Color on Remotely Sensed Imagery

Student Learning Outcome Explain why RS data look the way they do on digital grayscale or color image displays.

The Explanation The brightness values (BVs) of pixels in a remote sensing image vary across the image due to variations in the landscape represented by the image. 2009 NAIP Natural Color

The Explanation The BV of any given pixel is determined by two factors, the radiance recorded for that pixel and the quantization level of the remotely sensed data. BV (8-bit) 255 - white 127 - grey 0 - black 7-bit (0 – 127) Grayscale 8-bit (0 –255) 9-bit (0 –511) 12-bit (0 –1,023)

The Explanation Digital grayscale displays of remote sensing data show only one band of an image and BVs for that band are represented in shades of gray: the higher the BV, the lighter the shade of gray. Landsat TM Band 1 (Blue-Green) Landsat TM Band 2 (Green) Landsat TM Band 3 (Red) Landsat TM Band 4 (Near Infrared) Grayscale Low BVs High BVs

The Explanation Digital color displays of remote sensing data show up to three bands of an image using the RGB color model.

The Explanation RGB Color Model Based on additive color theory Applies to illuminant mode Light emitted from computer monitor, etc. Additive primary colors: Red, green blue Yield new colors when combined Red + Green = Yellow Red + Blue = Magenta Green + Blue = Cyan Red + Green + Blue = White Absence of color = Black

The Explanation RGB Color Model Visualized using RGB color cube Black (0, 0, 0) at origin Three axes (R, G, B) radiating outward from origin at 90° Each primary specified in steps of 256 increments ranging from 0 to 255 White (255, 255, 255) at maximum of all three primaries Blue Black * Green Red

The Explanation RGB Color Model Quantified using RGB color coordinate system

The Explanation Colors are classified using three descriptors Hue Name of color: red, green, blue, ... Saturation / chroma / intensity / purity Brightness or dullness of a color Value / tone Lightness or darkness of a color

The Explanation Digital color displays of remote sensing data, aka color composites, are generated by placing one image band in the Red color gun, another in the Green color gun, and yet another one in the Blue color gun, and by ultimately combining the three bands in a single image using additive color theory.

The Explanation In digital color displays of remote sensing data, BVs in each band are represented in varying saturation levels of R, G, and B: the higher the BV, the higher the saturation level. Low BVs High BVs Red Color Gun RGB = 255, 255, 255 RGB = 255, 130, 130 RGB = 255, 0, 0 Green Color Gun RGB = 255, 255, 255 RGB = 130, 255, 130 RGB = 0, 255, 0 Blue Color Gun RGB = 255, 255, 255 RGB = 130, 130, 255 RGB = 0, 0, 255

The Explanation So, in digital color displays, pixels with low BVs in all bands are black; pixels with high BVs in all bands are white; and pixels with intermediate BVs are red, green, blue, or a color resulting from the addition of these three primary colors, depending on the specific BVs in the different bands.

The Explanation Natural color (RGB = R, G, B) Color-infrared (RGB = NIR, R, G)

The Explanation Because there is often no one-to-one relationship between the BVs and the coordinate system used to represent the BVs (e.g., BVs may range from 0 to 100% but color values may range from 0 to 255), look-up tables (LUTs) – separate blocks of computer memory – are needed. LUTs basically specify the relationship between the original BVs and the gray level / color intensity of pixels in the output image. Color images have a set of three LUTs for each of the three R, G, and B guns. Grayscale images have identical LUTs for the three R, G, and B guns.

The Explanation Color LUTs: 8-Bit Example 1: Black-and-white display of Landsat TM band 4 of Charleston, SC. Example 2: Color density slice of Landsat TM band 5 of Charleston, SC, using the logic summarized on the following table.

The Explanation Color LUTs: 8-Bit Color Class Interval Visual Color Color LuT Values R, G, B BV Low High 1 Cyan 0, 255, 255 0 16 Shade of gray 17, 17, 17 17 17 18, 18, 18 18 18 19, 19, 19 19 19 * 59, 59, 59 59 59 2 Red 255, 0, 0 60 255

The Explanation Color LUTs: 8-Bit Black-and-white display of predawn TIR imagery of the Savannah River. Color density slice of the Savanna River TIR imagery.

The Explanation Color LUTs: 8-Bit Color Class Interval Visual Color Color LuT Values R, G, B Temperature Low High BV 1. Land Gray 127, 127 127 -3.0 11.6 0 73 2. River Ambient Dark blue 0, 0, 120 11.8 12.2 74 76 3. +1 C Light blue 0, 0, 255 12.4 13.0 77 80 4. 1.2 – 2.8 C Green 0, 255, 0 13.2 14.8 81 89 5. 3.0 – 5.0 C Yellow 255, 255, 0 15.0 17.0 90 100 6. 5.2 – 10.0 C Orange 255, 50, 0 17.2 22.0 101 125 7. 10.2 – 20 C Red 255, 0 , 0 22.2 32.0 126 176 8. > 20 C White 255, 255,255 32.2 48.0 177 255

The Explanation Color LUTs: 24-Bit R G B

4 3 2 1 NAIP Natural Color Orthophoto (RGB = 123) Landsat TM False Color Image (RGB = 432) Landsat TM Natural Color Image (RGB = 321) Landsat TM Band 1 (Blue-Green) Landsat TM Band 2 (Green) Landsat TM Band 3 (Red) Landsat TM Band 4 (Near Infrared)

AOI #1 Fallow Field Crops RGB = 4, 3, 2 Pecans Stream RGB = 3, 2, 1

Red Band 4 + Green Band 3 = RGB = 4, 3, 2 + Blue Band 2

Red Band 3 + Green Band 2 = RGB = 3, 2, 1 + Blue Band 1

AOI #2 RGB = 4, 3, 2 RGB = 3, 2, 1 Pond Rangeland 1 Rangeland 2 Exposed Soil RGB = 3, 2, 1

Red Band 4 + Green Band 3 = RGB = 4, 3, 2 + Blue Band 2

Red Band 3 + Green Band 2 = RGB = 3, 2, 1 + Blue Band 1

AOI #3 RGB = 4, 3, 2 RGB = 3, 2, 1 Baseball Outfield Parking Lot Baseball Infield RGB = 4, 3, 2 Commercial Roof RGB = 3, 2, 1

Red Band 4 + Green Band 3 = RGB = 4, 3, 2 + Blue Band 2

Red Band 3 + Green Band 2 = RGB = 3, 2, 1 + Blue Band 1

AOI #4 RGB = 4, 3, 2 RGB = 3, 2, 1 Residential New Grassy Field Exposed Soil Residential Old RGB = 3, 2, 1

Red Band 4 + Green Band 3 = RGB = 4, 3, 2 + Blue Band 2

Red Band 3 + Green Band 2 = RGB = 3, 2, 1 + Blue Band 1

Mangrove Ecosystem, Southern Florida G, R, NIR G R NIR