Working with RGB and HLS Color Coding Systems in SAS Software

Full-color displays have become commonplace with the advent of the Web. Typically they are rendered from desktop computers equipped with 16-bit color palettes capable of generating 65K unique colors. Thus a chart or scale is needed for making informed color selections for graphics applications. In SAS, the programmer has access to both the RGB (red/green/blue) and HLS (hue/light/saturation) coding systems. However, different algorithms are required for building charts and scales in the two systems. Before the technical aspects of chart and scale construction are presented in the paper, color spaces are defined to show how the full gamut of codes is mapped in both coding systems. For RGB, codes map to a cube, and a double-ended cone accommodates the Tektronix HLS system. With the color spaces as a foundation, instructions are then provided for building color charts with ODS and scales from PROC GPLOT. Both the charts and scales were introduced in the SUGI 27 paper Using ODS and the Macro Facility to Construct Color Charts and Scales for SAS Software Applications. RGB: The Color System for Screen Output The RGB (red/green/blue) Color Space Originally defined for color TV, the RGB system is the only system that is Web compatible (Weinman, 86). This means RGB codes are used in GIF, JPG and HTML files. In Figure 1, the color space for this screen-based system is depicted as a cube. While there are eight vertices in the cube, all are defined by combining the primary colors: red, green, and blue. Cyan(0,1,1), for example, contains a mixture of green and blue but no red, and red with (1,0,0) contains neither green nor blue only red. The cube in Figure 1 is constructed from an application of the G3D procedure that uses an ANNOTATE data set for coloring edges and vertices. The vertices are "dots" and the edges are marker font "U's" expressed as small parallelograms connected to form a "line" of color. Only one parameter is altered to traverse the cube. To go from red(1,0,0) to yellow(1,1,0), for example, green is added in incremental amounts along the base line. There is also a one-to-one correspondence between the X-Y-Z coordinates(0-1) and the RGB code values(0-255). Since SAS uses hexadecimal notation, yellow plotted at (1,1,0) translates to (255,255,0) or CXFFFF00 for actual coloring. The faces of the cube can be represented in 2-D space with an application of the GPLOT procedure. In Figure 2, one color coordinate is held constant and the other two are varied systematically to color all six sides of the RGB cube. Look at the top face in Figure 1. It corresponds to the second face rotated 90° in row 3 of Figure 2. Note that the value for the blue or third component at all four vertices is set to 1. The title "Blue=1" in Figure 2 should now make sense. To plot the diagonals in the RGB cube, all three components need to be systematically varied between two opposing vertices. The gray "line" connecting the black (0,0,0) and white (1,1,1) vertices in Figure 3 is created with an application of the LINES macro below: %macro Lines(SR,SG,SB,TR,TG,TB,n,ds); data &ds; length function font value color $8 text $12; retain function 'label' text 'U' style 'marker'; retain xsys ysys zsys '2'; %do i=0 %to %eval(&n-1); %let r=%sysevalf(&sr+(&tr.&sr.)*&i/&n.); Figure 1. RGB color space depicted as a cube. Coordinate ranges 0-1 directly map to color ranges 0-255.