Set Render->Grid to mm
Hi I have 2.10 and I'm having difficulty specifying mm in the
Filters>Render>Grid tool I can only use pixels which is slightly out
for what I need. I cant see a way of changing it, am I missing
something really obvious ?
What you are missing is that Gimp is a raster (bitmap) editor and works in pixels. Just because in some circumstances the size / rulers display real world units, say for print size, does not make Gimp work that way.
There are workarounds for that Render Grid tool although you will still use pixel values and you can get progressive errors as you found.
Image -> Print Size and set the resolution to a suitable pix / mm value for your use. (I used 30)
Render the grid, use a value of 30 (= 1 mm ) or multiples of 30
Change the units to mm (if required ) for the rulers to use mm
This quick animation example: https://i.imgur.com/JOtwCdu.mp4