Page 239 - Introduction to AI Robotics

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6 Common Sensing Techniques for Reactive Robots
int image[ROW][COLUMN][COLOR_PLANE];
...
red = image[row][col][RED];
green = image[row][col][GREEN];
blue = image[row][col][BLUE];
display_color(red, green, blue);

2. Separate. Some framegrabbers store an image as three separate two-di-
mensional arrays, as shown below. Some framegrabbers have functions
which return each color plane separately or interleaved. The equivalent
code fragment is:

int image_red[ROW][COLUMN];
int image_green[ROW][COLUMN];
int image_blue[ROW][COLUMN];
...
red = image_red[row][col];
red = image_green[row][col];
red = image_blue[row][col];
display_color(red, green, blue);

The RGB representation has disadvantages for robotics. Color in RGB is
a function of the wavelength of the light source, how the surface of the ob-
ject modifies it (surface reflectance), and the sensitivity of the sensor. The
first problem is that color is not absolute. RGB is based on the sensitivity
of the three color sensing elements to reflected light. An object may appear
to be at different values at different distances due to the intensity of the re-
flected light. Fig. 6.12 shows the same program and parameters to segment
an orange landmark that is serving as a “flag” for tracking a small robot just
outside of the image. The RGB segmentation in Fig. 6.12a is more correct than
in Fig. 6.12b. The only difference is that the flagged robot has moved, thereby
changing the incidence of the light. This degradation in segmentation qual-
VISUAL EROSION ity is called visual erosion, because the object appears to erode with changes
in lighting. Moreover, CCD devices are notoriously insensitive to red. This
means that one of the three color planes is not as helpful in distinguishing
colors.
Clearly a device which was sensitive to the absolute wavelength of the
reflected light (the hue) would be more advantageous than having to work