Visible Vegetation Index (VVI)
The Visible Vegetation Index (VVI) provides a measure of the amount of vegetation or greenness of an image using only information from the visible spectrum. It is well known that information in the near infrared is necessary to distinguish and separate vegetation from soil or ice surfaces in satellites imagery. Indeces like the Normalized Difference Vegetation Index (NDVI) are used for this purpose. The VVI is an alternative procedure that tries to use only information in the visible spectrum by measuring the amount of green in a region using similarity indices. The VVI is given by
where R, G, and B are the red, green, and blue components of the image, respectively, RGBo is vector of the reference green color, and w is a weight exponent to adjust the sensitivity of the scale. After various test with color calibrated satellite images, we found that RGBo= [30, 50, 0] works well for 24-bit images (256 colors per channel), and w = 1. It is also necessary to add some amount (about 10 worked best) to both the RGB channels and the RGBo to avoid a division by zero in the equation. The VVI is a project still under development and many future enhancements are expected. The VVI will be used to quickly measure the evolution of the amount of global vegetation in the images generated as part of the Visible Paleo-Earth Project.
Figure 1. Original true-color image of Earth without clouds (CREDIT: NASA Blue Marble - Next Generation).
Figure 2. VVI image of Figure 1 where lighter colors indicate more vegetation (green) and dark colors none. The VVI effectively filters the ocean, bare soil, and ice information from the original image. The lighter areas can be quickly added to estimate the amount of vegetation coverage.