Writer needs an .into_inner() function

[surferjeff]

I want to write a PNG to memory. I wrote code like this:

    let mut encoder = png::Encoder::new(Cursor::new(Vec::new()), info.width, info.height);
    encoder.set_color(png::ColorType::RGB);
    encoder.set_depth(info.bit_depth);
    let mut writer = encoder.write_header()?;
    writer.write_image_data(&frame)?;
    let cursor = writer.into_inner();
    let bytes = cursor.into_inner();

I can't get the bytes back out without an into_inner() function. The line let cursor = writer.into_inner(); fails to compile because there's no .into_inner() function.

[fintelia]

Couldn't you do:

let mut bytes = Vec::new();
{
    let mut encoder = png::Encoder::new(Cursor::new(&mut bytes), info.width, info.height);
    encoder.set_color(png::ColorType::RGB);
    encoder.set_depth(info.bit_depth);
    let mut writer = encoder.write_header()?;
    writer.write_image_data(&frame)?;
}

[surferjeff]

Yes, that worked. Thanks!

[ravenexp]

Couldn't you do:

let mut bytes = Vec::new();
{
    let mut encoder = png::Encoder::new(Cursor::new(&mut bytes), info.width, info.height);
    encoder.set_color(png::ColorType::RGB);
    encoder.set_depth(info.bit_depth);
    let mut writer = encoder.write_header()?;
    writer.write_image_data(&frame)?;
}

This workaround no longer works in 0.17:

error[E0597]: `pngbuf` does not live long enough
  --> src/png.rs:31:52
   |
31 |         let mut encoder = Encoder::new(Cursor::new(&mut pngbuf), self.width, self.height);
   |                                        ------------^^^^^^^^^^^-
   |                                        |           |
   |                                        |           borrowed value does not live long enough
   |                                        argument requires that `pngbuf` is borrowed for `'static`
...
50 |     }
   |     - `pngbuf` dropped here while still borrowed

I believe this is because Encoder::new() now enforces a 'static lifetime parameter for some reason:

image-png/src/encoder.rs

Lines 173 to 174 in c24fc93

	impl<'a, W: Write> Encoder<'a, W> {
	pub fn new(w: W, width: u32, height: u32) -> Encoder<'static, W> {

I think this issue should be reopened for version 0.17 and #299 may be revived as well.

[fintelia]

Could you check why the static bound was added? Might be that we could remove that instead

[ravenexp]

It was added in 7ed5169

I don't understand the motivation behind it well though...

[fintelia]

I don't quite understand that either, but it looks like the static bound isn't meant to apply to the writer at all!

[HeroicKatora]

It still works: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=769e84f77aae22624b26ec51df1a74c1

The return value does not force W: 'static. Can you link to full code?

[ravenexp]

I believe Rust Playground imports png version 0.16. The API I'm using in 0.17 is not there:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=037ccad055f3d5f8862707dacfbdd41a

[HeroicKatora]

Okay, that's a separate problem for StreamWriter then. As a workaround you can use stream_writer instead of into_stream_writer. This requires you to keep the underlying Writer alive and borrow from it like so:

            let mut writer = encoder
                .write_header()?;
            let mut writer = writer
                .stream_writer()?;

So it only works as long as you do not want to store the StreamWriter into a 'static struct itself but then the lifetimes work out. It seems that the requirement W: 'static is actually introduced implicitly as part of the StreamWriter type:

Specifically in this enum:

image-png/src/encoder.rs

Lines 981 to 984 in c24fc93

	enum ChunkOutput<'a, W: Write> {
	Borrowed(&'a mut Writer<W>),
	Owned(Writer<W>),
	}

This links the lifetime parameter to the Writer because the reference in the type definition implies Writer<W>: 'a which in turn requires W: 'a. Further the output type of into_stream_writer() is StreamWriter<'static, W> implying W: 'static due to the above portion. This does not apply to the Writer::new constructor because the type Encoder has no such link, its lifetime parameter is only used for an &'a Info structure and later lost when converting to Writer in writer_header.

[ravenexp]

Thanks!

Okay, that's a separate problem for StreamWriter then. As a workaround you can use stream_writer instead of into_stream_writer. This requires you to keep the underlying Writer alive and borrow from it like so:

            let mut writer = encoder
                .write_header()?;
            let mut writer = writer
                .stream_writer()?;

worked for me.

I'd happily avoid using StreamWriter if there was an easy way to pass 16-bit samples directly to Writer.
I also had to google to find out what sample data endianness PNG files use internally.
Ideally, such file format details should not leak through the high level library interface...