SFML Layer Mask

main.cpp

#include <SFML/Graphics.hpp>

int main()
{
	sf::Texture bg_tex;
	bg_tex.loadFromFile("bg.jpg");
	sf::Sprite bg{bg_tex};

	sf::Texture mask_tex;
	mask_tex.loadFromFile("mask.png");

	sf::Texture fg_tex;
	fg_tex.loadFromFile("fg.jpg");

	sf::Shader shader;
	shader.loadFromFile("Mix.frag", sf::Shader::Fragment);

	shader.setUniform("tex1", bg_tex);
	shader.setUniform("tex2", fg_tex);

	sf::CircleShape circle_shape{100.f};
	circle_shape.setOrigin(100.f, 100.f);

	sf::RenderTexture circle_tex;
	circle_tex.create(1280, 720);
	
	sf::Sprite sprite{circle_tex.getTexture()};


	sf::RenderWindow window({1280, 720}, "SFML Layer Mask");

	bool lmb_pressed{false};

	while (window.isOpen())
	{
		sf::Event event;
		while (window.pollEvent(event))
		{
			if (event.type == sf::Event::Closed)
				window.close();

			if (event.type == sf::Event::MouseButtonPressed && event.mouseButton.button == sf::Mouse::Left)
				lmb_pressed = true;

			if (event.type == sf::Event::MouseButtonReleased && event.mouseButton.button == sf::Mouse::Left)
				lmb_pressed = false;
		}

		if (lmb_pressed)
			circle_shape.setPosition(static_cast<sf::Vector2f>(sf::Mouse::getPosition(window)));

		circle_tex.clear();
		circle_tex.draw(circle_shape);
		circle_tex.display();

		window.clear();
		window.draw(sprite, &shader);
		window.display();
	}

	return 0;
}

Mix.frag

// Most credit to @fallahn

#version 120

uniform sampler2D tex1;
uniform sampler2D tex2;
uniform sampler2D maskTex;

void main()
{
	vec2 coord = gl_TexCoord[0].xy;
	coord.y = 1.0 - coord.y;

	vec4 mixed = texture2D(maskTex, gl_TexCoord[0].xy);
	vec4 color = mix(texture2D(tex1, coord), texture2D(tex2, coord), mixed);
	
	gl_FragColor = color;
}