Loading all texture data from HDD when we only need the lowest mipmap.

[bryanedds]

Here it seems we need to load all the bytes from HDD in order to make even a single lowest mipmap image -

Nu/Nu/Nu/OpenGL/OpenGL.Texture.fs

Lines 265 to 299 in 02663b3

	elif fileExtension = ".dds" then
	try let config = PfimConfig (decompress = false)
	use fileStream = File.OpenRead filePath
	use dds = Dds.Create (fileStream, config)
	let metadata = TextureMetadata.make dds.Width dds.Height
	if dds.Compressed then
	let mutable dims = v2i dds.Width dds.Height
	let mutable size = ((dims.X + 3) / 4) * ((dims.Y + 3) / 4) * 16
	let mutable index = 0
	let bytes = dds.Data.AsSpan(index, size).ToArray()
	let mipmapBytesArray =
	[|for _ in 1u .. dec dds.Header.MipMapCount do
	dims <- dims / 2
	index <- index + size
	size <- size / 4
	if size >= 16 then (dims, dds.Data.AsSpan(index, size).ToArray())|]
	if minimal then
	match Array.tryLast mipmapBytesArray with
	| Some (mipmapMinimalResolution, mipmapMinimalBytes) ->
	let metadataMinimal = TextureMetadata.make mipmapMinimalResolution.X mipmapMinimalResolution.Y
	Some (TextureDataMipmap (metadataMinimal, true, mipmapMinimalBytes, [||]))
	| None -> Some (TextureDataMipmap (metadata, true, bytes, mipmapBytesArray))
	else Some (TextureDataMipmap (metadata, true, bytes, mipmapBytesArray))
	else
	match TryFormatUncompressedPfimageData dds with
	| Some (bytes, mipmapBytesArray) ->
	if minimal then
	match Array.tryLast mipmapBytesArray with
	| Some (mipmapMinimalResolution, mipmapMinimalBytes) ->
	let metadataMinimal = TextureMetadata.make mipmapMinimalResolution.X mipmapMinimalResolution.Y
	Some (TextureDataMipmap (metadataMinimal, false, mipmapMinimalBytes, [||]))
	| None -> Some (TextureDataMipmap (metadata, false, bytes, mipmapBytesArray))
	else Some (TextureDataMipmap (metadata, false, bytes, mipmapBytesArray))
	| None -> None
	with _ -> None

I've made a query with the maintainer here -

nickbabcock/Pfim#116 (comment)

Let's follow up and see if we can't come up with a reasonable API!