1 files changed, 73 insertions, 0 deletions
diff --git a/src/mesa/drivers/dri/ffb/ffb_fog.c b/src/mesa/drivers/dri/ffb/ffb_fog.c
new file mode 100644
index 00000000000..e6eca2390ee
--- /dev/null
+++ b/src/mesa/drivers/dri/ffb/ffb_fog.c
@@ -0,0 +1,73 @@
+
+/* 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)
+ */