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/**
* Pure decision logic for attach-time image compression (the `Compress`
* toggle on the composer's staged-image thumbnails).
*
* The pipeline (re-encode to JPEG + rescale) runs at the browser edge
* (`../adapter/image-encode`); this module owns every DECISION so it stays
* pure `input → output` and unit-testable without a DOM:
*
* - `shouldConvertImage` — which source formats get converted.
* - `computeResizedDimensions` — the rescale math (cap the longest edge,
* never upscale, preserve aspect).
* - `useConvertedImage` — the smaller-wins rule.
*/
/** JPEG encode quality for the canvas re-encode. */
export const JPEG_QUALITY = 0.85;
/** Longest-edge cap (px) for the rescale. Never upscale past the source. */
export const MAX_IMAGE_EDGE_PX = 2048;
/**
* Source formats that get converted. Only LOSSLESS sources are re-encoded:
* PNG (the screenshot case — multi-MB PNGs are the reason this exists) plus
* BMP (trivially canvas-decodable in every browser). Already-lossy formats
* (JPEG/WebP) and animated ones (GIF) are sent verbatim — recompressing them
* degrades quality and saves nothing, and re-encoding a GIF destroys its
* animation. TIFF is NOT listed: browsers can't natively decode it, so the
* canvas path would always fall back to the original anyway. An oversized
* JPEG stays untouched (the backend's body-budget safety net handles the
* extreme case).
*/
const CONVERTIBLE_MIME_TYPES = new Set(["image/png", "image/bmp"]);
/**
* Whether a source MIME type is converted to JPEG at staging time.
* Unknown/absent MIME types are left alone (conservative — never gamble on
* bytes we can't classify).
*/
export function shouldConvertImage(mimeType: string | undefined): boolean {
return mimeType !== undefined && CONVERTIBLE_MIME_TYPES.has(mimeType);
}
/** Pixel dimensions of a raster image. */
export interface Dimensions {
readonly width: number;
readonly height: number;
}
/**
* Target dimensions after the rescale: cap the LONGEST edge at `maxEdge`
* (default `MAX_IMAGE_EDGE_PX`), never upscale (a smaller source passes
* through unchanged), and preserve aspect ratio.
*
* Pure math — the canvas draw itself lives at the browser edge.
*/
export function computeResizedDimensions(
width: number,
height: number,
maxEdge: number = MAX_IMAGE_EDGE_PX,
): Dimensions {
const longest = Math.max(width, height);
if (longest <= maxEdge) return { width, height };
const scale = maxEdge / longest;
// Rounding can never exceed the cap: each scaled edge is ≤ maxEdge before
// rounding, and the only values that round up land exactly ON the cap.
return { width: Math.round(width * scale), height: Math.round(height * scale) };
}
/**
* The smaller-wins rule: the converted JPEG is used only when it is actually
* FEWER bytes than the original (equal or larger → keep the original).
* Compared on raw byte counts (File/Blob `size`); base64 inflation affects
* both sides equally so the ratio is preserved.
*/
export function useConvertedImage(originalBytes: number, convertedBytes: number): boolean {
return convertedBytes < originalBytes;
}
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