Are JpegEncoder and other format encoders considered to be reusable for multiple images?

[br3aker]

If yes

There's a bug at least in jpeg:

JpegEncoder overwrites grayscale/colored images encoding so encoding combinations of different pixel types wouldn't work properly:

ImageSharp/src/ImageSharp/Formats/Jpeg/JpegEncoder.cs

Lines 66 to 83 in 9baba86

	private void InitializeColorType<TPixel>(Image<TPixel> image)
	where TPixel : unmanaged, IPixel<TPixel>
	{
	// First inspect the image metadata.
	if (this.ColorType == null)
	{
	JpegMetadata metadata = image.Metadata.GetJpegMetadata();
	this.ColorType = metadata.ColorType;
	}
	// Secondly, inspect the pixel type.
	if (this.ColorType == null)
	{
	bool isGrayscale =
	typeof(TPixel) == typeof(L8) || typeof(TPixel) == typeof(L16) ||
	typeof(TPixel) == typeof(La16) || typeof(TPixel) == typeof(La32);
	this.ColorType = isGrayscale ? JpegColorType.Luminance : JpegColorType.YCbCr;
	}

Steps to reproduce:

using var img_grayscale = Image.Load();
using var img_colored = Image.Load();

JpegEncoder encoder = new JpegEncoder { };
img_grayscale.SaveAsJpeg("path", encoder); // encoder is now grascale
img_colored.SaveAsJpeg("path", encoder); // saved as grayscale

If no

Current tests for jpeg assumes encoder can be used multiple times:

ImageSharp/tests/ImageSharp.Tests/Formats/Jpg/JpegEncoderTests.cs

Lines 178 to 222 in 9baba86

	[Fact]
	public void Quality_0_And_1_Are_Identical()
	{
	var options = new JpegEncoder
	{
	Quality = 0
	};
	var testFile = TestFile.Create(TestImages.Jpeg.Baseline.Calliphora);
	using (Image<Rgba32> input = testFile.CreateRgba32Image())
	using (var memStream0 = new MemoryStream())
	using (var memStream1 = new MemoryStream())
	{
	input.SaveAsJpeg(memStream0, options);
	options.Quality = 1;
	input.SaveAsJpeg(memStream1, options);
	Assert.Equal(memStream0.ToArray(), memStream1.ToArray());
	}
	}
	[Fact]
	public void Quality_0_And_100_Are_Not_Identical()
	{
	var options = new JpegEncoder
	{
	Quality = 0
	};
	var testFile = TestFile.Create(TestImages.Jpeg.Baseline.Calliphora);
	using (Image<Rgba32> input = testFile.CreateRgba32Image())
	using (var memStream0 = new MemoryStream())
	using (var memStream1 = new MemoryStream())
	{
	input.SaveAsJpeg(memStream0, options);
	options.Quality = 100;
	input.SaveAsJpeg(memStream1, options);
	Assert.NotEqual(memStream0.ToArray(), memStream1.ToArray());
	}
	}

While this currently does not break anything it gives false (or not) promise of being reusable through tests while not being so. Not sure if it wasn't expected so opened a discussion instead of an issue.

Question/Proposal: state explicitly in the docs if they are reusable or not and fix reusable usage for all encoders in tests.

[antonfirsov]

@br3aker good catch! I think this is a bug, encoder/decoder state should not be altered by encoding. We can implement an equivalent logic in JpegEncoderCore instead.

[br3aker]

We should check all of them I guess. I might be wrong but I think I saw something state-altering in the png encoder also. Will recheck tommorow and open an issue,

[JimBobSquarePants]

Yep, that code and similar should be in the underlying Core encoder which we new up every time.

[br3aker]

Looks like every other encoder doesn't suffer from similar issue, opened an issue.