add mask()

Author: lee2sman

docs/reference/assets/mask1.webp

@@ -80,6 +80,7 @@ An offline version of this documentation is available for [download](/download/#
 * [image()](image.md)
 * [imageMode()](imageMode.md)
 * [loadImage()](loadImage.md)
+* [mask()](mask.md)
 * [noTint()](noTint.md)
 * [tint()](tint.md)
 

docs/reference/index.md

@@ -0,0 +1,54 @@
+# mask()
+
+Masks part of the image with another.
+
+`mask()` uses another image's alpha channel as the alpha channel for this image. Masks are cumulative and can't be removed once applied. If the mask has a different pixel density from this image, the mask will be scaled.
+
+## Examples
+
+![mask example 1](assets/mask1.webp)
+
+```lua
+require 'L5'
+
+function setup()
+  size(100,100)
+  windowTitle('mask example')
+
+  -- Load the images
+  photo = loadImage('assets/rockies.jpg')
+  maskImage = loadImage('assets/mask2.png')
+  
+  -- Apply the mask
+  mask(photo, maskImage)
+  
+  -- Display the image
+  image(photo, 0, 0, width, height)
+  
+  describe('An image of a mountain landscape. The right side of the image has a faded patch of white.')
+end
+```
+
+## Syntax
+
+```lua
+mask(targetImage, maskImage)
+```
+
+## Parameters
+
+| Parameter   |                                                    |
+| -           | -------------------------------------------------- |
+| targetImage | image: texture that mask will be applied to.       |
+| maskImage   | image: texture that will be applied as a mask.     |
+
+
+## Related
+
+* [blend()](blend.md)
+* [copy()](copy.md)
+* [filter()](filter.md)
+
+---
+
+*This reference page contains content adapted from [p5.js](https://p5js.org/) and [Processing](https://processing.org) by [p5.js Contributors](https://github.com/processing/p5.js?tab=readme-ov-file#contributors) and [Processing Foundation](https://processingfoundation.org/people), licensed under [CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/).*