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

2. MATLAB Functions Command: image Command: imshow
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]

3. 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))

4. Notes on Color Map Mapping to fewer color than required.
Result: Brighter image or Darker image
Mapping to more color than required

5. 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.

6. 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

7. 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]

8. 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]

9. 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

10. 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 Plane
LSB  Least Significant Bit Plane

11. Bit Plane: Lena Bit Plane#0 Bit Plane#1 Bit Plane#2 Bit Plane#3

12. 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);
>>
>>
>>
>>

13. 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’

14. Decrease Spatial Resolution
E.g. Lower the resolution by half on x and y axis
>> imresize(x,1/2);

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

16. Interpolation Example
450% Scaled up
Nearest Neighbor
Bicubic
Bilinear

17. Quantization
Digitize the image so that the number of the unique value is within the available range 1 2 3 1 2 3

18. Uniform Quantization x Quantized value 3 2 1 0.25MAX 0.5MAX 0.75MAX
0.25MAX
0.5MAX
0.75MAX
MAX
Input value

19. Uniform Quantization Original values Output value 0 - 63 0 64 -127 1
MAX = 255
Original values Output value

20. Quantization: Example
2 level
3 level

21. 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);

22. Quantization in MATLAB: Method 1
Command Number of grayscales
uint8(floor(double(x)/2)*2)
uint8(floor(double(x)/4)*4)
uint8(floor(double(x)/8)*8)
uint8(floor(double(x)/16)*16)
uint8(floor(double(x)/32)*32)
uint8(floor(double(x)/64)*64)
uint8(floor(double(x)/128)*128)

23. Quantization in MATLAB: Method 2
Command Number of grayscales
imshow(grayslice(x,128),gray(128))
imshow(grayslice(x,64),gray(64))
imshow(grayslice(x,32),gray(32))
imshow(grayslice(x,16),gray(16))
imshow(grayslice(x,8),gray(8))
imshow(grayslice(x,4),gray(4))
imshow(grayslice(x,2),gray(2))
grayslice produces a uint8 version of image x,
gray(n) produces a color map of n values.