Transparent RenderTexture solution

transparent.cs

class TransparentRenderTexture
{
	#region Shaders
	private static string mainVertexShader = @"
		varying vec2 maskPos;
		void main()
		{
			gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
			gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
			gl_FrontColor = gl_Color;
			maskPos = gl_Position.xy / 2.0 + vec2(0.5, 0.5);
		}
	";

	private static string mainFragmentShader = @"
		uniform sampler2D texture;
		uniform sampler2D mask;
		varying vec2 maskPos;
		void main()
		{
			vec4 pixel = texture2D(texture, gl_TexCoord[0].xy) * gl_Color;
			vec4 mask = texture2D(mask, maskPos);
			gl_FragColor = pixel;
			if(mask.r > 0.5)
			{
				gl_FragColor.rgb = pixel.rgb * pixel.a;
			}
		}
	";

	private static string maskVertexShader = @"
		void main()
		{
			gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
			gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
			gl_FrontColor = gl_Color;
		}
	";

	private static string maskFragmentShader = @"
		uniform sampler2D texture;
		void main()
		{
			vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
			if(pixel.a < 0.001)
			{
				discard;
			}
			gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
		}
	";
	#endregion

	private RenderTexture surface;
	private RenderTexture mask;
	private Shader mainShader;
	private Shader maskShader;
	private RenderStates mainRenderState;
	private RenderStates maskRenderState;

	public TransparentRenderTexture(uint width, uint height)
	{
		surface = new RenderTexture(width, height);
		mask = new RenderTexture(width, height);

		mainShader = Shader.FromString(mainVertexShader, mainFragmentShader);
		mainShader.SetParameter("texture", Shader.CurrentTexture);
		mainShader.SetParameter("mask", mask.Texture);
		maskShader = Shader.FromString(maskVertexShader, maskFragmentShader);
		maskShader.SetParameter("texture", Shader.CurrentTexture);

		mainRenderState = new RenderStates(mainShader);
		mainRenderState.BlendMode = new BlendMode(BlendMode.Factor.One, BlendMode.Factor.OneMinusSrcAlpha);
		maskRenderState = new RenderStates(maskShader);
	}

	public void Draw(Drawable drawable)
	{
		surface.Draw(drawable, mainRenderState);
		mask.Draw(drawable, maskRenderState);
		mask.Display();
	}

	public void Clear()
	{
		surface.Clear(Color.Transparent);
		mask.Clear(Color.Black);
	}

	public void Display()
	{
		surface.Display();
	}

	public Texture Texture { get { return surface.Texture; } }
}