// 
// TODO:
// - what to do with ftransform? can it stay in the current form?
// - implement texture1DLod, texture2DLod, texture3DLod, textureCubeLod,
// - implement shadow1DLod, shadow2DLod,
// 

// 
// From Shader Spec, ver. 1.051
// 
// Some OpenGL operations still continue to occur in fixed functionality in between the vertex
// processor and the fragment processor. Other OpenGL operations continue to occur in fixed
// functionality after the fragment processor. Shaders communicate with the fixed functionality
// of OpenGL through the use of built-in variables.
// 
// The variable gl_Position is available only in the vertex language and is intended for writing
// the homogeneous vertex position. All executions of a well-formed vertex shader must write
// a value into this variable. It can be written at any time during shader execution. It may also
// be read back by the shader after being written. This value will be used by primitive assembly,
// clipping, culling, and other fixed functionality operations that operate on primitives after
// vertex processing has occurred. Compilers may generate a diagnostic message if they detect
// gl_Position is not written, or read before being written, but not all such cases are detectable.
// Results are undefined if a vertex shader is executed and does not write gl_Position.
// 
// The variable gl_PointSize is available only in the vertex language and is intended for a vertex
// shader to write the size of the point to be rasterized. It is measured in pixels.
// 
// The variable gl_ClipVertex is available only in the vertex language and provides a place for
// vertex shaders to write the coordinate to be used with the user clipping planes. The user must
// ensure the clip vertex and user clipping planes are defined in the same coordinate space. User
// clip planes work properly only under linear transform. It is undefined what happens under
// non-linear transform.
// 
// These built-in vertex shader variables for communicating with fixed functionality are
// intrinsically declared with the following types:
// 

vec4 gl_Position;                                       // must be written to
float gl_PointSize;                                     // may be written to
vec4 gl_ClipVertex;                                     // may be written to

// 
// If gl_PointSize or gl_ClipVertex are not written to, their values are undefined. Any of these
// variables can be read back by the shader after writing to them, to retrieve what was written.
// Reading them before writing them results in undefined behavior. If they are written more than
// once, it is the last value written that is consumed by the subsequent operations.
// 
// These built-in variables have global scope.
// 

// 
// The following attribute names are built into the OpenGL vertex language and can be used from
// within a vertex shader to access the current values of attributes declared by OpenGL. All page
// numbers and notations are references to the OpenGL 1.4 specification.
// 

// 
// Vertex Attributes, p. 19.
// 

attribute vec4 gl_Color;
attribute vec4 gl_SecondaryColor;
attribute vec3 gl_Normal;
attribute vec4 gl_Vertex;
attribute vec4 gl_MultiTexCoord0;
attribute vec4 gl_MultiTexCoord1;
attribute vec4 gl_MultiTexCoord2;
attribute vec4 gl_MultiTexCoord3;
attribute vec4 gl_MultiTexCoord4;
attribute vec4 gl_MultiTexCoord5;
attribute vec4 gl_MultiTexCoord6;
attribute vec4 gl_MultiTexCoord7;
attribute float gl_FogCoord;

// 
// Unlike user-defined varying variables, the built-in varying variables don’t have a strict
// one-to-one correspondence between the vertex language and the fragment language. Two sets are
// provided, one for each language. Their relationship is described below.
// 
// The following built-in varying variables are available to write to in a vertex shader.
// A particular one should be written to if any functionality in a corresponding fragment shader
// or fixed pipeline uses it or state derived from it. Otherwise, behavior is undefined.
//

varying vec4 gl_FrontColor;
varying vec4 gl_BackColor;
varying vec4 gl_FrontSecondaryColor;
varying vec4 gl_BackSecondaryColor;
varying vec4 gl_TexCoord[];                             // at most will be gl_MaxTextureCoordsARB
varying float gl_FogFragCoord;

// 
// For gl_FogFragCoord, the value written will be used as the “c” value on page 160 of the
// OpenGL 1.4 Specification by the fixed functionality pipeline. For example, if the z-coordinate
// of the fragment in eye space is desired as “c”, then that's what the vertex shader should write
// into gl_FogFragCoord.
// 
// As with all arrays, indices used to subscript gl_TexCoord must either be integral constant
// expressions, or this array must be re-declared by the shader with a size. The size can be
// at most gl_MaxTextureCoordsARB. Using indexes close to 0 may aid the implementation
// in preserving varying resources.
// 

// 
// The OpenGL Shading Language defines an assortment of built-in convenience functions for scalar
// and vector operations. Many of these built-in functions can be used in more than one type
// of shader, but some are intended to provide a direct mapping to hardware and so are available
// only for a specific type of shader.
// 
// The built-in functions basically fall into three categories:
// 
// • They expose some necessary hardware functionality in a convenient way such as accessing
//   a texture map. There is no way in the language for these functions to be emulated by a shader.
// 
// • They represent a trivial operation (clamp, mix, etc.) that is very simple for the user
//   to write, but they are very common and may have direct hardware support. It is a very hard
//   problem for the compiler to map expressions to complex assembler instructions.
// 
// • They represent an operation graphics hardware is likely to accelerate at some point. The
//   trigonometry functions fall into this category.
// 
// Many of the functions are similar to the same named ones in common C libraries, but they support
// vector input as well as the more traditional scalar input.
// 
// Applications should be encouraged to use the built-in functions rather than do the equivalent
// computations in their own shader code since the built-in functions are assumed to be optimal
// (e.g., perhaps supported directly in hardware).
// 
// User code can replace built-in functions with their own if they choose, by simply re-declaring
// and defining the same name and argument list.
// 

// 
// Geometric Functions
// 
// These operate on vectors as vectors, not component-wise.
// 

// 
// For vertex shaders only. This function will ensure that the incoming vertex value will be
// transformed in a way that produces exactly the same result as would be produced by OpenGL’s
// fixed functionality transform. It is intended to be used to compute gl_Position, e.g.,
// gl_Position = ftransform()
// This function should be used, for example, when an application is rendering the same geometry in
// separate passes, and one pass uses the fixed functionality path to render and another pass uses
// programmable shaders.
// 

vec4 ftransform () {
    return gl_ModelViewProjectionMatrix * gl_Vertex;
}

// 
// Texture Lookup Functions
// 
// Texture lookup functions are available to both vertex and fragment shaders. However, level
// of detail is not computed by fixed functionality for vertex shaders, so there are some
// differences in operation between vertex and fragment texture lookups. The functions in the table
// below provide access to textures through samplers, as set up through the OpenGL API. Texture
// properties such as size, pixel format, number of dimensions, filtering method, number of mip-map
// levels, depth comparison, and so on are also defined by OpenGL API calls. Such properties are
// taken into account as the texture is accessed via the built-in functions defined below.
// 
// If a non-shadow texture call is made to a sampler whose texture has depth comparisons enabled,
// then results are undefined. If a shadow texture call is made to a sampler whose texture does not
// have depth comparisions enabled, the results are also undefined.
// 
// In all functions below, the bias parameter is optional for fragment shaders. The bias parameter
// is not accepted in a vertex shader. For a fragment shader, if bias is present, it is added to
// the calculated level of detail prior to performing the texture access operation. If the bias
// parameter is not provided, then the implementation automatically selects level of detail:
// For a texture that is not mip-mapped, the texture is used directly. If it is mip-mapped and
// running in a fragment shader, the LOD computed by the implementation is used to do the texture
// lookup. If it is mip-mapped and running on the vertex shader, then the base texture is used.
// 
// The built-ins suffixed with “Lod” are allowed only in a vertex shader. For the “Lod” functions,
// lod is directly used as the level of detail.
// 

// 
// Use the texture coordinate coord to do a texture lookup in the 1D texture currently bound
// to sampler. For the projective (“Proj”) versions, the texture coordinate coord.s is divided by
// the last component of coord.
// 
// XXX
vec4 texture1DLod (sampler1D sampler, float coord, float lod) {
    return vec4 (0.0);
}
vec4 texture1DProjLod (sampler1D sampler, vec2 coord, float lod) {
    return texture1DLod (sampler, coord.s / coord.t, lod);
}
vec4 texture1DProjLod (sampler1D sampler, vec4 coord, float lod) {
    return texture1DLod (sampler, coord.s / coord.q, lod);
}

// 
// Use the texture coordinate coord to do a texture lookup in the 2D texture currently bound
// to sampler. For the projective (“Proj”) versions, the texture coordinate (coord.s, coord.t) is
// divided by the last component of coord. The third component of coord is ignored for the vec4
// coord variant.
// 
// XXX
vec4 texture2DLod (sampler2D sampler, vec2 coord, float lod) {
    return vec4 (0.0);
}
vec4 texture2DProjLod (sampler2D sampler, vec3 coord, float lod) {
    return texture2DLod (sampler, vec2 (coord.s / coord.p, coord.t / coord.p), lod);
}
vec4 texture2DProjLod (sampler2D sampler, vec4 coord, float lod) {
    return texture2DLod (sampler, vec2 (coord.s / coord.q, coord.t / coord.q), lod);
}

// 
// Use the texture coordinate coord to do a texture lookup in the 3D texture currently bound
// to sampler. For the projective (“Proj”) versions, the texture coordinate is divided by coord.q.
// 
// XXX
vec4 texture3DLod (sampler3D sampler, vec3 coord, float lod) {
    return vec4 (0.0);
}
vec4 texture3DProjLod (sampler3D sampler, vec4 coord, float lod) {
    return texture3DLod (sampler, vec3 (coord.s / coord.q, coord.t / coord.q, coord.s / coord.q),
        lod);
}

// 
// Use the texture coordinate coord to do a texture lookup in the cube map texture currently bound
// to sampler. The direction of coord is used to select which face to do a 2-dimensional texture
// lookup in, as described in section 3.8.6 in version 1.4 of the OpenGL specification.
// 
// XXX
vec4 textureCubeLod (samplerCube sampler, vec3 coord, float lod) {
    return vec4 (0.0);
}

// 
// Use texture coordinate coord to do a depth comparison lookup on the depth texture bound
// to sampler, as described in section 3.8.14 of version 1.4 of the OpenGL specification. The 3rd
// component of coord (coord.p) is used as the R value. The texture bound to sampler must be a
// depth texture, or results are undefined. For the projective (“Proj”) version of each built-in,
// the texture coordinate is divide by coord.q, giving a depth value R of coord.p/coord.q. The
// second component of coord is ignored for the “1D” variants.
// 
// XXX
vec4 shadow1DLod (sampler1DShadow sampler, vec3 coord, float lod) {
    return vec4 (0.0);
}
// XXX
vec4 shadow2DLod (sampler2DShadow sampler, vec3 coord, float lod) {
    return vec4 (0.0);
}
vec4 shadow1DProjLod(sampler1DShadow sampler, vec4 coord, float lod) {
    return shadow1DLod (sampler, vec3 (coord.s / coord.q, 0.0, coord.p / coord.q), lod);
}
vec4 shadow2DProjLod(sampler2DShadow sampler, vec4 coord, float lod) {
    return shadow2DLod (sampler, vec3 (coord.s / coord.q, coord.t / coord.q, coord.p / coord.q),
        lod);
}