Modifying transparency of Texture in LibGDX

Question

I want to have a foreground texture overlayed on a background texture. Besides these two textures, I also have a mask that indicates which parts of the foreground should be transparent. This is what I tried:

// Initially, the mask should have an alpha of 1
Pixmap mask = new Pixmap(128, 128, Pixmap.Format.Alpha);

// Cut a rectangle of alpha value 0
mask.setColor(new Color(0f, 0f, 0f, 0f));
mask.fillRectangle(0, 0, 32, 32);

// Load the foreground. The foreground should the same alpha values
// as the mask. If the mask has an alpha value of 1, then the foreground is
// visible. If the mask is 0, then the foreground is invisible.
Pixmap fg = new Pixmap(Gdx.files.internal("foreground.png"));
fg.drawPixmap(mask, fg.getWidth(), fg.getHeight());
Texture foreground = new Texture(fg);

Texture background = new Texture("background.png");

Needless to say, the results aren't what I want them to be. What should I change so that the background is visible wherever the mask has an alpha value of 0, and the foreground is visible wherever the mask has an alpha value of 1.

Answer 1

I think the problem here is blending. Pixmap.setBlending() is set to SourceOver by default. That means drawing a rectangle of alpha 0 results in no change at all, since you draw an invisible rectangle. Try setting it to Pixmap.Blending.None to really cut out the rectangle.

// Initially, the mask should have an alpha of 1
Pixmap mask = new Pixmap(128, 128, Pixmap.Format.Alpha);

// Cut a rectangle of alpha value 0
mask.setBlending(Pixmap.Blending.None);
mask.setColor(new Color(0f, 0f, 0f, 0f));
mask.fillRectangle(0, 0, 32, 32);

Pixmap fg = new Pixmap(Gdx.files.internal("foreground.png"));
fg.drawPixmap(mask, fg.getWidth(), fg.getHeight());
mask.setBlending(Pixmap.Blending.SourceOver);

Texture foreground = new Texture(fg);
Texture background = new Texture("background.png");

Actually you don't even need to create a mask, but you could directly "cut out" the rectangle on the foreground pixmap.