内置片元属性(fragment built-ins)
Godot片元函数的默认用途是设置对象材质的属性，并使内置渲染器处理最终的着色。然而你并不需要使用所有这些属性，如果你对其不做任何改写，Godot将优化掉相应的功能。
| Built-in | Description |
|---|---|
| in vec4 FRAGCOORD | Fragment coordinate, pixel adjusted. In screen space. |
| in mat4 WORLD_MATRIX | Model space to world space transform. |
| in mat4 INV_CAMERA_MATRIX | World space to view space transform. |
| in mat4 CAMERA_MATRIX | View space to world space transform. |
| in mat4 PROJECTION_MATRIX | View space to clip space transform. |
| in mat4 INV_PROJECTION_MATRIX | Clip space to view space transform. |
| in float TIME | Elapsed total time in seconds. |
| in vec2 VIEWPORT_SIZE | Size of viewport (in pixels). |
| in vec3 VERTEX | Vertex that comes from vertex function (default, in view space). |
| in vec3 VIEW | Vector from camera to fragment position (in view space). |
| in bool FRONT_FACING | True if current face is front face. |
| inout vec3 NORMAL | Normal that comes from vertex function (default, in view space). |
| inout vec3 TANGENT | Tangent that comes from vertex function. |
| inout vec3 BINORMAL | Binormal that comes from vertex function. |
| out vec3 NORMALMAP | Set normal here if reading normal from a texture instead of NORMAL. |
| out float NORMALMAP_DEPTH | Depth from variable above. Defaults to 1.0. |
| in vec2 UV | UV that comes from vertex function. |
| in vec2 UV2 | UV2 that comes from vertex function. |
| in vec4 COLOR | COLOR that comes from vertex function. |
| out vec3 ALBEDO | Albedo (default white). |
| out float ALPHA | Alpha (0..1); if written to, the material will go to the transparent pipeline. |
| out float METALLIC | Metallic (0..1). |
| out float SPECULAR | Specular. Defaults to 0.5, best not to modify unless you want to change IOR. |
| out float ROUGHNESS | Roughness (0..1). |
| out float RIM | Rim (0..1). If used, Godot calculates rim lighting. |
| out float RIM_TINT | Rim Tint, goes from 0 (white) to 1 (albedo). If used, Godot calculates rim lighting. |
| out float CLEARCOAT | Small added specular blob. If used, Godot calculates Clearcoat. |
| out float CLEARCOAT_GLOSS | Gloss of Clearcoat. If used, Godot calculates Clearcoat. |
| out float ANISOTROPY | For distorting the specular blob according to tangent space. |
| out vec2 ANISOTROPY_FLOW | Distortion direction, use with flowmaps. |
| out float SSS_STRENGTH | Strength of Subsurface Scattering. If used, Subsurface Scattering will be applied to object. |
| out vec3 TRANSMISSION | Transmission mask (default 0,0,0). Allows light to pass through object. Only applied if used. |
| out float AO | Strength of Ambient Occlusion. For use with pre-baked AO. |
| out float AO_LIGHT_AFFECT | How much AO affects lights (0..1; default 0). |
| out vec3 EMISSION | Emission color (can go over 1,1,1 for HDR). |
| sampler2D SCREEN_TEXTURE | Built-in Texture for reading from the screen. Mipmaps contain increasingly blurred copies. |
| sampler2D DEPTH_TEXTURE | Built-in Texture for reading depth from the screen. Must convert to linear using INV_PROJECTION. |
| out float DEPTH | Custom depth value (0..1). |
| in vec2 SCREEN_UV | Screen UV coordinate for current pixel. |
| in vec2 POINT_COORD | Point Coordinate for drawing points with POINT_SIZE. |
| out float ALPHA_SCISSOR | If written to, values below a certain amount of alpha are discarded. |
| in bool OUTPUT_IS_SRGB | True when calculations happen in sRGB color space (true in GLES2, false in GLES3). |
内置光属性(light built-ins)
自定义光函数完全是可选的。你可以将渲染模式render_mode设置为unshaded 从而忽略掉光函数。如果没有光函数，Godot将会使用片元函数中的材质属性来计算光照(取决于渲染模式render_mode)。
编写光函数实际上就是给DIFFUSE_LIGHT (慢反射光)或者SPECULAR_LIGHT (镜面反射光) 赋一些值。如果什么值都不赋，将意味着不处理任何光。
光函数会对每一个光源进行逐像素调用，并且会对每一种光源类型循环调用。
下面是一个使用兰博特光照模型(Lambertian lighting model)的自定义光函数的范例:
void light() {
DIFFUSE_LIGHT += dot(NORMAL, LIGHT) * ATTENUATION * ALBEDO;
}
如果你想让光照叠加，可以使用+=运算符将各个光叠加到DIFFUSE_LIGHT。
| 内置属性 | 描述 |
|---|---|
| in vec4 FRAGCOORD | Fragment coordinate, pixel adjusted. |
| in mat4 WORLD_MATRIX | Model space to world space transform. |
| in mat4 INV_CAMERA_MATRIX | World space to view space transform. |
| in mat4 CAMERA_MATRIX | View space to world space transform. |
| in mat4 PROJECTION_MATRIX | View space to clip space transform. |
| in mat4 INV_PROJECTION_MATRIX | Clip space to view space transform. |
| in float TIME | Elapsed total time in seconds. |
| in vec2 VIEWPORT_SIZE | Size of viewport (in pixels). |
| in vec3 NORMAL | Normal vector, in view space. |
| in vec3 VIEW | View vector, in view space. |
| in vec3 LIGHT | Light Vector, in view space. |
| in vec3 LIGHT_COLOR | Color of light multiplied by energy. |
| in vec3 ATTENUATION | Attenuation based on distance or shadow. |
| in vec3 ALBEDO | Base albedo. |
| in vec3 TRANSMISSION | Transmission mask. |
| in float ROUGHNESS | Roughness. |
| out vec3 DIFFUSE_LIGHT | Diffuse light result. |
| out vec3 SPECULAR_LIGHT | Specular light result. |
| in bool OUTPUT_IS_SRGB | True when calculations happen in sRGB color space (true in GLES2, false in GLES3). |
