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materialsystem/stdshaders/common_fog_ps_forcevertexfog_fxc.h

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+//
+// - This is the fog header to include for pixel shaders if the shader does support pixel-shader-blended vertex fog.
+//
+
+// -- PIXELFOGTYPE is 0 for RANGE FOG, 1 for WATER/HEIGHT FOG --
+// DYNAMIC: "PIXELFOGTYPE"	"0..0" [ = 0; ] //( pShaderAPI->GetSceneFogMode() == MATERIAL_FOG_LINEAR_BELOW_FOG_Z ) ? 1 : 0 ] 
+
+// -- DOPIXELFOG is 1 if the material has $vertexfog 0, or if the material forces pixel fog by default (lightmappedgeneric for instance.) --
+// STATIC: "DOPIXELFOG" "0..0" [ = 0 ]
+
+#if ( PIXELFOGTYPE == 1 )
+	// No matter what shader model we are, we do pixel fog for water and don't use fixed function hardware fog blending.
+	#undef DOPIXELFOG
+	#define DOPIXELFOG 1
+#endif
+
+#if defined( SHADER_MODEL_PS_2_0 ) && ( PIXELFOGTYPE == 0 )
+	#define HARDWAREFOGBLEND 1
+#else
+	#define HARDWAREFOGBLEND 0
+#endif
+
+// FIXME!  Will need to revisit this once we are doing water/height fog again.  Needs to be pixel for in the water case.