xtermjs/xterm.js

Text is abnormally thin when setting allowTransparency=true with webgl renderer (even when color is opaque)

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#4,212 opened on Oct 18, 2022

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area/addon/webglhelp wantedtype/bug

Description

Text rendering is abnormally thin when allowTransparency=true with the webgl renderer even when the background color is completely opaque.

allowTransparency=false: disallow_transparency

allowTransparency=true: allow_transparency

I think this is also mentioned in this comment in another bug.

Details

  • Browser and browser version: Chrome 106.0.5249.119
  • OS version: Linux (Debian)
  • xterm.js version: ToT @ 376f0d99721b6ec056b15551f46237b80e24b55b

Steps to reproduce

  1. Run the demo page
  2. Set theme to light and font family to "Noto Sans Mono"
  3. Execute echo -e '\033[33mhello world\033[m'
  4. Observe that the rendering is OK
  5. Check the "allowTransparency" toggle
  6. Reload the webgl addon because the rendering is messed up: Uncheck and then check the webgl checkbox under "Addon". (I believe reloading the addon is NOT what is causing problem here)
  7. Observe that the "hello world" text is abnormally thin

Note that the light theme have a completely opaque background, so the rendering theoretically should just be the same.

Contributor guide

Tech stack
typescript
Domain
frontend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScriptxterm.js project structureWebGL basicsCSS compositing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
Investigate the webgl renderer code, particularly how allowTransparency affects text rendering. Look at the shader programs and the render loop in the webgl addon (likely in src/renderer/webgl/). Compare the rendering path with allowTransparency=false vs true. The issue might be related to the alpha blending for the background even when the color is opaque. Check if the text bitmap is being incorrectly blended. Also review the comment in issue #1898 for additional context. Consider adding a test that verifies rendering with opaque background and allowTransparency=true.