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Image Display• MATLAB functions for displaying image• Bit Planes• Spatial resolution• Quantization • Dithering

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MATLAB Functions

• Command: image– display matrix as image– default: use current color map to assign color

• Command: imshow– display uint8 matrix as image– for double matrix, display in the range of [0,1]

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MATLAB: image Function

• Require mapping to display grayscale images• No mapping required for true color images• Commands to add after image:

– truesize : display one matrix element as one pixel– axis off : turn off axis labelling– colormap(gray(num_color)) : adjust color map

to grayscale– Note: Find the number of gray level by command

size(unique(matrix))

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Notes on Color Map

• Mapping to fewer color than required.– Result: Brighter image or Darker image

• Mapping to more color than required – Result: Brighter image or Darker image

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Notes on Color Map

• Mapping to fewer color than required produces brighter image. (pixel whose intensity higher than the defined value assigned the highest value (white))

• Mapping to more color than required produces darker image. (no pixels mapped to white)

• For indexed color image, map to the image’s palette.

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imshow for Double Matrix

• Range is [0,1] not [0,255].

• How to do it?– imshow(double_matrix/255)

• Vary the brightness of the image:– imshow(double_matrix/value)

• value > 255: darker; value < 255: brighter

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Conversion: Double to Uint Image

• Manually>> b = double(a);

>> c = b/255;

• Specific command: im2double>> d = im2double(a);

• Note: im2double automatically scale the output to the range of [0,1]

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Conversion: Double to Uint Image

• Manually>> a = uint8(c * 255);

• Specific command: im2uint8>> a = im2uint8(c);

• Note: im2uint8 automatically scale the input (range [0,1]) to the range of [0,255]

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Display Binary Image

• Use uint8 data type with logical flag on>> imshow(logical(binary_matrix))

Or

>> imshow(double(binary_matrix))

• For normal grayscale image>> logical_matrix = integer_matrix > threshold

• Conversion: logical to uint8

>> matrix = + matrix

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Bit Plane

• 8-bit unsigned integer

• Bit plane: binary image whose pixel is the value at the particular bit in the 8-bit unsigned integer

MSB Most Significant Bit PlaneLSB Least Significant Bit Plane

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Bit Plane: Lena

Bit Plane#0 Bit Plane#1 Bit Plane#2 Bit Plane#3

Bit Plane#4 Bit Plane#5 Bit Plane#6 Bit Plane#7

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Bit Plane Construction: Example>> c = imread(‘cameraman.tif’);>> cd = double(c);>> c0 = mod(cd,2);>> c1 = mod(floor(cd/2),2);>> c2 = mod(floor(cd/4),2);>> c3 = mod(floor(cd/8),2);>> c4 = mod(floor(cd/16),2);>> c5 = mod(floor(cd/32),2);>> c6 = mod(floor(cd/64),2);>> c7 = mod(floor(cd/128),2);

>>>>>>>>

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Spatial Resolution

• Density of the pixels over the image

• Higher spatial resolution = more pixels in the image

• Change spatial resolution by imresize command

Syntax: imresize(matrix, scale, method)

imresize(matrix, scale)

method: ‘nearest’ , ‘bilinear’, ‘bicubic’

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Decrease Spatial Resolution

• E.g. Lower the resolution by half on x and y axis

>> imresize(x,1/2);

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Increase Spatial Resolution

• E.g. Increase the spatial resolution by two on X and Y axes>> imresize(x,2);

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Interpolation Example450% Scaled up

Nearest Neighbor

Bilinear

Bicubic

http://www.dpreview.com/learn/?/key=interpolation

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Quantization

• Digitize the image so that the number of the unique value is within the available range

0

1

2

3

0 1 2 3

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Uniform QuantizationQuantized value

Input value0

1

2

3

MAX0.75MAX0.5MAX0.25MAX

x

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Uniform Quantization

Original values Output value 0 - 63 0 64 -127 1 128 -191 2 192 - 255 3

MAX = 255

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Quantization: Example

2 lev

el

3 level

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Quantization in MATLAB

• Quantization is simply applying flooring function after division

• Input image matrix = x• Number of output grayscales = n• Quantized output: f = floor(double(x)/n);

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Quantization in MATLAB: Method 1

Command Number of grayscales

uint8(floor(double(x)/2)*2) 128 uint8(floor(double(x)/4)*4) 64

uint8(floor(double(x)/8)*8) 32

uint8(floor(double(x)/16)*16) 16

uint8(floor(double(x)/32)*32) 8

uint8(floor(double(x)/64)*64) 4

uint8(floor(double(x)/128)*128) 2

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Quantization in MATLAB: Method 2 Command Number of grayscales

imshow(grayslice(x,128),gray(128)) 128 imshow(grayslice(x,64),gray(64)) 64

imshow(grayslice(x,32),gray(32)) 32

imshow(grayslice(x,16),gray(16)) 16

imshow(grayslice(x,8),gray(8)) 8

imshow(grayslice(x,4),gray(4)) 4

imshow(grayslice(x,2),gray(2)) 2

grayslice produces a uint8 version of image x, gray(n) produces a color map of n values.