1 Imaging Techniques for Flow and Motion Measurement Lecture 2 Lichuan Gui University of Mississippi 2011 Digital Image & Image Processing

2 Digital Image

3 Images constitute a continuous spatial distribution of the irradiance at a plane. Computers cannot handle continuous images but only arrays of digital numbers. Digital images consist of pixels. Each pixel represents a square region of the image on a square grid. Pixel value (gray value) represents intensity of the irradiance – 1-bit: 0  1 – 8-bit: 0  255 –10-bit: 0  1,023 –12-bit: 0  4,095 –24-bit: 0  16,777,215 Bitmap data in file. Digital Image

4 Sufficient pixels make image look continuous Digital Image

5 Color models - True color model: (Red,Green,Blue)=(0  255, 0  255, 0  255) - Others: Palettes/look-up table (LUT)/color map/index map/etc. Digital Image

6 Physical & logical pixels

7 Digital PIV Image Sample PIV image example ( 50  50 pixels)  Left: displayed on a PC screen  Right: presented as a 2D-function G(x,y)

8 Digital Image 2D & 3D digital images

9 Digital Image Histogram (PDF) of digital image PDF – Probability density function

10 Pixel operation  Changing gray value of single pixel without considering the neighborhood Filter operation  Changing gray value of single pixel considering the neighborhood of (2r+1)  (2r+1) pixels Many others Digital Image Processing

11 Linear transformation Pixel Operation

12 Binary operation Pixel Operation

13 Threshold Pixel Operation

14 Pixel Operation Invert Square Root

15 Smooth filter r =2 Digital Filter

16 Gradient filter Digital Filter

17 Digital Filter Laplace filter

18 Digital Filter Regional normalization

19 Digital Filter (Removing) unsharp mask G’(x,y)G(x,y)

20 Digital Filter Median Expansion Erosion

21 Digital image files Device independent image files - Microsoft Windows Bitmap “*.bmp” - Tag Image File Format “*.tif” - Graphics Interchange Format “*.gif” - JPEG File Interchange Format “*.jpg” - Many others Device dependent image files - Raw image formats (e.g. *.raww) - TSI image file Format “*.img” - DANTEC image file format “*.img” - LAVISION image file format “*.img” - Others

22 Digital image files Basic components of a digital image –Header, Palette, Bitmap Data, Footer etc. Information in a bitmap header –File Identifier –File Version –Number of Lines per Image –Number of Pixels per Line –Number of Bits per Pixel –Number of Color Planes –Compression Type –X & Y Origin of Image –Text Description –Others –Unused Space

23 Digital image files Microsoft Windows Bitmap –14-byte file header (Version 2.x +) Byte 1  2File type, always 4D42h (“BM”); Byte 3  6Size of the file in bytes; Byte 7  8Reserved 1, always 0; Byte 9  10Reserved 2, always 0; Byte 11  14Starting position of image data in bytes. –Bitmap header Byte 1  4Size of this header in bits; Byte 5  8Image width in pixels;(2 bytes for version 2.x) Byte 9  12Image height in pixels; (2 bytes for version 2.x) Byte 13  14Number of color planes; Byte 15  16Number of bits per pixel; (end of version 2.x) Byte 17  20Compression methods used; Byte 21  24Size of bitmap in Bytes; Byte 25  28Horizontal resolution in pixels per meter; Byte 29  32Vertical resolution in pixels per meter; Byte 33  36Number of colors in the image; Byte 37  40Minimum number of important colors. (end of version 3.x) Up to 108 bytes for Version 4.x

Microsoft Windows Bitmap –Color Palette One-, 4-, and 8-bit BMP files always contain a color palette 24-bit BMP files never contain color palettes –A 124×124-pixel uncompressed 8-bit gray-scale BMP file File header:14 bytes Bitmap header:40 bytes Color palette:1024 bytes Bitmap data:15376 bytes Total size:16454bytes 24 Digital image files Homework: write a program to read gray values at the center and 4 corners of image “image01.bmp”