Libgdx blending contrast issue in render();

Question

In a Libgdx 1.x Stage, my goal is to apply a dark semi transparent mask on my screen except an area that should display the original screen.

My global render method:

public void render() {
    Gdx.gl.glClearColor(0, 0, 0, 1);        
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT); 
    batch.setShader(null); 
    update();
    stage.act(Gdx.graphics.getDeltaTime());     
    stage.draw();       

    batch.begin();      
    darkener.draw(batch, 0.7f);     
    spotlight.draw(batch, 1f);      
    batch.end();                
}

Both instances of objects Darkener and Spotlight are Sprites made from a pure white 4x4 texture:

spotlight = new Sprite(new Texture(Gdx.files.internal("data/ui/whitepixel.png")));

darkener = new Sprite(new Texture(Gdx.files.internal("data/ui/whitepixel.png")));

I give my Darkener object the size of the screen, and my Spotlight object the size of the area I want not to be darkened.

Here is the draw method of my object darkener:

public void draw(Batch batch, float alpha) {        
    darkener.draw(batch, alpha);        
}

And the draw method of my Spotlight object:

public void draw(Batch batch, float alpha) {
    batch.setBlendFunction(GL20.GL_DST_COLOR, GL20.GL_SRC_ALPHA);
    spotlight.draw(batch, alpha);
    batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
}

My problem is about blending: With the parameters:

darkener.draw(batch, 0.7f);     
spotlight.draw(batch, 1f);  

The preserved (spotlight) area is darker than the original.

With the parameters:

darkener.draw(batch, 0.3f);     
spotlight.draw(batch, 1f);  

The preserved (spotlight) area is brighter (saturated) than the original.

With the paremeters:

darkener.draw(batch, 0.5f);     
spotlight.draw(batch, 1f);  

The preserved (spotlight) area is exactly like the original (so, as I want it). The problem is that I want to be able to set different values for my Darkener either < 0.5f or > 0.5f.

What I am doing wrong?

Answer 1

The blend function you're using for the spotlight takes the current pixel brightness and multiplies it by (1+alpha).

So if you darken to 0.7 of brightness (using a darkener alpha of 0.3), you want the spotlight to multiply the brightness by 1/0.7 = 1.429 so you should use a spotlight alpha of 0.429. So:

spotlightAlpha = 1/(1-darkenerAlpha) - 1; //assuming darkener RGB is black

The problem is if you darken to less than 0.5 brightness (darkner alpha > 0.5), because alpha cannot be greater than 1. You would have to run the spotlight with alpha 1 repeatedly until the brightness is above 0.5 and then one more time with the appropriate value.

You will lose color precision the darker you go so this method may not be suitable. Might want to consider drawing 8 dark boxes instead. And if you want the spotlight to be non-rectangular you can make its sprite inverted to fill in the middle rectangle.