What is texture?
The GLCM Practical
Notes More Information and
Calculating texture measures from the GLCM
Most texture calculations are weighted averages of
the normalized GLCM cell contents.
A weighted average multiplies each value to be used by a
factor (a weight) before summing and dividing by the number of values. The
weight is intended to express the relative importance of the value.
Example: the most common weighted average that
students encounter is the term grade. Exams usually have a higher weight than
quizzes. The weights are the % of course grade assigned to each mark.
Creating a texture image
The result of a texture calculation is a single number
representing the entire window. This number is put in the place of the centre
pixel of the window, then the window is moved one pixel and the process is
repeated of calculating a new GLCM and a new texture measure. In this way an
entire image is built up of texture values. More information
Edge of image problems Each cell in a window must
sit over an occupied image cell. This means that the centre pixel of the
window cannot be an edge pixel of the image. If a window has dimension N x N,
a strip (N-1)/2 pixels wide around the image will remain unoccupied. The usual
way of handling this is to fill in these edge pixels with the nearest texture
Example: For a 5x5 window, the outer 2 rows and
columns of the image receive the texture values calculated in row 3 (top
edge), column 3 (left edge), row L-2 (bottom edge) and column P-2 (right edge)
where P,L are the dimensions in pixels and lines of the original image. For
the illustrated image, L=P=10, so values are calculated from row 3 and column
3 through row 8 and column 8.
Image edge pixels usually represent a very small
fraction of total image pixels, so this is only a minor problem. However, if
the image is very small or the window is very large, the image edge effect
should be remembered when examining the texture image.
Edge effects can be a problem in classification. For an
excellent discussion, see Ferro and
Groups of texture measures
This tutorial groups the texture measures according to
the purpose of the weights in the equations. The major groups are briefly
listed here. Details for each group are in the links at the top of this page.
Another common way to classify textures is according
to their degree, meaning the highest exponent used. Most measures -
and all used here - are first or second degree.
Example: If a squared term is used, the measure
is second degree. If a cubed term is used, it is third degree.
1. Contrast group:
Measures related to contrast use weights related to the
distance from the GLCM diagonal.
2. Measures related to orderliness
3. Group using
descriptive statistics of the GLCM
To go to a particular group click below.
To continue through the tutorial, click the next button to go
to the contrast group.