12.6 CC - Color Combiner

The color combiner (CC) combines texels from TX and stepped RGBA pixel values from RS. The CC is the ultimate paint mixer. It can take two color values from many sources and linearly interpolate between them. The CC basically performs this equation:

newcolor = (A-B)*C+D

Here, A, B, C, and D can come from many different sources. Notice that if D=B, then this is a simple linear interpolator.

Figure 12-9 Color Combiner State and Input/Output

Most of CC programming involves setting the desired sources for (A,B,C,D) of the equation above. There are also programmable color registers within CC that can be used to source (A,B,C,D) input of the interpolator.

The Figure 12-10 and the Figure 12-11 describes all possible input selection of a general purpose linear interpolator for RGB and Alpha color combination. The input in the shaded boxes are CC internal state that you can set. Most are programmable color registers.

Figure 12-10 RGB Color Combiner Input Selection

Figure 12-11 Alpha Combiner Input Selection

There are two internal color registers in the CC: primitive and environment color. The primitive color can be used to set a constant polygon face color. The environment color can be used to represent the ambient color of the environment. Both can be used as source for linear interpolation. The names "primitive" and "environment" are purely arbitrary; you can use them for any purpose you wish.

Many of the typical RGB and alpha input selections are predefined in Table 12-12. In 1 cycle mode both mode1 and mode2 should be same. Please see the man page for gDPSetCombineMode for a description of each mode setting.

Note: In one-cycle mode, mode1 and mode2 should be the same value.

Color Combiner (CC) can perform two linear interpolation arithmetic computations in two-cycle pipeline mode. Typically, the second cycle is used to perform texture and shading color modulation (in other words, all those modes you saw in one-cycle mode). However, the first cycle can be used for another linear interpolation calculation; for example, LOD interpolation between the two bilinear filtered texels from two mipmap tiles.

Color Combiner (CC) can be programmed more specifically when you design your own color combine modes. To define a new mode use the format:

#define G_CC_MYNEWMODE a,b,d,d, A,B,C,D

Where the color output will be (a-b)*c+d and the alpha output will be (A-B)*C+D. The values you can use for each of a, b, c, d, A, B, C, and D are:

COMBINED        combined output from cycle 1 mode
TEXEL0          texture map output
TEXEL1          texture map output from tile+1 
PRIMITIVE       PrimColor
SHADE           Shade color
ENVIRONMENT     Environment color
CENTER          chroma key center value
SCALE           key scale value
COMBINED_ALPHA  combined alpha output from cycle 1
TEXEL0_ALPHA    texture map alpha
TEXEL1_ALPHA    texture map alpha from tile+1
PRIMITIVE_ALPHA PrimColor Alpha
SHADE_ALPHA     Shade alpha
ENV_ALPHA       Environment color alpha
LOD_FRACTION    LOD fraction
PRIM_LOD_FRAC   Prim LOD fraction
NOISE           noise (random)
K4              color convert constant K4
K5              color convert constant k5
1               1.0
0               0.0

Then you can use your new mode just like a regular mode:

gDPSetCombineMode(G_CC_MYNEWMODE, G_CC_MYNEWMODE);

The color combiner can be used to perform "chroma keying" which is a process where areas of a certain color are taken out and replaced with a texture. This is a similar effect to "blue screen photography" or as seen on the television news weather maps.

The theory is quite simple; a key color is provided, and all pixels of this color are replaced by the texel color requested. The key color is actually specified as a center and width, allowing soft-edge chroma keying (for blended colors):

Chroma Key Equations

KeyR = clamp(0, (-abs((R - RCen) * RScl) + RWd), 255)
KeyG = clamp(0, (-abs((G - Gen) * GScl) + GWd), 255)
KeyB = clamp(0, (-abs((B - BCen) * BScl) + BWd), 255)
KeyA = min(KeyR, KeyG, KeyB)

The center, scale, and width parameters have the following meanings:

In two-cycle mode, the keying operation must be specified in the second cycle (key alpha is not available as a combine operand). The combine mode G_CC_CHROMA_KEY2 is defined for this purpose.

gsDPSetCombineKey(G_CK_KEY)

The command above enables chroma keying.

gsDPSetKeyR(cR, sR, wR)
gsDPSetKeyGB(cG, sG, wG, cB, sB, wB)

The commands above allow you to set the parameters for each channel.