| /* FFB fog support: | |
| * | |
| * There are two levels of support for FOG in the Creator3D series. | |
| * Both involve a depth cue unit and 1 or 4 slope factors and scales | |
| * for varying the pixel intensity. | |
| * | |
| * Chips prior to FFB2 only have a single set of such settings, FFB2 | |
| * and later have 4 settings. | |
| * | |
| * The basic depth cueing equation is: | |
| * | |
| * C_final = dcsf(z) * C_orig + (1 - dcsf(z)) * C_fog | |
| * | |
| * C_final -- The final color passed to blend unit or frame | |
| * buffer (if blending is disabled). | |
| * | |
| * C_orig -- The color we start with, which comes either from | |
| * the raster processor or cpu writes to the smart | |
| * framebuffer aperture. | |
| * | |
| * C_fog -- This is the "fog" color, ie. the desired color | |
| * at the deepest Z. | |
| * | |
| * dcsf(z) -- The depth cue scale as a function of Z. | |
| * | |
| * With pre-FFB2 chips there are four parameters to control the depth | |
| * cue scaling. Here is a diagram: | |
| * | |
| * 1.0 ------------- | |
| * | | | | | |
| * | | | | | |
| * Sfront XXXXX---+---+ | |
| * | |X | | | |
| * dcsf(z) | | X | | | |
| * | | X| | | |
| * Sback +---+---XXXXX | |
| * | | | | | |
| * 0.0 ------------- | |
| * 0.0 Zf Zb 1.0 | |
| * | |
| * z | |
| * Therefore: | |
| * | |
| * for Zf < z < Zb | |
| * | |
| * dcsf(z) = Sback + ((Sfront - Sback) / (Zf - Zb)) * (Zb - z) | |
| * | |
| * for z <= Zf | |
| * | |
| * dcsf(z) = Sfront | |
| * | |
| * for z >= Zb | |
| * | |
| * dcsf(z) = Sback | |
| * | |
| * With FFB2 and later, 3 more slope regions are provided, the first of | |
| * them starts at the end of the region defined above and ends at a | |
| * specified depth value, the next slop region starts there and ends | |
| * at the next specified depth value, and so on. Each of the 3 slope | |
| * regions also have scale and slope settings of their own. | |
| * | |
| * The C_fog color is programmed into the alpha blending unit color1 | |
| * and color2 registers as follows: | |
| * | |
| * color1: -(C_fog) | |
| * color2: C_fog - bg | |
| * | |
| * If alpha blending is disabled, the bg factor is zero. Note that | |
| * the alpha blending color registers specify each of the RGB values | |
| * as 9 bit 1:8 signed numbers in the range -1.00 to 0.ff inclusive. | |
| * (ie. 0x100 == -1.00 and 0x0ff == +0.ff) | |
| */ |