Apart from the shader functions and constants defined in the OpenGL ES Shading Language (GLSL ES) Reference Pages, there are also a number of shader constants available to you that are unique to GameMaker Studio 2.

Matrices

The following display matrix constants can be used as array indices when using the shader array constant gm_Matrices:

Constant	Description
MATRIX_VIEW	This array index constant holds the index to the current view matrix. The index returned would be used as an array value when calling the gm_Matrices constant within the shader code.
MATRIX_PROJECTION	This array index constant holds the index to the current projection matrix. The index returned would be used as an array value when calling the gm_Matrices constant within the shader code.
MATRIX_WORLD	This array index constant holds the index to the current world matrix. This can be used for things like lighting if you have light information in world-space. The index returned would be used as an array value when calling the gm_Matrices constant within the shader code.
MATRIX_WORLD_VIEW	This array index constant holds the index to the result of the world and view matrices multiplied together. This is often used for things like fog. The index returned would be used as an array value when calling the gm_Matrices constant within the shader code.
MATRIX_WORLD_VIEW_PROJECTION	This array index constant holds the index to the result of the world, view and projection matrices multiplied together. This is the normal transformation matrix used for vertex positions. The index returned would be used as an array value when calling the gm_Matrices constant within the shader code.
MATRIX_MAX	This is not an array index, but rather returns the size of the matrix array in the vertex shader.
MAX_VS_LIGHTS	This is not an array index, but rather returns the number of the lights in the vertex shader.

Other Constants

The following pre-defined matrix uniforms and constants can be used in your shader to access GameMaker Studio 2 specific values:

Constant	Description
gm_Matrices[matrix]	This array constant returns a transform matrix and is one of the available pre-defined uniforms that GameMaker Studio 2 creates for you to use within the shader code editor. The array index is chosen from one of the above listed constants.
gm_BaseTexture	This is a 2D sampler constant that returns the texture of the current object, as set by GameMaker Studio 2. So it would be the current sprite, surface or texture that would normally be used when drawing without the shader being called.
gm_LightingEnabled	This can be used to get or set the GameMaker Studio 2 lighting when using 3D.
gm_FogStart	This can be used to get the distance where polygons start to be blended with the fog colour.
gm_RcpFogRange	This can be used to get the distance at which fog is maximal and nothing can be seen anymore.
gm_PS_FogEnabled	This will return true or false if the GPU has pixel fog enabled or not.
gm_FogColour	This can be used to get the fog colour used by GameMaker Studio 2.
gm_VS_FogEnabled	This will return true or false if the GPU has vertex fog enabled or not.
gm_AlphaTestEnabled	This can be used to get alpha testing in the shader. See gpu_set_alphatestenable for more information on alpha testing.
gm_AlphaRefValue	This can be used to get the current alpha testing reference value. See gpu_set_alphatestref for more information on the alpha test reference.