Writing Lua Shaders

Parent: Shaders

Lua Shaders

When you write your shaders in Lua, they will be automatically translated to the relevant shader Language for the platform you are using (eg GLSL, HLSL or MSL). This is the recommended way to write your shaders in Gideros.

The Shader API allows the creation of Shader objects from within Lua. The Shader.lua() constructor takes five arguments:

• the Vertex shader code (a lua function)
• the Fragment shader code (a lua function)
• a set of numerical flags or 0 if none
• an array of uniforms/constants descriptors
• an array of attributes descriptors

With Lua Shaders, it is assumed that the code is within the Vertex and Fragment parameter functions.

A typical Lua Vertex and Fragment shader code looks like:

function vssaturate(vVertex, vColor, vTexCoord) : Shader -- Vertex Shader
	local vertex = hF4(vVertex, 0.0, 1.0)
	fTexCoord = vTexCoord
	return vMatrix * vertex
end

function fssaturate() : Shader -- Fragment Shader
	local frad = (1 + sin(fTime)) * 0.5
	local frag = lF4(fColor) * texture2D(fTexture, fTexCoord)
	local coef = lF3(0.2125, 0.7154, 0.0721)
	local dp = dot(frag.rgb, coef)
	frag.rgb = mix(frag.rgb, frag.rgb / dp, frad)
	if (frag.a == 0.0) then discard() end
	return frag
end

Once both functions are created, you pass them in Shader.lua:

local saturate = Shader.lua(vssaturate, fssaturate, 0,
	{
	{name = "vMatrix", type = Shader.CMATRIX, sys = Shader.SYS_WVP, vertex = true},
	{name = "fColor", type = Shader.CFLOAT4, sys = Shader.SYS_COLOR, vertex = false},
	{name = "fTexture", type = Shader.CTEXTURE, vertex = false},
	{name = "fTextureInfo", type = Shader.CFLOAT4, sys = Shader.SYS_TEXTUREINFO, vertex = false},
	{name = "fTime", type = Shader.CFLOAT, sys = Shader.SYS_TIMER, vertex = false},
	},
	{
	{name = "vVertex", type = Shader.DFLOAT, mult = 2, slot = 0, offset = 0},
	{name = "vColor", type = Shader.DUBYTE, mult = 4, slot = 1, offset = 0},
	{name = "vTexCoord", type = Shader.DFLOAT, mult = 2, slot = 2, offset = 0},
	},
	{
	{name = "fTexCoord", type = Shader.CFLOAT2},
	}
)

Changing Uniforms/Constants

In order to change the value of a uniform from Lua, you use the setConstant function on the Shader, it takes four arguments:

• the uniform name
• the type of data to set: one of the Shader.Cxxx constants
• the number of elements of the given type to set (number)
• the actual data to set, either as a table or as multiple arguments

saturate:setConstant("fSwitch", Shader.CINT, 1, 0) -- set fSwitch value to be 0 (off)

This will change the value of a Uniform for the whole Shader.

You can also change the value of a Uniform on a sprite to sprite basis using Sprite:setShaderConstant:

bmp:setShaderConstant("fSwitch", Shader.CINT, 1, 22) -- set fSwitch value to be 0 (on)

Associating a Shader to a Sprite

The Sprite API has a new call to deal with that: Sprite:setShader(shader) tells Gideros to use the specified shader for rendering the sprite. Setting back the shader to nil actually revert to the default shader.

bmp:setShader(saturate)

Lua Shader Functions

(Please note that these functions may or may not be available, the Lua shader is still under development)

Language Helpers

Conversion and Constructors

Angle and Trigonometry

Exponential

Common

Geometric

Matrix

Vector Relational

Texture Lookup

Fragment Processing

Noise

Geometric

Restrictions

Don't rely on anything external, basically no globals except for attributes, uniforms and varying.

• No Do..While
• No Repeat..Until
• No tables. The only allowed form of table lookup is for vector component swizzling and access to uniform arrays
• No strings, userdata, etc. Basically only numbers are supported
• Mind your types! Although lua is not a typed language, shaders need types
• Always check on all platforms: Gideros will translate your lua code to GLSL,HLSL or MSL code, but some platforms are more strict than others about what you can write, or how many arguments a common function is supposed to take

Lua Shader Examples

Rain drops demo - by Hgy29

Further Reading

OpenGL Shading Language: https://www.khronos.org/opengl/wiki/OpenGL_Shading_Language
Common compatibility pitfalls: https://www.opengl.org/wiki/GLSL_:_common_mistakes
The Book of Shaders: https://thebookofshaders.com/
ShaderToy: https://www.shadertoy.com/

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