HLS Colour Model
This model has the double-cone representation shown in Figure 3.40. The three colour parameters in this model are called hue (H), lightness (L), and Saturation (S). Hue has the same meaning as in the HSV model. It specifies an angle about the vertical axis that locates a chosen hue. In this model, H = 00 corresponds to blue. The remaining colours are specified around the perimeter of the cone in the same order as in the HSV model. Magenta is at 60°, red is at 120°, and cyan is located at H = 180°. Again, complementary colours are 180° apart on the double cone.
The vertical axis in this model is called lightness, L. At L = 0, we have black, and white is at L = 1. Gray scale is along the L axis, and the "pure hues" lie on the L = 0.5 plane. Saturation parameter S again specifies relative purity of a colour. This parameter varies from 0 to 1, and pure hues are those for which S = 1 and L = 0.5. As S decreases, the hues are said to be less pure. At S = 0, we have the gray scale. As in the HSV model, the HLS system allows a user to think in terms of making a selected hue darker or lighter. A hue is selected with hue angle H, and the desired shade, tint, or tone is obtained by adjusting L and S. Colours are made lighter by increasing L and made darker by decreasing L. When S is decreased, the colours move toward gray.