the function doesn't need to be continuous (observablehq#1601)

(its rectangular domain is ‘continuous’ though—or more precisely in the general case we have no guarantee that we're querying integer coordinates)

Author: Fil

docs/marks/raster.md

@@ -30,7 +30,7 @@ function mandelbrot(x, y) {
 To produce contours instead of a heatmap, see the [contour mark](./contour.md).
 :::
 
-The **raster mark** renders a [raster image](https://en.wikipedia.org/wiki/Raster_graphics)—that is, an image formed by discrete pixels in a grid, not a vector graphic like other marks. And whereas the [image mark](./image.md) shows an *existing* image, the raster mark *creates* one from abstract data, either by [interpolating spatial samples](#spatial-interpolators) (arbitrary points in **x** and **y**) or by sampling a continuous function *f*(*x*,*y*) along the grid.
+The **raster mark** renders a [raster image](https://en.wikipedia.org/wiki/Raster_graphics)—that is, an image formed by discrete pixels in a grid, not a vector graphic like other marks. And whereas the [image mark](./image.md) shows an *existing* image, the raster mark *creates* one from abstract data, either by [interpolating spatial samples](#spatial-interpolators) (arbitrary points in **x** and **y**) or by sampling a function *f*(*x*,*y*) along the grid.
 
 For example, the heatmap below shows the topography of the [Maungawhau volcano](https://en.wikipedia.org/wiki/Maungawhau), produced from a {{volcano.width}}×{{volcano.height}} grid of elevation samples.