Vtyjkyuk

I was connecting a line to a circle node, when I noticed that (under a microscope) the line does not meet the circular node's radius perfectly. This wouldn't normally be visible, but with an ultra thick line and small node radius, the gap becomes obvious.

documentclass[preview]standalone
usepackagetikz
begindocument
begintikzpicture
draw (0,-3pt) -- (0,3pt);
draw (0.3,-3pt) -- (0.3,3pt);
node (leftEnd) at (0,0) [draw,circle,inner sep=0pt,minimum size=3pt] ;
node (rightEnd) at (0.3,0) [draw,circle,inner sep=0pt,minimum size=3pt,fill=white] ;
draw[ultra thick] (leftEnd) -- (rightEnd);
endtikzpicture
enddocument

How do I make the line meet without any gap?

EDIT: Ideally the circle nodes should be empty, so as to allow lines behind them to be visible or not. I have updated the MWE and picture to reflect that.

Anohter option is to simply connect the center points and then redraw with a fill=white option, or, as per marmot's suggestion, you can do the drawing all at once:

Code:

documentclass[preview, border=0.5pt]standalone
usepackagetikz

tikzsetNode Style/.style=thick, draw,fill=white,circle,inner sep=0pt,minimum size=3pt

begindocument
begintikzpicture
node (leftEnd) at (0,0) [Node Style] ;
node (rightEnd) at (0.3,0) [Node Style] ;

draw [ultra thick] (leftEnd.center) -- (rightEnd.center);

node (leftEnd) at (0,0) [Node Style] ;
node (rightEnd) at (0.3,0) [Node Style] ;
endtikzpicture
%
begintikzpicture
draw[ultra thick] 
 (0,0) node[Node Style] 
 -- (0.3,0) node[Node Style] ;
endtikzpicture%
enddocument

You can adjust the shorten value ,shorten >=1.5pt,shorten <=1.5pt, or use the arrows.meta library to get the same result.

RESULT: Zoom 1000%

MWE:

documentclass[tikz]standalone
usetikzlibraryarrows.meta
begindocument
begintikzpicture[
 >=Circle[fill=none,length=3.5pt,line width=0.5]
]
node (leftEnd) at (0,0) [draw,circle,inner sep=0pt,minimum size=3pt] ;
node (rightEnd) at (0.3,0) [draw,circle,inner sep=0pt,minimum size=3pt] ;
draw[ultra thick,shorten >=1.5pt,shorten <=1.5pt] (leftEnd.center) -- (rightEnd.center);
draw[ultra thick,<->,shorten >=-1.75pt,shorten <=-1.75pt] (0,-10pt) -- ++(0.3,0);

endtikzpicture
enddocument

to the some line width it helps if you set in nodes' style outer sep=0pt:

documentclass[tikz, margin=3mm]standalone

begindocument
begintikzpicture[
circ/.style = circle, draw, inner sep=0pt, minimum size=3pt, outer sep=0pt
 ]
node (leftEnd) [circ] ;
node (rightEnd) [circ] at (0.5,0) ;
draw[ultra thick] (leftEnd) -- (rightEnd);
draw[line width=6pt, gray, semitransparent] (leftEnd) -- (rightEnd);
endtikzpicture
enddocument