expose webp_write API that returns WebpBox directly

[Follpvosten]

...instead of requiring the user to pass in a &mut W: Write.
While being able to directly have it write to a file or network stream is nice, if you want to just collect the image's bytes into a Vec, it's not an ideal solution:

let mut vec = Vec::new();
webp_write(&img, &mut vec)?;

I would imagine an interface similar to what the webp_read_from_memory functions already provide; functions named webp_write_to_memory or similar which just return the inner WebpBox<[u8]> instead of writing its contents.
That API could then be used like this:

webp_write_to_memory(&img)?.to_vec();

which feels more rusty to me. Alternatively, since DynamicImage already lives in RAM and you're allocating a WebpBox anyways, this could be the only API that's exposed, as it more clearly shows the cost of the developer's actions; in this case there would be no new functions, but the existing ones would change. Users can then of course just call .write_all(&webp_box) on where they want to write the image.

Which changes would you prefer? I'm open to discussion and ready to implement them myself.

[qnighy]

Only the simplest APIs of libwebp (C) use WebpBox (i.e. WebPFree). We can get rid of it using lower-level APIs. Though libwebp-rs might lack the corresponding bindings for now.

[qnighy]

For decoding, we can read the header first, allocate the Vec, and then call a DecodeInto-family function.

[Follpvosten]

What I meant was, instead of taking a &mut W: Write, like fn webp_write(img: &DynamicImage, w: &mut W), change function signatures (or add new functions) to return WebpBox<[u8]> instead. That's slightly more efficient and less indirection for the in-memory usecase.

Example:

pub fn webp_write_rgba<W: Write, C>(img: &ImageBuffer<Rgba<u8>, C>, w: &mut W) -> ImageResult<WebpBox<[u8]>>
where
    C: Deref<Target = [u8]>,
{
    libwebp::WebPEncodeRGBA(&img, img.width(), img.height(), img.width() * 4, 0.75)
        .map_err(|_| EncodingError::new(ImageFormatHint::Unknown, "Webp Format Error".to_string()))
        .map_err(ImageError::Encoding)
}

However, now that I know you prefer following the image API over exposing more efficient interfaces, I'm pretty sure you will not want that, so you are free to close this issue.

[qnighy]

I think the right direction is to use streaming encoding APIs exposed by libwebp (C), keeping the current interface. It is only an implementation detail that fn webp_write internally allocates WebpBox<[u8]>.