CS112: Lab 11 :: Finding the stars

Star gazing

Background information on bwlabel()

bwlabel identifies parts of a binary matrix that are connected. There is an optional second argument to bwlabel that is either 4 or 8. With a second argument of 4, bwlabel looks for objects connected along the four directions shown (up,down,left,right). With a second argument of 8, bwlabel looks for objects connected along the diagonal as well as those along the standard up, down, left and right directions.
bwlabel(BW,4) finds connections in these 4 directions bwlabel(BW,8) finds connections in these 8 directions 


BW = [ 1  1  1  0  0  0  0  0  0 
       1  1  1  0  0  1  1  0  0
       1  1  1  0  0  1  1  0  0
       1  1  1  0  1  1  1  0  0
       1  1  1  0  0  0  0  1  1
       0  0  0  0  0  0  0  1  1  
       0  0  0  0  0  0  1  1  1  
       0  0  0  0  0  0  0  1  1  ];

L = bwlabel(BW,4)

L =  [ 1  1  1  0  0  0  0  0  0 
       1  1  1  0  0  2  2  0  0
       1  1  1  0  0  2  2  0  0
       1  1  1  0  2  2  2  0  0
       1  1  1  0  0  0  0  3  3
       0  0  0  0  0  0  0  3  3  
       0  0  0  0  0  0  3  3  3  
       0  0  0  0  0  0  0  3  3  ];


Ls = bwlabel(BW,8)

Ls = [ 1  1  1  0  0  0  0  0  0 
       1  1  1  0  0  2  2  0  0
       1  1  1  0  0  2  2  0  0
       1  1  1  0  2  2  2  0  0
       1  1  1  0  0  0  0  2  2
       0  0  0  0  0  0  0  2  2  
       0  0  0  0  0  0  2  2  2  
       0  0  0  0  0  0  0  2  2  ];
Note: colors are used in the example above to illustrate how bwlabel works. bwlabel identifies objects only by number; not color.

Notes about imshow

  1. imshow(image,[min max])
    displays a re-scaled version of image (without actually changing the contents of the original image). Sometimes this comes in handy, especially with 8-bit images. Recall that MATLAB's default color range for an 8-bit image is 0-255 (where 0 is black and 255 is white).

    >> block = uint8(zeros(50,50));
    >> imshow(block);

    >> block(20:40,20:40) = 1; % intending to set a subsection to white

    >> imshow(block); % hmmm, where is the white part??


    >> imshow(block,[0,1]); % ahhh! There it is!


     
  2. imshow(X,map)
    displays the indexed image X with the colormap map. All colormaps have exactly 3 columns for the red, green and blue components of each color in the map (specified on the interval between 0.0 and 1.0) A colormap may have any number of rows, and each row is interpreted as a color.

    Colormaps and bwlabel mesh particularly well together. The labels that bwlabel returns serve as indices into the colormap, creating what is called an indexed image. Here is an example: 

    

X = [1  1  0  0  0  0  0  0  0  0
     1  1  0  0  0  0  0  0  0  0
     0  0  2  2  0  0  0  0  0  0 
     0  0  2  2  0  0  0  0  0  0 
     0  0  0  0  3  3  0  0  0  0 
     0  0  0  0  3  3  0  0  0  0 
     0  0  0  0  0  0  4  4  0  0 
     0  0  0  0  0  0  4  4  0  0 
     0  0  0  0  0  0  0  0  5  5
     0  0  0  0  0  0  0  0  5  5];

 redmap = zeros(5,3);
 redmap(1,1)= 0.20;
 redmap(2,1)= 0.40;
 redmap(3,1)= 0.60;
 redmap(4,1)= 0.80;
 redmap(5,1)= 1.0;

>> redmap
redmap =
    0.2000         0         0
    0.4000         0         0
    0.6000         0         0
    0.8000         0         0
    1.0000         0         0

% making a greenish bluish map
 aquamap = zeros(5,3);
 aquamap(1,2) = 0.20;  
 aquamap(1,3) = 0.20;
 aquamap(2,2) = 0.40;  
 aquamap(2,3) = 0.40;
 aquamap(3,2) = 0.60;  
 aquamap(3,3) = 0.60;
 aquamap(4,2) = 0.80;  
 aquamap(4,3) = 0.80;
 aquamap(5,2) = 1.0;
 aquamap(5,3) = 1.0;

>> aquamap
 aquamap = 
         0    0.2000    0.2000
         0    0.4000    0.4000
         0    0.6000    0.6000
         0    0.8000    0.8000
         0    1.0000    1.0000
>>
    We can display the image X above by specifying a display range as the second input:
    Food for thought: what would you expect imshow(X) to produce?

    >> imshow(X, [0 5]);


    Or, we could display it with the colormap redmap:

    >> imshow(X,redmap);

    With the redmap colormap, the labels in the image X correspond to rows in the colormap from top to bottom. So the first label 1 (in the upper left corner) takes the color from the first row of redmap which is [0.20 0 0] which is a pale red. This pattern continues, with the last label 5 (in the lower right corner) taking the color from the fifth row of redmap which is [1.0 0 0] or pure red.

    Or, we could display it with the colormap aquamap:

    >> imshow(X,aquamap);

    An analogy that is often used for indexed images is that of paint-by-numbers. The image has areas with color indicated by different numbers. Each paint tub has a number on it, linking the colors and the numbers in the image. The same thing is happening here with the labeled image and the colormap, although the indexing is implicit (row i of the colormap corresponds to the ith label in the image).

    Notes:

And here is the link to the assignment:

Assignment 8