feat: measure exact color coverage before image resizing

[wafflestomper08]

Summary

Add opt-in source-pixel retention and Image.color_pixel_percentage(colors, bounds?, original=False) for exact RGBA palette measurements. Original pixels are retained from the existing decode, without extra requests or decoding; normal rendering is unchanged.

Weather Map currently counts all nontransparent resized radar pixels. Weak radar returns can spread during downsampling and incorrectly keep the app in rotation. A saved Portland 13-frame sample measured 26.27% with the existing method, versus 0.0153% when measuring native map pixels at >=15 dBZ. This API lets the app apply the provider's rain/snow intensity palette before resizing, without changing its displayed animation.

Companion Weather Map PR: tronbyt/apps#733 (draft). It requires this API to be released in the server's embedded Pixlet before enabling its new filtering path.

Tests

• go test -tags gzip_fonts ./... passed.
• Additional boundary tests at 4.9/5/5.1% and 19.9/20/20.1%, including widespread excluded faint pixels, passed in ./render ./runtime.
• Tests cover clipped bounds, duplicate colors, alpha matching, invalid arguments, missing source retention, and identical rendered pixels with retention on/off.
• Offline 13-frame Portland regression: peak 26.2695% -> 0.01526%.
• Synthetic rain/snow fixture distinguishes 14 from 15 dBZ and excludes pixels outside the map crop.
• Weather Map app check passes with the new binary.

Generated bindings and widget documentation are included.

Summary by CodeRabbit

• New Features
  • Added image color-coverage measurement, with optional color matching, region selection, and measurement against original image pixels.
  • Added an option to retain original image pixels before resizing. This is disabled by default.
• Documentation
  • Documented color-coverage measurement, region and denominator behavior, and original-image retention.

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📝 Walkthrough

Walkthrough

The change adds exact-color pixel percentage measurements to render images and exposes them through the runtime Image API. It also adds an opt-in option to retain the first decoded frame for measurements before resizing.

Changes

Image Pixel Coverage

Layer / File(s)	Summary
Source retention and color measurement render/image.go, render/image_coverage_test.go	Image can retain its first decoded frame and report the percentage of pixels matching supplied colors. Measurements support clipped bounds and exact NRGBA matching, including alpha. Tests cover retention, empty inputs, bounds, and percentage thresholds.
Runtime API and usage runtime/modules/render_runtime/generated.go, runtime/render_test.go, docs/widgets.md	The runtime Image constructor accepts retain_original and exposes it as an attribute. The color_pixel_percentage method validates colors and optional bounds, then returns a percentage or an error. Runtime tests and documentation describe the API.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant StarlarkImage
  participant imageColorPixelPercentage
  participant RenderImage as render.Image
  StarlarkImage->>imageColorPixelPercentage: call color_pixel_percentage
  imageColorPixelPercentage->>RenderImage: ColorPixelPercentage(colors, bounds, original)
  RenderImage-->>imageColorPixelPercentage: percentage or error
  imageColorPixelPercentage-->>StarlarkImage: Starlark float or error

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 4 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: measuring exact color coverage using original image pixels before resizing.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 4 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @render/image.go:
- Line 122: Update the palette construction around NRGBAModel.Convert to detect
and unwrap *colorutil.Color values, using their embedded NRGBA channels
directly; keep conversion for other color types unchanged so wrapped palette
colors retain exact channels.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: f69a0966-4730-4ac0-a2be-0a591d748e9b

📥 Commits

Reviewing files that changed from the base of the PR and between b5844b5 and 9146248.

⛔ Files ignored due to path filters (3)

• runtime/gen/docs/render.tmpl is excluded by !**/gen/**
• runtime/gen/header/render.tmpl is excluded by !**/gen/**
• runtime/gen/type.tmpl is excluded by !**/gen/**

📒 Files selected for processing (5)

• docs/widgets.md
• render/image.go
• render/image_coverage_test.go
• runtime/modules/render_runtime/generated.go
• runtime/render_test.go

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[wafflestomper08]

@gabe565 Could you review this API addition when you have a chance? The wrapped-NRGBA review finding is fixed with a reproducing regression test, and the full Pixlet test suite passes. The dependent Weather Map change (tronbyt/apps#733) is held in draft pending runtime availability; it corrects faint-return/resize false positives and now also skips provider HTTP error frames when filtering is enabled. Thank you!