DIP/Matlab

[Ch3] Images and MATLAB

jeong_reneer 2022. 1. 28. 22:25

3.1 Greyscale images

1) imread

>> w = imread('wombats.tif');

(1) w : A matrix variable (which has grey values of all the pixels in greyscale image)

(2) imread : To read the pixel values from an image file, and return a matrix of all the pixel values

- It ends in a semicolon ( ; ) : not displaying the result of command to screen

The result of imread command is a matrix of size 256x256, or with 65536 elements

- The name wombats.tif is given in single quote marks ( ' ' )

Without them, Matlab would assume that wombats.tif was the name of a variable, rather than the name of a file

 

2) imshow

>> figure, imshow(w), pixval on

(1) figure : To create a figure window on the screen 

(2) imshow(w) : To display the matrix w as an image

(3) pixval on : To turn on the pixel values in our figure

- A display of the grey values of the pixels in the image

- Appeared at the bottom of the figure in the form 'c x r = p'    imfinfo

  (c, r, p : column, row, grey value of the given pixel)

  Ex. 'wombats.tif' : 8-bit greyscale img → pixel values appear as integers in the range 0-255

 

(4) impixel(matrix, row, col) : returns all 3 same values (g : A single two-dimensional matrix)

 

 

3.2 RGB Images

1) imread, imshow 24-bit RGB image

2) size of RGB image : ( number of rows, columns, 'pages' of a )

a : A three-dimensional matrix = multidimensional array = stack of three matrices, each of the same size

>> a=imread('autumn.tif');
>> figure,imshow(a),pixval on

>> size(a)
ans =
   206   345     3

 

3) Indexing to obtain any of the RGB values at a given location

4) impixel function ★ column 먼저 !

>> a(100,200,2) # matrix(row,col,RGB)
ans =
  uint8
   25

>> a(100,200,:)
  1×1×3 uint8 배열
ans(:,:,1) =
   75
ans(:,:,2) =
   25
ans(:,:,3) =
   30

>> impixel(a,200,100) # (matrix, col, row)
ans =
    75    25    30

 

 

3.3 Indexed color images

1) imread, imshow indexed color image

>> em=imread('emu.tif');
>> figure,imshow(em)

Result : A dark, barely distinguishable image, with single integer grey values

em : a single greyscale image

 

2) [em, emap] 

>> figure,imshow('emu.tif')

>> [em,emap]=imread('emu.tif');
>> figure,imshow(em,emap)

Result : A color image

emap : a color map

emu.tif : An indexed image, consisting of two matrices (color map + index to color map)

MATLAB stores RGB values of an Indexed image as double values between 0 and 1

 

3) imfinfo

>> imfinfo('emu.tif')

ans = 

  다음 필드를 포함한 struct:

                     Filename: 'C:\Users\82104\Downloads\emu.tif'
                  FileModDate: '28-1-2022 15:00:45'
                     FileSize: 119804
                       Format: 'tif'
                FormatVersion: []
                        Width: 331
                       Height: 384
                     BitDepth: 8
                    ColorType: 'indexed'
              FormatSignature: [73 73 42 0]
                    ByteOrder: 'little-endian'
               NewSubFileType: 0
                BitsPerSample: 8
                  Compression: 'PackBits'
    PhotometricInterpretation: 'RGB Palette'
                 StripOffsets: [1×16 double]
              SamplesPerPixel: 1
                 RowsPerStrip: 24
              StripByteCounts: [1×16 double]
                  XResolution: 72
                  YResolution: 72
               ResolutionUnit: 'Inch'
                     Colormap: [256×3 double]
          PlanarConfiguration: 'Chunky'
                    TileWidth: []
                   TileLength: []
                  TileOffsets: []
               TileByteCounts: []
                  Orientation: 1
                    FillOrder: 1
             GrayResponseUnit: 0.0100
               MaxSampleValue: 255
               MinSampleValue: 0
                 Thresholding: 1
                       Offset: 117938

 

cf) MATLAB does not distinguish between greyscale and binary images

A binary image is just a special case of a greyscale image which has only two intensities

However, we can see that 'text.tif' is a binary image since the number of bits per pixel is only one!

→ 더 간단한 Binary 인데 MATLAB 에서는 Greyscale이랑 구분을 X

 

 

3.4 Data types and conversions

1) The most common Data Types in MATLAB

 

Data Type Description Range
int8 8-bit integer -128 ~ 127
uint8 8-bit unsigned integer 0 ~ 255
int16 16-bit integer -32768~ 32767
uint16 16-bit unsigned integer 0 ~ 65535
double Double precision real number Machine specific

 

 

2) Conversion

>> a=23;
>> b=uint8(a);

>> b
b =
  uint8
   23
   
>> whos a b
  Name      Size            Bytes  Class     Attributes

  a         1x1                 8  double              
  b         1x1                 1  uint8

 

Arithmetic operations

- are not permitted on int8, int16, uint8, uint16 types

- are permitted on only double type! 

→ Conversion functions 통해 double type으로 conversion 해야 Arithmetic operations 가능 !

Ex) Grayscale image : uint8 (efficient storage space, since each pixel requires only 1 byte)

→ gray2rgb functions 통해 uint8에서 double type으로 conversion 

 

Function Use Format
ind2gray Indexed to Greyscale y=ind2gray(x,map);
gray2ind Greyscale to Indexed [y,map]=gray2ind(x);
rgb2gray RGB to Greyscale y=rgb2gray(x);
gray2rgb Greyscale to RGB y=gray2rgb(x);
rgb2ind RGB to Indexed [y,map]=rgb2ind;
ind2rgb Indexed to RGB y=ind2rgb(x,map);