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    _renderBuffer() {
        const engine = this.scene.getEngine();
        engine.setDepthFunctionToLessOrEqual();
 
        //depth write
        engine.setColorWrite(false);
        this.effect.setFloat('alphaTest', 0.9);
        engine.drawElementsType(Material.TriangleFillMode, 0, this.sprites.length * 6);
        engine.setColorWrite(true);
 
        //diffuse
        this.effect.setFloat('alphaTest', -0.9);
        engine.setAlphaMode(Engine.ALPHA_COMBINE);
        engine.drawElementsType(Material.TriangleFillMode, 0, this.sprites.length * 6);
        engine.setAlphaMode(Engine.ALPHA_DISABLE);
        this.effect.setFloat('alphaTest', 0);
    }