TIFF Encoder support saving grayscale using 16 bits (L16)

[ptasev]

We have a large grayscale image (1 gigapixels) of the L16 pixel format. Currently it appears the TIFF encoder does not support saving 16 bits per sample and 1 sample per pixel images. At the moment this setup is saved as 8 bits per sample and 3 samples per pixel.

ImageSharp/src/ImageSharp/Formats/Tiff/TiffBitsPerPixel.cs

Lines 54 to 59 in 03cc807

	/// <summary>
	/// 16 bits per pixel, for gray images.
	///
	/// Note: The TiffEncoder does not yet support 16 bits per color channel and will default to 24 bits per pixel instead.
	/// </summary>
	Bit16 = 16,

ImageSharp/src/ImageSharp/Formats/Tiff/TiffEncoderCore.cs

Lines 376 to 390 in 03cc807

	case TiffBitsPerPixel.Bit8:
	this.SetEncoderOptions(bitsPerPixel, photometricInterpretation ?? TiffPhotometricInterpretation.BlackIsZero, compression, predictor);
	break;
	case TiffBitsPerPixel.Bit6:
	case TiffBitsPerPixel.Bit10:
	case TiffBitsPerPixel.Bit12:
	case TiffBitsPerPixel.Bit14:
	case TiffBitsPerPixel.Bit16:
	case TiffBitsPerPixel.Bit30:
	case TiffBitsPerPixel.Bit36:
	case TiffBitsPerPixel.Bit42:
	case TiffBitsPerPixel.Bit48:
	// Encoding not yet supported bits per pixel will default to 24 bits.
	this.SetEncoderOptions(TiffBitsPerPixel.Bit24, TiffPhotometricInterpretation.Rgb, compression, TiffPredictor.None);
	break;

This feature would allow us to properly save the image as grayscale and not use an extra 8-bits per pixel when using the following encoder settings.

var image = new Image<L16>(4, 2);
image.SaveAsTiff("tmp.tiff", new TiffEncoder() { BitsPerPixel = TiffBitsPerPixel.Bit16, PhotometricInterpretation = TiffPhotometricInterpretation.BlackIsZero});

From doing a quick look through the code on GitHub, it should be fairly straightforward to implement this. I can give implementing this a shot if the team would like. Ideally it can go in both 2.1.X and v3.

[JimBobSquarePants]

Hi @ptasev

It would be awesome if you could implement this, thanks!

We wouldn't backport a feature into 2.1.x though only critical (i.e. security driven) defects. V3 is very close to completion and will be released hopefully within the next few weeks.

[ptasev]

Added in PR #2339