[dmwong25]

Summary

Document the existing channel normalization in write_png, including the normalization of the alpha channel and of uint8 RGB input after an alpha channel is added. Show how callers can preserve RGB colors by preparing a uint8 RGBA array.

Also replace the Python 2 base64 example with an executable Python 3 example and add focused pixel-level tests for the documented behavior. The test helper uses only the standard library to decode the generated PNG scanlines.

This is a documentation and test contribution related to #195. It does not change normalization behavior, add an option, or resolve the request to change that behavior.

Verification

• python -m pytest -vv -rxs tests -m 'not headless': 39 passed, 2 deselected.
• All 9 examples in the write_png docstring execute successfully.
• Focused Sphinx build of the changed docstring passes with warnings treated as errors.
• Full documentation HTML build succeeds with the existing missing _static directory warning.
• Black, isort, Flake8, mypy, and git diff --check pass.
• Browser/headless tests, notebook tests, and the full multi-platform CI matrix were not run locally.

Prepared with AI assistance. The documentation, decoded pixel results, examples, and local checks were reviewed; no claim of maintainer endorsement is intended.