Apply the grey colour to inactive screen when showing popup

Question

I am trying to show a pop up screen in my LibGdx based Android Game. While doing so I have to blur of apply grey color to the background screen which is inactive while showing the popup.

As of now To achieve this I have created a texture with the black background and alpha 70 %. This texture is drawn on top of the main background and hence it is achieved.

But the above approach need a Texture of big size same as the main background. It adds weight of the App and to the render method as well.

Kindly help me a suitable approach.

Sample picture of my requirement is shown below. This is the effect I want in LibGdx not in specific to android

Answer 1

Your texture can be a 1x1 white image that you stretch to fill the screen. (White so it is more versatile.) And if you don't want to fool with calculating how to stretch it relative to where the camera is, you can do this:

private static final Matrix4 IDT = new Matrix4();

batch.setProjectionMatrix(camera.combined);
batch.begin();
//draw stuff in background
batch.setProjectionMatrix(IDT);
batch.setColor(0, 0, 0, 0.7f);
batch.draw(singlePixelTexture, -1, -1, 2, 2); //coordinates and size of the screen when identity projection matrix is used
batch.setProjectionMatrix(camera.combined);
//draw your front window
batch.end();

However, you could also leave the projection matrix alone and instead calculate where and how big the background should be to fill the screen with the existing matrix:

float camX = camera.position.x;
float camY = camera.position.y;
float camWidth = camera.viewportWidth;
float camHeight = camera.viewportHeight;

batch.setProjectionMatrix(camera.combined);
batch.begin();
//draw stuff in background
batch.setColor(0, 0, 0, 0.7f);
batch.draw(singlePixelTexture, camX - camWidth/2, camY - camHeight/2, camWidth, camHeight);
//draw your front window
batch.end();

And FYI, although a large texture does use more memory and load time, it does not slow down rendering unless it is drawn shrunk down without mip mapping.

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Edit: How to generate single pixel texture at runtime:

singlePixelPixmap = new Pixmap(1, 1, Pixmap.Format.RGBA8888);
singlePixelPixmap.setColor(1, 1, 1, 1);
singlePixelPixmap.fill();
PixmapTextureData textureData = new PixmapTextureData(singlePixelPixmap, Pixmap.Format.RGBA8888, false, false, true);
singlePixelTexture = new Texture(textureData);
singlePixelTexture.setFilter(TextureFilter.Nearest, TextureFilter.Nearest);

Note that the pixmap must be disposed in the dispose method, but you don't want to do it earlier than that, or the texture cannot automatically reload if there is a GL context loss.