vkd3d-shader/ir: Allow controlling fog through parameters.

Fog requires several parameters to even implement the most basic of
functionality correctly, so this commit is relatively large.

include/vkd3d_shader.h

@@ -476,6 +476,45 @@ enum vkd3d_shader_binding_flag
     VKD3D_FORCE_32_BIT_ENUM(VKD3D_SHADER_BINDING_FLAG),
 };
 
+/**
+ * The factor used to interpolate the fragment output colour with fog.
+ *
+ * See VKD3D_SHADER_PARAMETER_NAME_FOG_FRAGMENT_MODE for specification of the
+ * interpolation factor as defined here.
+ *
+ * The following variables may be used to determine the interpolation factor:
+ *
+ * c = The fog coordinate value output from the vertex shader. This is an
+ *     inter-stage varying with the semantic name "FOG" and semantic index 0.
+ * E = The value of VKD3D_SHADER_PARAMETER_NAME_FOG_END.
+ * k = The value of VKD3D_SHADER_PARAMETER_NAME_FOG_SCALE.
+ *
+ * \since 1.15
+ */
+enum vkd3d_shader_fog_fragment_mode
+{
+    /**
+     * No fog interpolation is applied;
+     * the output colour is passed through unmodified.
+     * Equivalently, the fog interpolation factor is 1.
+     */
+    VKD3D_SHADER_FOG_FRAGMENT_NONE = 0x0,
+    /**
+     * The fog interpolation factor is (E - c) * k.
+     *
+     * In order to implement traditional linear fog, as present in Direct3D and
+     * OpenGL, i.e.
+     *
+     *     (end - c) / (end - start)
+     *
+     * set
+     *
+     *     E = end
+     *     k = 1 / (end - start)
+     */
+    VKD3D_SHADER_FOG_FRAGMENT_LINEAR = 0x3,
+};
+
 /**
  * The manner in which a parameter value is provided to the shader, used in
  * struct vkd3d_shader_parameter and struct vkd3d_shader_parameter1.
@@ -739,6 +778,73 @@ enum vkd3d_shader_parameter_name
      * \since 1.14
      */
     VKD3D_SHADER_PARAMETER_NAME_POINT_SPRITE,
+    /**
+     * Fog mode used in fragment shaders.
+     *
+     * The value specified by this parameter must be a member of
+     * enum vkd3d_shader_fog_fragment_mode.
+     *
+     * If not VKD3D_SHADER_FOG_FRAGMENT_NONE, the pixel shader colour output at
+     * location 0 is linearly interpolated with the fog colour defined by
+     * VKD3D_SHADER_PARAMETER_NAME_FOG_COLOUR. The interpolation factor is
+     * defined according to the enumerant selected by this parameter.
+     * The interpolated value is then outputted instead of the original value at
+     * location 0.
+     *
+     * An interpolation factor of 0 specifies to use the fog colour; a factor of
+     * 1 specifies to use the original colour output. The interpolation factor
+     * is clamped to the [0, 1] range before interpolating.
+     *
+     * The default value is VKD3D_SHADER_FOG_FRAGMENT_NONE.
+     *
+     * The data type for this parameter must be
+     * VKD3D_SHADER_PARAMETER_DATA_TYPE_UINT32.
+     *
+     * Only VKD3D_SHADER_PARAMETER_TYPE_IMMEDIATE_CONSTANT is supported in this
+     * version of vkd3d-shader.
+     *
+     * \since 1.15
+     */
+    VKD3D_SHADER_PARAMETER_NAME_FOG_FRAGMENT_MODE,
+    /**
+     * Fog colour.
+     * See VKD3D_SHADER_PARAMETER_NAME_FOG_FRAGMENT_MODE for documentation of
+     * fog.
+     *
+     * The data type for this parameter must be
+     * VKD3D_SHADER_PARAMETER_DATA_TYPE_FLOAT32_VEC4.
+     *
+     * The default value is transparent black, i.e. the vector {0, 0, 0, 0}.
+     *
+     * \since 1.15
+     */
+    VKD3D_SHADER_PARAMETER_NAME_FOG_COLOUR,
+    /**
+     * End coordinate for linear fog.
+     * See VKD3D_SHADER_PARAMETER_NAME_FOG_FRAGMENT_MODE for documentation of
+     * fog.
+     *
+     * The data type for this parameter must be
+     * VKD3D_SHADER_PARAMETER_DATA_TYPE_FLOAT32.
+     *
+     * The default value is 1.0.
+     *
+     * \since 1.15
+     */
+    VKD3D_SHADER_PARAMETER_NAME_FOG_END,
+    /**
+     * Scale value for linear fog.
+     * See VKD3D_SHADER_PARAMETER_NAME_FOG_FRAGMENT_MODE for documentation of
+     * fog.
+     *
+     * The data type for this parameter must be
+     * VKD3D_SHADER_PARAMETER_DATA_TYPE_FLOAT32.
+     *
+     * The default value is 1.0.
+     *
+     * \since 1.15
+     */
+    VKD3D_SHADER_PARAMETER_NAME_FOG_SCALE,
 
     VKD3D_FORCE_32_BIT_ENUM(VKD3D_SHADER_PARAMETER_NAME),
 };