Aliasing artifacts in rounded corners rendering (rectangles and arrows)

[sullicsullic]

Describe the bug
Great work, kitty is my favorite terminal.
When using Kitty terminal, I've noticed consistent rendering artifacts around rounded elements, specifically:
1- Rounded corners of rectangles appear jagged and unsmooth
2- Arrow components with rounded edges show similar aliasing issues
Hope to achieve smoother rendering for these rounded corners.

To Reproduce

• Rounded corners in nvim cmd line
• Arrows when error occurs

Screenshots
1-This is rounded corners

2-This is rounded arrow

Environment details

kitty 0.39.1 (93dc28aa78) created by Kovid Goyal
Darwin SullisBlackPearl.local 24.2.0 Darwin Kernel Version 24.2.0: Fri Dec  6 19:04:03 PST 2024; root:xnu-11215.61.5~2/RELEASE_ARM64_T8132 arm64
ProductName:		macOS ProductVersion:		15.2 BuildVersion:		24C101
OpenGL: '4.1 Metal - 89.3' Detected version: 4.1
Frozen: True
Fonts:
  medium: ComicCode-Regular: /Users/sullic/Library/Fonts/Comic Code.otf
          Features: ()
    bold: ComicCode-Bold: /Users/sullic/Library/Fonts/Comic Code Bold.otf
          Features: ()
  italic: ComicCode-Italic: /Users/sullic/Library/Fonts/Comic Code Italic.otf
          Features: ()
      bi: ComicCode-BoldItalic: /Users/sullic/Library/Fonts/Comic Code Bold Italic.otf
          Features: ()
Paths:
  kitty: /Applications/kitty.app/Contents/MacOS/kitty
  base dir: /Applications/kitty.app/Contents/Resources/kitty
  extensions dir: /Applications/kitty.app/Contents/Resources/Python/lib/kitty-extensions
  system shell: /bin/zsh
System color scheme: light. Applied color theme type: none
Loaded config files:
  /Users/sullic/.config/kitty/kitty.conf

Config options different from defaults:
bold_font                  Comic Code Bold
bold_italic_font           Comic Code Bold Italic
cursor_blink_interval      (0.5, kitty.options.utils.EasingFunction(), kitty.options.utils.EasingFunction())
cursor_shape               3
cursor_trail               1
cursor_underline_thickness 3.0
font_family                Comic Code
font_size                  16.0
italic_font                Comic Code Italic
scrollback_lines           200000
shell_integration          frozenset({'no-cursor'})
symbol_map:
	U+3400 - U+4dbf → Hannotate SC
	U+4e00 - U+9fff → Hannotate SC
	U+f900 - U+faff → Hannotate SC
	U+20000 - U+2a6df → Hannotate SC
	U+2a700 - U+2b73f → Hannotate SC
	U+2b740 - U+2b81f → Hannotate SC
	U+2b820 - U+2ceaf → Hannotate SC
Changed shortcuts:
	cmd+k →  clear_terminal_and_scrollback
	ctrl+cmd+, →  reload_config
	ctrl+cmd+l →  clear_screen
	opt+cmd+k →  clear_scrollback
	opt+cmd+r →  reset_terminal
	shift+cmd+/ →  open_kitty_website
Colors:
	background                 #fafafa   
	color1                     #ff3333   
	color10                    #b8e532   
	color11                    #ffc849   
	color12                    #73d7ff   
	color13                    #ffa3aa   
	color14                    #7ff0cb   
	color2                     #86b200   
	color3                     #f19618   
	color4                     #41a6d9   
	color5                     #f07078   
	color6                     #4cbe99   
	color7                     #ffffff   
	color8                     #323232   
	color9                     #ff6565   
	cursor                     #ff6900   
	foreground                 #5b6673   
	selection_background       #f0ede4   
	selection_foreground       #fafafa   

Important environment variables seen by the kitty process:
	PATH                                /Applications/kitty.app/Contents/MacOS:/usr/bin:/bin:/usr/sbin:/sbin
	LANG                                zh_CN.UTF-8
	SHELL                               /bin/zsh
	USER                                sullic

[kovidgoyal]

Looks fine to my admittedly old eyes. If you wish to improve the rendering, a PR is welcome, relevant code is in decorations.c see the rounded_corner() function.

[gmr458]

in @leavesullic 's screenshots the rounded corners look thikcker than the lines, in my case the have a differente color

Kitty:

I was looking at other terminals and Ghostty is the one that renders rounded borders the best, then Wezterm follows, in Alacritty they look pixelated.

Ghostty:

WezTerm:

Alacritty:

[kovidgoyal]

I have pushed improved code for drawing parametrized curves, should fix the thickness issues, though as I said given my eyesight I cant really tell, it looks better to me, but YMMV.

generated with:

kitty -o font_size=36 sh -c 'printf "\e[40m\e[m \e[40m╮\e[m\n╭╮\n╰╯\n╭─╮\n│ │\n╰─╯\n\n"; read'

[Dragon-Hatcher]

This seems to have caused a regression for me. I agree that the curves look smoother but for some reason the curved segments are also much lighter now? Or maybe the lines are too thin now? It is hard for me to tell.

Old on the left, new on the right.

I think you can actually see the same affect in the screenshot you posted. Though it is a bit subtler.

[kovidgoyal]

Cant see the effect on my screenshots, on yours its likely caused by being a low res screen, where the antialiasing will dim out the curves a bit. I dont know of a good way to fix that, if you do PR is welcome.

[Dragon-Hatcher]

Ok but this still feels like a bug to me. Because even in the straight parts of the curve the anti-aliasing still makes it thinner than it needs to be. Here it is zoomed in

[Dragon-Hatcher]

Just tested it out on my mac and I agree it looks every so slightly better. Just doesn't really feel worth it to me since it looks way worse on my monitor. I don't see the tradeoff in a slight improvement for high resolution displays and a massive downgrade for lower resolution ones.

[kovidgoyal]

It's not really possible to draw curves that look the same as straight lines in two pixels of width. Either they will look wider and same weight or same width and less weight. I prefer better rendering on high res display since high res displays are the future. You could simply detect the case of a low res display and revert to the old algorithm for them, a PR for that is welcome, should be fairly trivial, check the value of thickness/supersample_factor and if its <= 2 use the old algorithm.