How to Create a Binary Image

Binary Image = Thresholded Original Image

binary(r,c) = binary image pixel (r,c)

f(r,c) = original image pixel (r,c)

T = threshold value

at every pixel location (r,c) in the image set:

binary(r,c) =

Example

Save have a greyscale image with 256 greylevels

i.e. p(r,c) = {0, 1, 2, .... 255}

say T = 100

then if p(r,c) <= 100 -----> binary(r,c)=1

else if p(r,c) > 100 -----> binary(r,c)=0

How to pick T?

Want T such that only pixels belonging to object you want to highlight are greater than T.

• Trial and Error
• Can use the Image's Histogram to Pick T.

Pick T Threshold using Histogram

• Look at a plot of h(m) and look for a valley to separate the objects pixels from the rest of the image. For example, suppose h(m) takes on the following bi-modal shape.

What about a Color Image?

Color image: f(r,c) = (red,green,blue) 

e.g. f(r,c) = (255,255,255)= white color 

      f(r,c) = (0, 0, 0) = black color 

          

1) Create a histogram for each color and create three binary images one for each color field: h red(), h green(), h blue()

2) Convert color to greyscale first as shown below and then threshold the new greyscale image:

greyscale value = f'(r,c) = (red + green + blue)/

Example: Find the coin

Histogram and selected threshold level, Pick Threshold right where it seperates lighter backbround from darker coin pixels.