Paintbrush paints noisy strokes at low sizes

[cactushead]

When I zoom in to 2x zoom and draw with the mouse with the tool options as pictured, the quality of the stroke looks noisy and bad. In the long diagonal stroke, the centreline is noticeably darker in some parts and it makes the width look uneven. It's a little less noticeable at 1x zoom. There are sometimes 'bites' taken out of the line, when there's a light-coloured pixel where I'd expect one of the same grey colour as as its diagonal neighbours along the line. These bites are noticeable even at 1x zoom. As well as these issues, when I draw a nearly vertical line by dragging the mouse down and slightly to the right, it seems as though the brush dabs are jittering from side to side down the length of the stroke. Why do these things happen? In drawing programs such as Krita or Paint Tool SAI, I've not reproduced such issues.

The three dots are just for reference---they are single brush strokes in a pixel corner, a pixel edge, and a pixel centre.

At 4x zoom and showing brush settings:
   

At 1x zoom:
   

[rich2005]

Linux Mint - Gimp 2.10.14 perhaps.

I can not replicate that, at least not with a mouse ( a little closer with a graphics tablet but not with your brush settings)
You have a dynamic set pressure-size but with a mouse there is constant pressure, so it should have no effect. Set it back to dynamics off.
The brush size is 41, spacing 12 and the drawn line must only be 3 or 4 pixels. Beats me. Should be like the wide line. I can get a jittery line with a small brush size and a user dynamic  but not your settings.

   

In case some unseen setting in place, you could try deleting the file devicerc in your Gimp user profile. It holds mouse/tablet settings. Gimp will make a new one if you change input device settings.

[cactushead]

Thanks for your advice. I realise now that I gave a misleading picture of my brush settings because I changed them between drawing the strokes and taking the screenshot.

Here's a picture that represents the settings I used.

   

I've deleted the devicerc file and changed dynamics to 'Dynamics Off' as you suggested, but I'm still seeing the behaviour. The smoother diagonal stroke at the bottom was drawn with 'Smooth stroke' turned on, quality 100 and weight around 500.

[rich2005]

Check the grid spacing. A small spacing + View -> Snap to Grid gives that effect. Toggle View -> Snap to Grid off.

   

[Ofnuts]

• You are using a soft brush, did you try with Hardness 100?
  
• When you use the brush when zoomed, Gimp receives coordinates for the mouse that aren't exact pixel coordinates for the picture, so it has to interpolate. As far as I can tell, a small amount of stroke smoothing makes things look a lot better.