HSI and HSV color space

Question

What is the difference between HSI and HSV color space? I want to use HSI color space but I did not find any useful material for HSI. Is HSI the same as HSV?

Answer 1

HSI, HSV, and HSL are all different color spaces. Hue computation is (as far as I can find) identical between the three models, and uses a 6-piece piece-wise function to determine it, or for a simpler model that is accurate to within 1.2 degrees, atan((sqrt(3)⋅(G-B))/2(R-G-B)) can be used. For the most part, these two are interchangeable, but generally HSV and HSL use the piece-wise model, where HSI usually uses the arctan model. Different equations may be used, but these usually sacrifice precision for either simplicity or faster computation.

For lightness/value/intensity, the three spaces use slightly different representations.

• Intensity is computed by simply averaging the RGB values: (1/3)⋅(R+G+B).
• Lightness averages the minimum and maximum values for RGB: (1/2)⋅(max(R,G,B) + min(R,G,B)).
• Value is the simplest, being the value of the maximum of RGB: max(R,G,B).

When used in subsequent calculations, L/V/I is scaled to a decimal between 0 and 1.

Saturation is where the three models differ the most. For all 3, if I/V/L is 0, then saturation is 0 (this is for black, so that its representation is unambiguous), and HSL additionally sets saturation to 0 if lightness is maximum (because for HSL maximum lightness means white).

• HSL and HSV account for both the minimum and maximum of RGB, taking the difference between the two: max(R,G,B) - min(R,G,B), this value is sometimes referred to as chroma (C).
• HSV then takes the chroma and divides it by the value to get the saturation: C/V.
• HSL divides chroma by an expression taking lightness into account: C/(1-abs(2L-1)).
• HSI doesn't use chroma, instead only taking min(R,G,B) into account: min(R,G,B)/I.

Sources

• Wikipedia: HSL and HSV
• Wikipedia: Hue