[R] What is the threshold for `useRaster` of graphics::image()?

[cd@@ek m@iii@g oii posteo@de]

 Hi R-help list

What is the threshold for the `useRaster` argument of graphics::image()
to decide whether a grid is "regular" or not (i.e. the usage of
useRaster=T is allowed or not). My
`dev.capabilities("rasterImage")$rasterImage` is "yes".

I could not find this information in the internet. I also could not find
the source code of the graphics::image() function (I would answer the
question by myself then). Where can I find the source code?

Thanks a lot for any help and kind regards,
Chris

packageDescription("graphics")
Package: graphics
Version: 4.0.2
Built: R 4.0.2; x86_64-pc-linux-gnu; 2020-06-22 08:33:40 UTC; unix

 
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