Investigation 4-1. Spatial Analysis: Area
• AnalyzingDigitalImages software
• Plant Leaf image
Method 1: Rectangle Tool
Click and drag to draw a rectangle around
the object of interest. To adjust the size of the box, either click and
drag the blue or red corner of the box or click the small arrows that
appear to the left of the image.
As one would expect, this would not yield very accurate area measurement for objects that aren’t rectangles.
Method 2: Polygon Tool
1. Draw a polygon around the object using
the Polygon tool. Click on the image to place each corner of the
polygon. Note: you may draw up to 20 corners to define a polygon. A
warning appears after the 19th corner is drawn.
2. To adjust the size or shape of the polygon, click near a corner and drag.
If this method provides a sufficient precision for your needs, save your measurements and process your next picture.
Method 3: Masking Colors
This method provides the most precise area
measurement provided the color of the feature is sufficiently uniform
and different from the surrounding features.
Selecting a color range or relationship to isolate specific features is called “masking”.
1. To set a color range, go to the “Mask
Colors” window and draw a rectangle inside the feature of interest. To
adjust the size of the box, click and drag the blue or red corner of the
box. The range of colors inside the rectangle is automatically
TIP: To adjust the selected range of colors,
click and drag the minimum (min) or maximum (max) lines for red,
green, or blue intensities.
TIP: Using the histograms of colors, which
represent the frequency of color intensities for the complete image, you
may decide only one or two colors are best to use to mask the image.
To ignore a color during masking, toggle the check box to off by clicking on it.
2. Click “Apply Mask” to blacken all pixels
with colors within the selected range of colors. Pixels with colors
outside this range will turn white.
TIP: To see if the desired feature has been
accurately masked, click on the radiobutton labeled “Original”. To see
the mask again, click the “Show Mask” button. Compare boundaries of
the feature in both images.
3. Another way to mask colors is to create a
mathematical relationship between red, green, and/or blue at each
pixel. Use this feature if the range of colors cannot isolate the
desired feature. Note that this option takes much longer to process the
image than the first masking option, so be patient when applying.
When satisfied that the mask isolates the feature you want to measure,
return to the “Spatial Analysis” window and use either the Rectangle or
Polygon tool to draw around just the highlighted feature. Notice that
the blackened pixels of the ruler are not inside the rectangle drawn
below – these will not be counted in the area calculation.
Note that when using the Masked Image (see
highlighted below), the area of the masked pixels inside the box or
polygon is calculated (the white pixels are ignored). Using the other
images, Original and Enhanced, the area tools calculate the area of the
total pixels inside the drawn shape.
Comparing the area using the polygon (11.49
in2) and the masking tool (11.11 in2), one would use the masking tool to
study daily growth of leaves during the springtime.
Augmenting the Masking Colors by Enhancing Colors First
If it is difficult to isolate the feature
from the surrounding colors using the mask tools, use the tools on the
“Enhance Colors” window to change the colors of the image prior to using
the mask tools. There are two sets of color enhancement options: first
are the pre-defined options and the second are to create your own color
There are 7 preset enhancements to view the
digital image, and these are very useful to survey the colors of an
image: RGB, Red (gray), Green (gray), Blue (gray), Red v Green
(normalized), Red v Blue (normalized), and Green v Blue (normalized)
Red-Green-Blue (RGB): Standard color composite of digital
imagery in which the color intensities of red, green, and blue are
displayed in the computer display’s red, green, and blue, respectively.
The image should look the same as the object you photographed.
|Red, Green, or Blue as shades of gray: Gray shade images
allow you to examine the intensities of values without biasing your
sensitivity to red, green, or blue. If blue is being displayed as gray
(shown in image on right) then high intensities appear white, and low
intensities appear black.
|Red versus Green, Red vs. Blue, or Green vs. Blue (normalized):
Provides a comparison of two color intensities. The difference of the
color intensities is divided by the sum of the color intensities. The
displayed color is the greater of the two intensities. Equal intensities
are displayed as black. This enhancement minimizes the effects of
shadows and uneven lighting across the image.
For example, suppose Red vs. Green
(normalized) is selected (shown above). If a pixel has red, green, and
blue (RGB) values of 40%, 80%, and 60%, respectively, the difference
between red and green is 40% in the green, and the sum of the red and
green intensities is 120%. The normalized value will be 40% / 120% or
0.33, which is converted back to percent values of 33% for display
purposes. The displayed color for that pixel will be a dark shade of
green (RGB values of 0%, 33%, 0%). Compare this value to another pixel
with a difference between red and green of 40%, but in this case the
pixel has RGB values of 0%, 40%, 20%. The normalized difference is 40% /
(0% + 40%) which equals 1. This value is converted back to a
percentage of 100%. The pixel will be displayed as a very bright green
(RGB value of 0%, 100%, 0%).
2. Custom Enhancements
The second option to enhance the colors of
an image is to select a range of colors and either limit or stretch this
range of intensities.
Limiting to only that range of selected intensities turns off that color for pixels outside this threshold.
Clicking and dragging the yellow and cyan
lines selected the red intensities between 12 and 53. The reds within
this range represent the leaf.
By displaying only those red colors within
this range (the limit option), the leaf remains the same but the
background turns a cyan color because the more intense red values of the
background have been turned off.
Stretching a range of color produces a
similar effect for colors outside the selected range – those colors are
turned off, but the selected colors are linearly expanded from 0% for
the minimum intensity to 100% for the maximum intensity.
When the same values are stretched, the
selected reds in the leaf become exaggerated, changing just the color of
the leaf to redder hues.
Turning “off” a color for enhancement speeds up the processing time since it will be ignored.
Regardless of which enhancement method is
used, the color enhanced image may be manipulated in the masking tool by
selecting the Enhanced image button.