ENG Introduction to Engineering

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2 ENG Introduction to Engineering Lecture # 9 Files, Sounds, Images and Movies Koç University ENG Slide #1

3 File Handling MATLAB has two general ways of importing/exporting data from the disk: Loading/Saving workspace variables using load and save commands. More general file operations using fopen, fread, fwrite, fscanf, fprintf. In addition to these general file operations, there are specialized commands for reading, writing images, sounds, movies, which will be covered separately later. ENG Slide #2

4 Saving Work Space save command saves workspace variables on disk. Usage: save by itself saves all workspace variables to matlab.mat. save( filename ) saves all workspace variables to filename.mat. save( filename, var1, var2,...) saves only the specified workspace variables in filename.mat. save(..., format ) enables you to make use of other data formats available with the save function. ENG Slide #3

5 Saving Work Space ENG Slide #4

6 Loading Work Space load command loads workspace variables from disk Usage: load by itself loads all the variables from the MAT-file matlab.mat, if it exists, or returns an error if the file doesn t exist. load filename loads all the variables from the file specified by filename. filename is an unquoted string specifying a file name, and can also include a file extension and a full or partial pathname. If filename has no extension, load looks for a file named filename.mat and treats it as a binary MAT-file If filename has an extension other than.mat, load treats the file as ASCII data load filename X Y Z... loads just the specified variables X, Y, Z. load -ascii filename forces load to treat the file as an ASCII file, regardless of file extension ENG Slide #5

7 Loading Work Space ENG Slide #6

8 Loading Work Space ENG Slide #7

9 Loading Work Space ENG Slide #8

10 Low-level File Handling In addition to load, save commands for work space variables, MATLAB provides special functions for low-level file treatment: fopen fclose fread fwrite fscanf fprintf These commands provides lower level treatment of files. Their format is similar to C functions with the same names. ENG Slide #9

11 Opening files: fopen fopen is used to open a file for read/write operations. fopen can also be used to obtain information about an already open file. fid = fopen(filename) opens the file filename for read access. fid is a scalar MATLAB integer, called a file identifier. You use the fid as the first argument to other file input/output routines. If fopen cannot open the file, it returns -1. ENG Slide #10

12 Opening files: fopen fid = fopen(filename, mode) opens the file filename in the specified mode. Modes are defined as follows: ENG Slide #11

13 Reading from files: fread fread reads binary data from files A = fread(fid) reads data in binary format from the file specified by fid into matrix A. Open the file using fopen before calling fread. The fid argument is the integer file identifier obtained from the fopen operation. MATLAB reads the file from beginning to end, and then positions the file pointer at the end of the file ENG Slide #12

14 Reading from files: fread A = fread(fid, count) reads the number of elements specified by count. At the end of the fread, MATLAB sets the file pointer to the next byte to be read. A subsequent fread will begin at the location of the file pointer. A = fread(fid, count, precision) reads the file according to the data format specified by the string precision. precision argument commonly contains a data type specifier such as int or float, followed by an integer giving the size in bits. ENG Slide #13

15 Reading from files: fread Specifying Precision: Any of the strings in the following table, either the MATLAB version or their C or Fortran equivalent, can be used for precision ENG Slide #14

16 Reading from files: fread ENG Slide #15

17 Reading from files: fread Example 1 Continued ENG Slide #16

18 Reading from files: fread ENG Slide #17

19 Reading from files: fscanf fscanf is used to read formatted data from data files. A = fscanf(fid, format) reads all the data from the file specified by fid, converts it according to the specified format string, and returns it in matrix A. format string consists of ordinary characters and/or conversion specifications. Conversion specifications indicate the type of data to be matched and involve the character %, optional width fields, and conversion characters, organized as shown below ENG Slide #18

20 Reading from files: fscanf Conversion characters used in the format field; ENG Slide #19

21 Reading from files: fscanf [A,count] = fscanf(fid, format, size) reads the amount of data specified by size, converts it according to the specified format string, and returns it along with a count of elements successfully read. size is an argument that determines how much data is read. Valid options are: n: Read n elements into a column vector. inf: Read to the end of the file, resulting in a column vector containing the same number of elements as are in the file. [m,n]: Read enough elements to fill an m-by-n matrix, filling the matrix in column order. n can be specified as inf, but m cannot. ENG Slide #20

22 Reading from files: fscanf Example Suppose we have the following ASCII text file exp.txt We use the following MATLAB code to read data into MATLAB: ENG Slide #21

23 Writing to files: fwrite and fprintf fwrite is the dual of fread for writing to files, and its format is very similar: ENG Slide #22

24 Writing to files: fwrite and fprintf fprintf is the dual of fscanf for writing to files, and its format is very similar: ENG Slide #23

25 Standard File Formats The low-level file handling functions are intended to be used for file formats you specify. For the standard file formats for images, sound, movies, datasheets, MATLAB provides easy to use functions, so you don t need to know the format of these files and bother using fopen, fscanf etc. But what are the file formats for which MATLAB provides functions? You can use fileformats command to display them! ENG Slide #24

26 MATLAB Supported File Formats ENG Slide #25

27 MATLAB Supported File Formats ENG Slide #26

28 Sound Files Wave files are one of the most common formats of storing sound files. These files by convention have extension.wav Wave files keep the sound files at a higher quality than MP3 format but they require more disk space! MATLAB provide wavread function for reading wave files: Here Fs: The number of samples taken from the sound in a second. bits: The number of bits used for each sound sample. ENG Slide #27

29 Sound File Example To illustrate the use of this function, lets look at the following example, where we read a wave file containing 9 second piece of Diana Krall s song "Temptation": ENG Slide #28

30 Sound File Example Resulting plot for the 9-second piece of the the song is shown below: Time (seconds) Now to listen to this song-piece we use another MATLAB command sound: sound(y,fs) ENG Slide #29

31 Writing Sound Files Suppose you somehow modified the song piece in MATLAB (by doing some Signal Processing operations!). And you want to save it in the wave file format so that your friends can listen to it. MATLAB has a special treat: You can write it in the wave file format by using MATLAB s wavwrite function: wavwrite(y, filename ) wavwrite(y,fs, filename ) wavwrite(y,fs,n, filename ) ENG Slide #30

32 Image File Formats There are many file formats for storing images. To name some: JPEG, TIFF, PNG etc. MATLAB provides imread function that allows you to read most of the available formats. The most common syntax for imread is [X,map] = imread(filename,fmt) Here X is a matrix containing the image. map is the colormap information related to image. ENG Slide #31

33 Image File Example Lets read an image using imread command [X,map]=imread( peppers.png ); peppers.png is an image file that comes with MATLAB. Lets display the image using imshow imshow(x,map} The result is.. ENG Slide #32

34 Writing Image Files If you create your own image file, or if you modify an existing image (through some Image Processing algorithms) you can write the resulting image to a file using imwrite command of MATLAB: imwrite(a,filename,fmt) By choosing the fmt appropriately, you can create an image file of any supported type. ENG Slide #33

35 Movies You can read AVI type video files to MATLAB using aviread command, e.g., mov=aviread( bassguitarmovie ); You can display the load movie using movie command: movie(mov) The screen shot of the resulting movie is shown below: ENG Slide #34