# Alpha mask filter

Source: https://ref.dm-lang.org/notes/filters/alpha
Added in BYOND 513.

**Format**

> filter(type="alpha", ...)

**Args**

> x: Horizontal offset of mask (defaults to 0)

Uses an icon or render target as a mask over this image. Every pixel that is transparent in either the image or the mask, is transparent in the result.

The `x` and `y` values can move the mask from its normal position. By default, the mask is centered over the center of the image.

The `MASK_INVERSE` flag will invert the alpha mask so that opaque areas in the mask become transparent, and vice-versa. There is also a `MASK_SWAP` flag which treats the source image as the mask and vice-versa, which might be useful for some effects.

> [!note]
> 
> Unlike many other filters, this filter **is** taken into account for mouse-hit purposes.

### See also
- [icon var (atom)](https://ref.dm-lang.org/atom/var/icon.md)
- [render_target var (atom)](https://ref.dm-lang.org/atom/var/render_target.md)