src/lib/client/downscale.ts

/**
 * Downscales an image file in the browser to at most `maxEdge` px on its longest side
 * and re-encodes it as JPEG. Returns null when no processing is needed or possible.
 */
export async function downscaleImage(file: File, maxEdge = 1600, quality = 0.85): Promise<Blob | null> {
	if (typeof createImageBitmap === 'undefined' || typeof document === 'undefined') return null;
	try {
		const bitmap = await createImageBitmap(file);
		const scale = Math.min(1, maxEdge / Math.max(bitmap.width, bitmap.height));
		if (scale === 1 && file.type === 'image/jpeg') {
			bitmap.close();
			return null;
		}
		const canvas = document.createElement('canvas');
		canvas.width = Math.max(1, Math.round(bitmap.width * scale));
		canvas.height = Math.max(1, Math.round(bitmap.height * scale));
		const ctx = canvas.getContext('2d');
		if (!ctx) return null;
		ctx.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
		bitmap.close();
		return await new Promise<Blob | null>((resolve) => canvas.toBlob(resolve, 'image/jpeg', quality));
	} catch {
		return null;
	}
}