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* Closes <https://github.com/emilk/egui/issues/4776>
* [x] I have followed the instructions in the PR template
I've been meaning to look into this for a while but finally bit the
bullet this week. Contrary to what I initially thought, the problem of
blurry lines is unrelated to feathering because it also happens with
feathering disabled.
The root cause is that lines tend to land on pixel boundaries, and
because of that, frequently used strokes (e.g. 1pt), end up partially
covering pixels. This is especially noticeable on 1ppp displays.
There were a couple of things to fix, namely: individual lines like
separators and indents but also shape strokes (e.g. Frame).
Lines were easy, I just made sure we round them to the nearest pixel
_center_, instead of the nearest pixel boundary.
Strokes were a little more complicated. To illustrate why, here’s an
example: if we're rendering a 5x5 rect (black fill, red stroke), we
would expect to see something like this:
The fill and the stroke to cover entire pixels. Instead, egui was
painting the stroke partially inside and partially outside, centered
around the shape’s path (blue line):
Both methods are valid for different use-cases but the first one is what
we’d typically want for UIs to feel crisp and pixel perfect. It's also
how CSS borders work (related to #4019 and #3284).
Luckily, we can use the normal computed for each `PathPoint` to adjust
the location of the stroke to be outside, inside, or in the middle.
These also are the 3 types of strokes available in tools like Photoshop.
This PR introduces an enum `StrokeKind` which determines if a
`PathStroke` should be tessellated outside, inside, or _on_ the path
itself. Where "outside" is defined by the directions normals point to.
Tessellator will now use `StrokeKind::Outside` for closed shapes like
rect, ellipse, etc. And `StrokeKind::Middle` for the rest since there's
no meaningful "outside" concept for open paths. This PR doesn't expose
`StrokeKind` to user-land, but we can implement that later so that users
can render shapes and decide where to place the stroke.
### Strokes test
(blue lines represent the size of the rect being rendered)
`Stroke::Middle` (current behavior, 1px and 3px are blurry)
`Stroke::Outside` (proposed default behavior for closed paths)
`Stroke::Inside` (for completeness but unused at the moment)
### Demo App
The best way to review this PR is to run the demo on a 1ppp display,
especially to test hover effects. Everything should look crisper. Also
run it in a higher dpi screen to test that nothing broke 🙏.
Before:
After (notice the sharper lines):
f2815b423e