SPOT 4.1 Basic and Advanced Software Manual for SPOT Insight ...

Ch. 8 - Viewing, Editing, and Annotating ImagesThe color space options and their suggested uses are as follows:• Adjust RGB (Red Green Blue) – Use for monochrome images• Adjust HSL (Hue Saturation Luminance) – Use for all color images except fluorescence• Adjust HSV (Hue Saturation Value) – Use for fluorescence imagesEach color space is described below.RGBHSLThe RGB color space operates by assigning a red, green, and blue value to each pixel in animage. Because red, green and blue are the primary colors for emissive light, this is the methodthat color computer monitors use to display images. However, the RGB color space is notintuitive to users.The HSL color space is a more intuitive model based on the three attributes that are common toall colors: hue, saturation, and luminance. Instead of assigning a red, blue, and green value toeach pixel, the HSL color space assigns a value for each color attribute (e.g., hue, saturation, andluminance). These attributes are defined as follows:• Hue – Hue is the designation for the actual color, as distinguished from others in the colorspectrum. Hue is derived from a color wheel and is expressed in the color's angular location(in degrees) on the wheel. (e.g., +180° to –180° for the SPOT software).• Saturation – Saturation is the proportion of perceived pure hue in the color. Saturation (inthe SPOT software) is measured on a numerical scale of 0 to 100, where zero is pure grayand 100 is pure hue (e.g., day glow).• Luminance – Luminance is the relative brightness or darkness of a hue. Luminance (in theSPOT software) is measured on a numerical scale of 0 to 100. A saturated color starts out asblack at a luminance of 0, reaches full brightness as a saturated color at a luminance of 50,and then transitions through lighter pastel shades to white at a luminance of 100. Thismimics the human perception of increasing brightness for images that are illuminated withlight that contains a broad spectrum of wavelengths.HSVThe HSV color space was added to the SPOT software to optimize fluorescence image editing.Both the RGB and the HSL color spaces are problematic when used to edit color fluorescenceimages. Increasing gamma in RGB can cause dimmer sections of the image to lose saturation(look “washed out”) and change hue. Increasing gamma or brightness in HSL causes the brighterareas of an image to look unnaturally pastel or even white. The HSV color space avoids bothproblems.The HSV color space is similar to the HSL color space in that it also uses the intuitive conceptsof hue and saturation. However, instead of luminance for brightness, it uses Value. Value is likeLuminance in that a value of 0 is black. Value differs from Luminance in that a Value of 100 (inthe HSV color space) is the brightest version of the original hue whereas a Luminance of 100 (inthe HSL color space) is always white. For example, a dim cherry red changes to a bright cherryred when its Value is increased from 30 to 100. When its Luminance is increased from 30 to100, a dim cherry red turns white.HSV works well for fluorescence images because the dyes in fluorescence images emit singlewavelength light. This light is, by definition, highly saturated. Editing in the HSV color spaceensures that all areas of the image retain their saturated colors. Thus, using the HSV color space,images can be modified without causing the bright areas to look unnaturally pastel or white.User Guide to the SPOT Insight Camera 143