LibGDX: MaskGroup 🎭

A group that knows how to disguise their children

``Photo by John Noonan on Unsplash

To understand how this group works, read this article: https://veldan1202.medium.com/libgdx-masking-using-blending-function-81c46fdfac33

mask.png | child.png

Android:

MaskGroup use Mask:

private val mask = Texture("mask.png")

private val maskGroup = MaskGroup(mask)

MaskGroup unuse Mask:

private val maskGroup = MaskGroup(null)

MaskGroup:

import com.badlogic.gdx.Gdx
import com.badlogic.gdx.graphics.GL20
import com.badlogic.gdx.graphics.Texture
import com.badlogic.gdx.graphics.g2d.Batch
import com.badlogic.gdx.math.Vector2
import com.badlogic.gdx.scenes.scene2d.Group
import com.badlogic.gdx.utils.viewport.Viewport

class MaskGroup(
    val mask: Texture? = null,
) : Group() {

    private lateinit var localToScreenCoordinates: Vector2
    private lateinit var localToScreenSize       : Vector2

    private var mX: Float = 0f
    private var mY: Float = 0f
    private var mW: Float = 0f
    private var mH: Float = 0f



    override fun draw(batch: Batch?, parentAlpha: Float) {
        if (stage != null) {
            updateCoordinatesAndSize()
            batch?.endbegin()
            drawScissor(batch, parentAlpha)
        }
    }



    private fun drawScissor(batch: Batch?, parentAlpha: Float) {
        Gdx.gl.glEnable(GL20.GL_SCISSOR_TEST)
        Gdx.gl.glScissor(mX.toInt(), mY.toInt(), mW.toInt(), mH.toInt())

        if (mask != null) batch?.drawMask(parentAlpha) else super.draw(batch, parentAlpha)

        Gdx.gl.glDisable(GL20.GL_SCISSOR_TEST)
    }



    private fun updateCoordinatesAndSize() {
        localToScreenCoordinates = localToScreenCoordinates(Vector2())
        localToScreenSize        = stage.viewport.project(Vector2(width, height))

        mX = localToScreenCoordinates.x
        mY = Gdx.graphics.height - localToScreenCoordinates.y
        mW = localToScreenSize.x - stage.viewport.zeroScreenVector.x
        mH = localToScreenSize.y - stage.viewport.zeroScreenVector.y
    }


    
    private fun Batch.drawMask(parentAlpha: Float) {
        Gdx.gl.glColorMask(false, false, false, true)

        setBlendFunction(GL20.GL_ONE, GL20.GL_ZERO)
        draw(mask, x, y, width, height)

        drawMasked(parentAlpha)
    }

    private fun Batch.drawMasked(parentAlpha: Float) {
        setBlendFunction(GL20.GL_ZERO, GL20.GL_SRC_ALPHA)
        super.draw(this, parentAlpha)
        flush()

        Gdx.gl.glColorMask(true, true, true, true)
        setBlendFunction(GL20.GL_DST_ALPHA, GL20.GL_ONE_MINUS_DST_ALPHA)
        super.draw(this, parentAlpha)
        flush()

        setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA)
    }
    
}

val Viewport.zeroScreenVector: Vector2 get() = project(Vector2(0f, 0f))
fun Batch.endbegin(block: Batch.() -> Unit = {  }) {
    end()
    block()
    begin()
}

PS. Vel_daN: Love what You DO 💚.