To expand on @BillKerr comment, "Lock brush to view" scales the brush with the zoom: the ratio brush size/window area
remains constant, so that at high zoom factors you don't get a brush that is bigger than your image window. In effect this shrinks the brush as you zoom in. If course if you start with a 3px brush, at 3800% zoom it is 3/38=0.1 pixel and is inoperative.
Further more, the 1. Pixel
brush is "parametric". Gimp has two kinds of brushes:
- "parametric" brushes (
.vbr
files), that are simple shapes defined by a few parameters: shape (circle/square/diamond), radius, and sides. These are the ones that you can edit with the brush editor. Under the hood they are close to vector graphics.
- "bitmap" brushes, that are plain bitmap images (the
z. Pepper
brush, for instance)(.gbr
files)
So, when you look closely, the 1. Pixel
brush will always be at least 3px (one pixel for center, and 2x1 pixels due to radius=1. If you want a real 1-pixel brush, it has to be a bitmap brush. Gimp comes with such a brush, called Pixel (1x1 square)
.