Image Processing Toolbox

Displaying Binary Images

To display a binary image, the syntax is

• BW = imread('circles.tif');
  imshow(BW)
  
  
  

In MATLAB, a binary image is of class logical. Binary images contain only 0's and 1's. Pixels with the value 0 display as black; pixels with the value 1 display as white.

Note    For the toolbox to interpret the image as binary, it must be of class logical. Intensity images that happen to contain only 0's and 1's are not binary images.

Changing the Display Colors of a Binary Image

You may prefer to invert binary images when you display them, so that 0 values display as white and 1 values display as black. To do this, use the NOT (~) operator in MATLAB. (In this example, a box is drawn around the image to show the image boundary.) For example,

• imshow(~BW)
  
  
  

You can also display a binary image using a colormap. For example, the following command displays 0's as red and 1's as blue.

• imshow(BW,[1 0 0; 0 0 1])
  
  
  

Reading and Writing Binary Images

In certain file formats, a binary image can be stored in a 1-bit format. When you read in a binary image in 1-bit format, MATLAB represents it in the workspace as a logical array.

By default, MATLAB writes binary images as 1-bit images, if the file format supports it.

• imwrite(BW,'test.tif'); % MATLAB supports writing 1-bit TIFFs.
  

To verify the bit depth of test.tif, call imfinfo. As you will see, the BitDepth field indicates that it has been saved as a 1-bit image, with the beginning of your output looking something like this.

• imfinfo('test.tif')
  ans = 
                       Filename: 'd:\mystuff\grid.tif'
                    FileModDate: '25-Nov-1998 11:36:17'
                       FileSize: 340
                         Format: 'tif'
                  FormatVersion: []
                          Width: 20
                         Height: 20
                       BitDepth: 1
                      ColorType: 'grayscale'
                FormatSignature: [73 73 42 0]
                      ByteOrder: 'little-endian'
                 NewSubfileType: 0
                  BitsPerSample: 1
                    Compression: 'CCITT 1D'
                       ...
  

Note    You may have noticed that the ColorType field of the binary image queried above has a value of 'grayscale'. MATLAB sets this field to one of three values: 'grayscale', 'indexed', and 'truecolor'. When reading an image, MATLAB evaluates the image type by checking both the BitDepth and the ColorType fields.

The Image and Axes Properties of a Binary Image

imshow sets the Handle Graphics properties that control how colors display, as follows:

• The image CData is set to the data in BW.
• The image CDataMapping property is set to direct.
• The axes CLim property is set to [0 1].
• The figure Colormap property is set to a grayscale colormap whose values range from black to white.

Displaying Intensity Images

Displaying RGB Images