Support RLE encoding for TGA

[marvin-j97]

I would like to be able to save images as run-length encoded TGA to save space.
The TGA decoder seems to support RLE, but the encoder doesn't.

Sources

• http://tfc.duke.free.fr/coding/tga_specs.pdf
• https://www.fileformat.info/format/tga/egff.htm#:~:text=information%2C%20if%20any.-,Image%20Data%20Encoding,-Image%20data%20stored
• http://www.paulbourke.net/dataformats/tga/

Considerations

I'm wondering, from an outside perspective, if I have a DynamicImage, how would one set the encoded = yes/no flag when using img.save("img.tga"). Would something like (pseudo API) img.save_with(path, tga::SaveOptions::encoded()) be required for control over RLE encoding...

[fintelia]

The API could use DynamicImage::write_with_encoder. Analogous to how you can control compression parameters for JPEG:

let encoder = JpegEncoder::new_with_quality(&mut writer, 95);
img.write_with_encoder(encoder)?;

That said, TGA compression is not great by modern standards. If you can use a different format, you'll likely get better compression

[marvin-j97]

That said, TGA compression is not great by modern standards. If you can use a different format, you'll likely get better compression

It's not my choice, unfortunately 😐

The API could use DynamicImage::write_with_encoder

So a possible API could be:

pub struct TgaEncoder<W: Write> {
    writer: W,
    use_rle: bool,
}

impl<W: Write> for TgaEncoder<W> {
  // ... new

  pub fn with_rle(w: W) -> TgaEncoder<W> {
    TgaEncoder {
      writer: w,
      use_rle: true,
    }
  }

  pub fn use_rle(mut self) -> TgaEncoder<W> {
    self.use_rle = true;
    self
  }
}

let encoder = TgaEncoder::with_rle(&mut writer);
// or
let encoder = TgaEncoder::new(&mut writer).use_rle();
img.write_with_encoder(encoder);

[fintelia]

Yeah. Though, I'd probably suggest calling it "rle" or "compression" or something like that. Even the uncompressed version still counts as "encoding".

Another question is whether it makes more sense to add the new option as a constructor ("with_rle") or as a setter ("set_rle"). I think the existing encoders all use constructors, but that doesn't seem as sustainable if we add more options...

[marvin-j97]

Yeah. Though, I'd probably suggest calling it "rle" or "compression" or something like that. Even the uncompressed version still counts as "encoding".

Another question is whether it makes more sense to add the new option as a constructor ("with_rle") or as a setter ("set_rle"). I think the existing encoders all use constructors, but that doesn't seem as sustainable if we add more options...

Updated 👍

[marvin-j97]

I'm trying to read a bit into the code, and just documenting my thoughts.

Looks like tga::Header::from_pixel_data would need another parameter use_rle, and extend the match from:

match color_type {
  ColorType::Rgba8 => (8, 24, ImageType::RawTrueColor),
  // ...
}

to

match (color_type, use_rle) {
  (ColorType::Rgba8, false) => (8, 24, ImageType::RawTrueColor),
  (ColorType::Rgba8, true) => (8, 24, ImageType::RunTrueColor),
  // ... etc
}

Need to check the spec to see if this is the only change required for header.rs.

[marvin-j97]

Here's my first pass for TGA RLE support:

#2056