diff --git a/compare.glsl b/compare.glsl
new file mode 100644
index 0000000..90c7bf6
--- /dev/null
+++ b/compare.glsl
@@ -0,0 +1,19 @@
+// Compute the pixelwise diff between two textures.
+// Computes the diff for each color chanel separately (r, g, b).
+// Assumes both images are not transparent.
+
+// try isampler2D, ivec4 etc.? Probably not
+uniform vec2 size;
+uniform sampler2D base;
+uniform sampler2D curr;
+
+void main(void)
+{
+	vec2 pos = gl_FragCoord.xy/size;
+// 	vec4 color = texture(curr, pos);  // fails for some reason!?
+// 	vec4 basecolor = texture(base, pos);  // fails too...
+// 	color.xyz = abs(color.xyz - basecolor.xyz);
+// 	gl_FragColor = color;
+// 	gl_FragColor = basecolor;
+	gl_FragColor = vec4(1.0);
+}
diff --git a/src/main.cpp b/src/main.cpp
index ab8a65d..c8a4d32 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -42,6 +42,75 @@
  *  - invisible?
  */
 
+int main() {
+	const float winW = 480;
+	const float winH = 480;
+	
+	sf::RenderWindow window(sf::VideoMode(winW, winH), "gross");
+	window.setMouseCursorVisible(false); // hide the cursor
+	
+	// load images
+	sf::Texture base;
+	sf::Texture comp;
+	base.loadFromFile("tom-face.png");
+	comp.loadFromFile("tom-face-christmas.png");
+	sf::Sprite sprbase(base);
+	sf::Sprite sprcomp(comp);
+	
+	sf::Shader compshdr;
+	compshdr.loadFromFile("compare.glsl", sf::Shader::Fragment);
+	
+	if (!compshdr.isAvailable()) {
+		std::cout << "The shader is not available\n";
+		return 1;
+	}
+	
+	compshdr.setUniform("base", base);
+	compshdr.setUniform("curr", comp);
+	compshdr.setUniform("size", 240, 240);
+	
+	// Set the resolution parameter (the resoltion is divided to make the fire smaller)
+// 	shader.setParameter("resolution", sf::Vector2f(winW / 2, winH / 2));
+	
+	// Use a timer to obtain the time elapsed
+// 	sf::Clock clk;
+// 	clk.restart(); // start the timer
+	
+	while (window.isOpen()) {
+		// Event handling
+		sf::Event event;
+		
+		while (window.pollEvent(event)) {
+			// Exit the app when a key is pressed
+			if (event.type == sf::Event::KeyPressed)
+				window.close();
+		}
+		
+		// Set the others parameters who need to be updated every frames
+// 		shader.setParameter("time", clk.getElapsedTime().asSeconds());
+		
+// 		sf::Vector2i mousePos = sf::Mouse::getPosition(window);
+// 		shader.setParameter("mouse", sf::Vector2f(mousePos.x, mousePos.y - winH));
+		
+		// Draw the sprites, one with the shader on it
+		window.clear();
+		
+		window.draw(sprbase);
+		
+		sprcomp.setPosition(240, 0);
+		window.draw(sprcomp);
+		
+		sprcomp.setPosition(0, 240);
+		window.draw(sprcomp, &compshdr);
+		
+		window.display();
+	}
+	
+	return 0;
+}
+
+// old main - keep for testing purposes
+/*
 int main() {
 	const float winW = 800;
 	const float winH = 600;
@@ -92,3 +161,4 @@ int main() {
 	
 	return 0;
 }
+*/