fix the first chunk's lower triangle
Author: Line290
Pull request: yes
Labels: name: wontfix

Correct the first chunk's lower triangle.
For details, please look at Page 4 in a google shared document.
Link is: https://docs.google.com/document/d/12rv879j2m5VkfTvk0F-WSOPFF5gqbE0kgk60PvY5nHc/edit?usp=sharing

# What does this PR do?

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
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## Who can review?

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Comments:
Sorry, I made a mistake, ignore this.

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