What is a 'Mask' in Game development?

Question

• What does it mean a 'mask' in 2D (and possibly 3D) Game development?
• What would an experience Game Dev understands if I say ´make this Rect a mask´?

I know it could be a very broad term and it may depend on the context, if is so then just a bullet list of possible meaning would do.

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My understanding is that is something that 'masks' with the purpose to cover some other entity on the game world, however what are the most use cases and why would you use it?

I research a bit online, but because of the naming 'mask' the search result gets easily polluted with unreleated topic (especial after covid).

Answer 1

Conceptually, masking involves selectively ignoring something, usually to isolate something else.

The two main uses I've familiar with are:

1. Masking data such as bit fields or flags for bitwise operations.
2. Masking images, often for compositing or other effects.

Data Masking

For an example of the first, let's say you have a bit field such as the 6502 status register:

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
Negative flag	oVerflow flag	-	Break flag	Decimal flag	Interrupt-disable flag	Zero flag	Carry flag

ANDing the register with 01000000 would mask out everything except the overflow flag. So something like if(R & 01000000) would allow you to check for the overflow condition. Specifically, the logical operation preserves (masks) only the 6th bit for evaluation.

Image Masking

This question examines ways to use an alpha mask to ignore unwanted portions of a texture in order to get a more complex result:

Mask	Addition of a red hexagon sprite	Result of masking the sprite

In the above example, the mask preserves the portion of the sprite in the round center area. You might use this form of masking to implement a circular field of view in a top down game as it allows the player to see things in the field of view, but not outside it.

It's probably worth mentioning, sometimes it can be a bit ambiguous regarding whether mask is being used to refer to what is kept versus what is discarded.

Answer 2

Maybe I'm too old, but back in the day a mask meant an area of blank pixels around the outer shape of a sprite. This was to isolate the sprite from the background and prevent the background colours mixing with or showing through the sprite colours. I'm sure it's still used in 2D games to give sprites separation from the background. In fact, you can see a mask at work when you look at your mouse cursor - the white area around the black arrow icon is a mask.

The mask data was often stored alongside the sprite data, and applied first to the background to mask out a "hole" for the sprite, before the sprite was drawn on top.

Answer 3

This is how Wikipedia describes it.

This technique is used for painting pointing device cursors, in typical 2-D videogames for characters, bullets and so on (the sprites), for GUI icons, and for video titling and other image mixing applications.

Although, personally, I mostly only use masks for UI. Very often you'll have one UI element sliding under another and a mask is an easy way to prevent something from being visible when it shouldn't.

Answer 4

There are some decent answers so far, but I think this part deserves a bit more attention.

What would an experience Game Dev understands if I say ´make this Rect a mask´?

To a game dev, there would be no doubt that if you are using a Rect (short for rectangle in a lot of game engines), that you would clearly be asking about an image mask. You would generally not be creating a data mask from a polygon.

However, if the developer is unfamiliar with the project, there may be some ambiguity about if you want a vector or raster mask. 2d graphics can generally be saved in 2 ways, a vector image which is a composition defined by lines drawn from points procedurally filled with colors, or a raster image which is an image made from a grid of pixels. In most cases it does not matter that much which kind of map you use, but it can. If you are trying to mask an image that has to be highly scalable, you will generally want to use a vector mask to keep from loosing detail to poor resolutions. If you need to make an especially complex mask, like for doing photographic selective tinting, then a raster mask is often the better choice.