src/mesa/drivers/dri/ffb/ffb_fog.c

   1
   2 /* FFB fog support:
   3  *
   4  * There are two levels of support for FOG in the Creator3D series.
   5  * Both involve a depth cue unit and 1 or 4 slope factors and scales
   6  * for varying the pixel intensity.
   7  *
   8  * Chips prior to FFB2 only have a single set of such settings, FFB2
   9  * and later have 4 settings.
  10  *
  11  * The basic depth cueing equation is:
  12  *
  13  *      C_final = dcsf(z) * C_orig + (1 - dcsf(z)) * C_fog
  14  *
  15  *      C_final --      The final color passed to blend unit or frame
  16  *                      buffer (if blending is disabled).
  17  *
  18  *      C_orig  --      The color we start with, which comes either from
  19  *                      the raster processor or cpu writes to the smart
  20  *                      framebuffer aperture.
  21  *
  22  *      C_fog   --      This is the "fog" color, ie. the desired color
  23  *                      at the deepest Z.
  24  *
  25  *      dcsf(z) --      The depth cue scale as a function of Z.
  26  *
  27  * With pre-FFB2 chips there are four parameters to control the depth
  28  * cue scaling.  Here is a diagram:
  29  *
  30  *                 1.0  -------------
  31  *                      |   |   |   |
  32  *                      |   |   |   |
  33  *              Sfront  XXXXX---+---+
  34  *                      |   |X  |   |
  35  *      dcsf(z)         |   | X |   |
  36  *                      |   |  X|   |
  37  *              Sback   +---+---XXXXX
  38  *                      |   |   |   |
  39  *                 0.0  -------------
  40  *                      0.0 Zf  Zb 1.0
  41  *
  42  *                            z
  43  * Therefore:
  44  *
  45  *      for Zf < z < Zb
  46  *
  47  *              dcsf(z) = Sback + ((Sfront - Sback) / (Zf - Zb)) * (Zb - z)
  48  *
  49  *      for z <= Zf
  50  *
  51  *              dcsf(z) = Sfront
  52  *
  53  *      for z >= Zb
  54  *
  55  *              dcsf(z) = Sback
  56  *
  57  * With FFB2 and later, 3 more slope regions are provided, the first of
  58  * them starts at the end of the region defined above and ends at a
  59  * specified depth value, the next slop region starts there and ends
  60  * at the next specified depth value, and so on.  Each of the 3 slope
  61  * regions also have scale and slope settings of their own.
  62  *
  63  * The C_fog color is programmed into the alpha blending unit color1
  64  * and color2 registers as follows:
  65  *
  66  *      color1:         -(C_fog)
  67  *      color2:         C_fog - bg
  68  *
  69  * If alpha blending is disabled, the bg factor is zero.  Note that
  70  * the alpha blending color registers specify each of the RGB values
  71  * as 9 bit 1:8 signed numbers in the range -1.00 to 0.ff inclusive.
  72  * (ie. 0x100 == -1.00 and 0x0ff == +0.ff)
  73  */