fix(imageOutline): fix the bug where outline was not showing without filling

[sedghi]

Context

The labelmap outline in the volume/image mapper used the condition tColor.a > 0.01 to improve performance by skipping the background. However, this meant that we couldn't render the outline when the fill is 0 (or a small number).You can see the bug here in this code sand box

I changed the logic to not rely on that and only work on segment Index.

Results

Now we can render them fine if the opacity is set to 0

Changes

Changed the logic of outline detection to not rely on the color opacity but rather the segment index itself.

• Documentation and TypeScript definitions were updated to match those changes

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

• This change adds or fixes unit tests
• Tested environment:
  • vtk.js: Latest
  • OS: macOS
  • Browser: Latest Chrome Stable

[sankhesh]

LGTM 👍

Thank you @sedghi

[github-actions]

🎉 This PR is included in version 29.7.2 🎉

The release is available on:

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• npm package (@latest dist-tag)
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