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Chapter 2: Images and M ATLAB 2.1 Grayscale Images

Chapter 2: Images and M ATLAB 2.1 Grayscale Images. M ATLAB is a data analysis software package with powerful support for matrices and matrix operations Command window Reads pixel values from an image. >> help imdemos. >> w=imread(‘caeraman.tif’); >> figure, imshow(w); impixelinfo.

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Chapter 2: Images and M ATLAB 2.1 Grayscale Images

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  1. Chapter 2:Images and MATLAB2.1 Grayscale Images • MATLAB is a data analysis software package with powerful support for matrices and matrix operations • Command window • Reads pixel values from an image >> help imdemos • >> w=imread(‘caeraman.tif’); • >> figure, imshow(w); impixelinfo

  2. imshow(w) figure impixelinfo 2.1 Grayscale Images • Creates a figure on the screen • A figure is a window in which a graphics object can be placed • displays the matrix w as an image • turns on the pixel values in our figure • They appear at the bottom of the figure in the form • Pixel Info: (X, Y) intensity

  3. 2.1 Grayscale Images cameraman.tif

  4. 2.2 RGB Images • >> a=imread(‘autumn.tif’); • >> figure, imshow(a), impixelinfo

  5. FIGURE 2.2

  6. 2.2 RGB Images • Multidimensional array • >>size(a) • 206 (rows) 345 (columns) 3 (pages) • >> a(100,200,2) • 25 • >> a(100,200,1:3) or >> a(100,200,:) • or functionimpixel • >> impexel(a,200,100) • 75 25 30

  7. 2.3 Indexed Color Images • >> figure, imshow(‘forest.tif’), impixelinfo • >> c=imread(‘forest.tif’); • >> figure, imshow(c), • impixelinfo

  8. 2.3 Indexed Color Images • >> [c, cmap]=imread(‘forest.tif’); • >> figure, imshow(c, cmap), impixelinfo • Information about Your Image • A great deal of information can be obtained with the imfinfo function • >> imfinfo(‘forest.tif’)

  9. 2.4 Data Types and Conversions

  10. 2.4 Data Types and Conversions

  11. 2.5 Images Files and Formats • You can use MATLAB for image processing very happily without ever really knowing the difference between GIF, TIFF, PNG, etc. • However, some knowledge of the different graphics formats can be extremely useful in order to make a reasoned decision • Header information • This will, at the very least, include the size • of the image in pixels (height and width) • It may also include the color map, compression used, and a description of the image

  12. 2.5 Images Files and Formats • The imread and imwrite functions of MATLAB currently support the following formats • JPEG • These images are created using the Joint Photographics Experts Group compression method (ch14) • TIFF • A very general format that supports different compression methods, multiple images per file, and binary, grayscale, truecolor, and indexed images

  13. 2.5 Images Files and Formats • GIF • A venerable format designed for data transfer. It is still popular and well supported, but is somewhat restricted in the image types it can handle • BMP • Microsoft Bitmap format has become very popular and is used by Microsoft operating systems • PNG, HDF, PCX, XWD, ICO, CUR

  14. FIGURE 2.3 • A HEXADECIMAL DUMP FUNCTION

  15. 2.5.1 Vector versus Raster Images • We may store image information in two different ways • Vector images: a collection of lines or vectors • Raster images: a collection of dots • The great bulk of image file formats store images as raster information • As well as containing all pixel information, an image file must contain some header information • this must include the size of the image, but may also include some documentation, a color map, and the compression used • e.g. PGM format was designed to be a generic format used for conversion between other formats

  16. 2.5.3 Microsoft BMP

  17. 2.5.3 Microsoft BMP

  18. 2.5.3 Microsoft BMP • The image width is given by bytes 18–21; they are in the second row • 42 00 00 00 • To find the actual width, we reorder these bytes back-to-front: • 00 00 00 42 • Now we can convert to decimal • (4×161)+(2×160) = 66 which is the image width in pixels • The image height • 1F 00 00 00 (1×161)+(F×160) = 31

  19. 2.5.4 GIF and PNG • GIF • Colors are stored using a color map. The GIF specification allows a maximum of 256 colors per image • GIF doesn’t allow binary or grayscale images, except as can be produced with RGB values • The pixel data is compressed using LZW (Lempel-Ziv-Welch) compression • The GIF format allows multiple images per file. This aspect can be used to create animated GIFs

  20. 2.5.4 GIF and PNG • PNG • The PNG format has been more recently designed to replace GIF and to overcome some of GIF’s disadvantages • Does not rely on any patented algorithms, and it supports more image types than GIF • Supports grayscale, true color, and indexed images • Moreover, its compression utility, zlib, always results in genuine compression

  21. 2.5.5 JPEG • The JPEG algorithm uses lossy compression, in which not all the original data can be recovered >> dumphex('football.jpg',4) ans = FF D8 FF E0 00 10 4A 46 49 46 00 01 01 00 00 01 ......JFIF...... 00 01 00 00 FF DB 00 43 00 03 02 02 03 02 02 03 .......C........ 03 03 03 04 03 03 04 05 08 05 05 04 04 05 0A 07 ................ 07 06 08 0C 0A 0C 0C 0B 0A 0B 0B 0D 0E 12 10 0D ................

  22. 2.5.5 JPEG

  23. 2.5.6 TIFF • One of the most comprehensive image formats • Can store multiple images per file • Allows different compression routines and different byte orderings • Allows binary, grayscale, truecolor or indexed images, and opacity or transparency • An excellent format for data exchange

  24. 2.5.6 TIFF

  25. 2.5.6 TIFF • This particular image uses the little-endian byte ordering • The first image in this file (which is in fact the only image), begins at byte • Because this is a little-endian file, we reverse the order of the bytes: 00 01 01 E0. This works out to 66016. E0 010100

  26. 2.5.7 DICOM • DICOM (Digital Imaging and Communications in Medicine) • Like GIF, may hold multiple image files • May be considered as slices or frames of a three dimensional object • The DICOM specification is huge and complex. Drafts have been published on the World Wide Web

  27. 2.5.8 Files in MATLAB • Which writes the image stored in matrix X with color map map (if appropriate) to file filename with format fmt • Without the map argument, the image data is supposed to be grayscale or RGB e.g. >>imwrite(c,cmap,’forest.png’,’png’); >>imwrite(c,’forest1.png’,’png’);

  28. Exercise • The following shows the hexadecimal dump of a BMP file: 42 4D 7E 05 02 00 00 00 00 00 36 04 00 00 28 00 00 00 23 01 00 00 C2 01 00 00 01 00 08 00 00 00 00 00 48 01 02 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 01 01 01 00 02 02 Determine the height and width of this image (in pixels) and state whether it is a grayscale or color image.

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