Image is the reference to the image structure to be written to an image file. File type describes the file type to be written. File Path is the complete pathname, including drive, directory, and filename, of the file to be written. Table 1.2 indicates the image types that each image file format supports. Table 1.2 Image Types Supported by Each File Format File Type BMP JPEG PNG TIFF AIPD Image Types Supported 8, RGB 8, RGB 8, 16, RGB 8, 16, RGB 8, 16, F, C, RGB, HSL As IMAQ WriteFile there are other 4 reading/writing functions of the images in different standard formats: IMAQ Write BMP File, IMAQ Write JPEG File, IMAQ Write PNG File, IMAQ Write TIFF File. Using these functions you can set various specific parameters. (Please refer to IMAQ Vision for G Reference Manual). 1.5 Basic display functions The Display (basics) library contains VIs that control the display of images in image windows as well as the positioning, opening, and closing of these windows on the display screen. IMAQ GetPalette It selects a display palette. Five predefined palettes are available. To activate a color palette choose a code for Palette Number and connect the Color Palette output to the input Color Palette of IMAQ WindDraw. Palette Number gives you a choice of five predefined palettes. Gray is the default palette. Binary is designed especially for binary images. 5

6 1. Introduction to IMAQ Vision. Structure of Digital Image Processing Applications Using IMAQ Vision Gradient Gradient palette Rainbow Rainbow palette Temperature Temperature palette error in is a cluster that describes the error status before this VI executes error out error out is a cluster that describes the error status after this VI executes. Color Palette indicates an array of clusters composed of 256 elements for each of the three color planes. IMAQ WindClose It closes an image window. Window Number (0..15) Specifies the image window to close. It is specified by a number from 0 to 15. Close All Windows (N) specifies whether all the image windows are to be closed. error in is a cluster that describes the error status before this VI executes. error out is a cluster that describes the error status after this VI executes Note: Remember to close all windows at the end of your application. IMAQ WindDraw It displays an image in an image window. The image window appears automatically when the VI is executed. Note that the default image window does not have scrollbars. You can add scrollbars by using the IMAQ WindSize VI. IMAQ WindDraw function offers support for one of the following 8, 16, F, C, RGB, HSL image formats. Window Number (0..15) specifies the image window in which the image is displayed. Up to 16 windows can be displayed simultaneously. Each window is specified with an indicator ranging from 0 to 15. Only the specified image window is affected, and all other image windows remain the same. The default value is 0. Image specifies the image reference for the displayed image. Title is an image window name. When a string is attached to this input, the image window automatically takes that name. Color Palette is used to apply a color palette to an image window error in is a cluster that describes the error status before this VI executes. error out is a cluster that describes the error status before this VI executes. Resize to Image size? (Y) specifies whether the user wants to resize the image window automatically to fit the image size. There are other functions which are not presented here, as: (Please refer to IMAQ Vision for G Reference Manual): IMAQ WindMove Indicates and sets the position of an image window. IMAQ WindShow Shows or hides an image window. IMAQ WindSize Indicates and sets the size of an image window. 6

7 1.6 Practice How to open an image using the IMAQ functions Open a LabView session Create a new project In the diagram window put the following IMAQ functions: IMAQ Create, IMAQ ReadFile and IMAQ WindDraw to obtain the chart presented below: To complete the diagram use the functions: File Dialog and String Constant the constant may be obtained by selecting select mode(2) input of the File Dialog function right click (when the mouse cursor is on ), and then select Create/Constant function Save the created project Run the LabView application (Run or CTRL+R) Try to open different types of images. What do you remark? Add the IMAQ Status function to the chart. Run again the application. In the window that corresponds to the diagram double-click on the IMAQ Status icon. A new window will open, which contains the status table of the application. Push the Examine button and determine the spatial resolution of the image and its size in bytes Using other IMAQ functions Using the File type output of the IMAQ ReadFile function, a String constant and the concatenation function of two streams display as title of the image window You loaded an xxx image where xxx represents the image type. Using the previously created diagram, create an application that contains 2 buttons, one for loading an image and the other one for closing the application. The closing button will have to release the memory area, to close the window and to stop the application execution. The chart you have to obtain is similar to the one presented bellow: 7

8 1. Introduction to IMAQ Vision. Structure of Digital Image Processing Applications Using IMAQ Vision Insert two boolean/square buttons in the control panel window and modify the label and the boolean text in order to distinguish them. (E.g. Load Image and Close). Modify the mechanical action of the buttons in order to set them on the Latch When Released position. For this select by right-click the button that you wish to modify and choose from the submenu Mechanical Action. In the chart insert two CASE sequences and link to each one of them one of the buttons created previously. Attention! Select by right click the area of the CASE structure, then choose Make this Case TRUE from the appearing submenu to obtain. Introduce both buttons in a while loop. Using the previous charts introduce for both buttons for the TRUE case the component elements as you can remark in the upper figure. To execute the STOP function is necessary to close successfully first the image window. To do this is needed the information from the error out output of the IMAQ WindClose function, which will be the status variable for the session closure. You ll have to extract from the output cluster with Cluster/Unbundle By Name function, only the status element (STATUS). This element must be complemented (negated) because it is TRUE only if there are some errors, so if IMAQ WindClose is successfully executed the output will be FALSE. After the logical NOT, the parameter will be linked to STOP and as well to IMAQ Dispose, which releases the memory area, but only if the image windows are closed. Run the application by executing Run Continuously, command. 8

9 1.7 Questions and exercises 1. Modify the chart from in order to be able to load color images. (Note. You should use Create/Constant function) 2. Complete the chart at 1.6.1, to display images with different color palette (gray, binary, gradient, rainbow, temperature) 3. Modify the chart from in order to be able to load images which are displayed at the monitor s x=100, y=50 coordinates. 1.8 Bibliography [1]. IMAQ Vision for G Reference Manual, National Instruments, 1999 [2]. IMAQ Vision User Manual, National Instruments, 1999 [3]. IMAQ PCI/PXI 1411 User Manual, National Instruments,