docs: add design spec for pure-go webp exif injector

2026-07-15 17:01:21 +07:00
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 # Pure-Go WebP EXIF RIFF Injector
 ## 1. Goal
 Completely remove file-based IO and the `webpmux` dependency by weaving the EXIF chunk into the WebP RIFF byte struct physically in memory.
 ## 2. Architecture
 Instead of using `webpmux` passing paths around, `helper/exif.go` will implement `InjectExif(webpBytes []byte, packName, publisher string) ([]byte, error)`.
 ## 3. RIFF Parsing Logic
 1. Validate `RIFF` and `WEBP` signatures.
 2. Read all chunks to extract Canvas Width/Height (from `VP8X`, `VP8L`, or `VP8 `).
 3. Identify if `VP8X` is missing, and if so, construct it. We must ensure the `EXIF` bit flag is set.
 4. Construct the custom `EXIF` chunk (which holds the exact binary array and JSON metadata we validated via F-BOT_GO earlier).
 5. Append `EXIF` to the end of the WebP chunks, update the main `RIFF` file size pointer, and return the aggregated `[]byte` slice.
 ## 4. Main Command Refactor
 `cmds/media/sticker.go` will stop requesting files via `ResolveExif()`. It will simply pass the raw Webp bytes received from `ConvertImageToWebp` or `ConvertVideoToWebp` into `InjectExif` and instantly upload the resulting byte array.