GLSL shaders development in Kotlin

The sources provide a way of creating GLSL-like code in Kotlin.

Shader source in Kotlin:

class FragmentShader(useAlphaTest: Boolean) : ShaderBuilder() {
    private val alphaTestThreshold by uniform(::GLFloat)
    private val texture by uniform(::Sampler2D)
    private val uv by varying(::Vec2)
    init {
        var color by vec4()
        color = texture2D(texture, uv)
        // static branching
        if (useAlphaTest) {
            // dynamic branching
            If(color.w lt alphaTestThreshold) {
                discard()
            }
        }
        gl_FragColor = color
    }
}

You can get your GLSL source in two ways:

1. Runtime generation

FragmentShader(useAlphaTest = true).getSource()

Pros: easy to generate sources.

Cons: GLSL swizzling functionality may affect performance, as it creates lots of objects at runtime.

2. Generation using @ShaderProgram annotation

This functionality can be used as follows:

@ShaderProgram(VertexShader::class, FragmentShader::class)
class ShaderProgramName(alphaTest: Boolean)

Annotation processor generates new sources during gradle kaptKotlin task which look as follows:

class ShaderProgramNameSources {
    enum class Sources(vertex: String, fragment: String): ShaderProgramSources {
        Source0("<vertex code>", "<fragment code>")
        ...
    }
    fun get(alphaTest: Boolean) {
        if (alphaTest) return Source0
        else return Source1
    }
}

After generation you can get both vertex and fragment sources as follows:

val sources = ShaderProgramNameSources.get(replaceAlpha = true)
println(sources.vertex)
println(sources.fragment)

Pros: no need to generate sources at runtime, as you will have direct access to them.

Cons: shader source classes have to be accessible for AP to load them, thus can't be stored just anywhere in the project.

Generated GLSL

Both methods produce the same result:

uniform sampler2D texture;
uniform float alphaTestThreshold;
varying vec2 uv;
void main(void) {
    vec4 color;
    color = texture2D(texture, uv);
    if ((color.w < alphaTestThreshold)) {
        discard;
    }
    gl_FragColor = color;
}

How to use

You will need gradle to build sources.

You have two options here. If you do not need @ShaderProgram annotation functionality in the above example, then you can simply import the KotlinGlsl project (and may remove ProgramExample module).

If you do want to use @ShaderProgram, then you will have to store your Kotlin shader sources either directly in the KotlinGlsl or in a project accessible for it during annotations processing stage, as AP will try to load classes provided with @ShaderProgram.

License

MIT