Using ICC Profile converter in V4

[Socolin]

Hello,

With #1567 done, I'm interested in start testing this features. But I'm hitting a few difficulty. (I'm using 4.0.0-alpha.0.46)

1. I was not able to find a way to use the ColorProfileConverter most of the related code seems to be internal. Since I'm doing composition between multiple image that will be printed later, I'm curious if it would be possible to load the images, work with them in CIELAB, or CIEXYZ, and then choose the profile when saving (for example a CMYK profile), so there is minimum loss, instead of converting everything to sRGB, then converting it again for printing.

2. I also found that it should be possible to convert the profile to sRGB during decoding. But when I tried with the images from here: https://displaycal.net/icc-color-management-test/ the pixels were not converted (Should I open an issue for that, or did I miss something ?)
   EDIT: It seems IsLikelySrgb is true for those tests profiles
   EDIT2: It works if I add return false in IsLikelySrgb()

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Formats;
using SixLabors.ImageSharp.PixelFormats;

var decoderOptions = new DecoderOptions
{
	ColorProfileHandling = ColorProfileHandling.Convert,
};
using var image = Image.Load<Rgba32>(decoderOptions, "sRGB_Gray.jpg");
Console.WriteLine(image[0,0]);
image.Save("sRGB_Gray_out.jpg");

[JimBobSquarePants]

Hi @Socolin

I was not able to find a way to use the ColorProfileConverter most of the related code seems to be internal. Since I'm doing composition between multiple image that will be printed later, I'm curious if it would be possible to load the images, work with them in CIELAB, or CIEXYZ, and then choose the profile when saving (for example a CMYK profile), so there is minimum loss, instead of converting everything to sRGB, then converting it again for printing.

All operations on images in ImageSharp require conversion to TPixel format that can be easily converted to an RGBA compatible buffer of floats typically expressed by Vector4 so you are not able to load an image and work in color profiles like CIELAB, or CIEXYZ.

The ColorProfileConverter allows conversion between any IColorProfile<TSelf, TProfileSpace> instances or spans with or without a color profile. The public API to do this is available, and the process will automatically apply itself on supply of a source and target profile.

ColorConversionOptions options = new()
{
    SourceIccProfile = profileIn,
    TargetIccProfile = profileOut,
};
ColorProfileConverter converter = new(options);
converter.Convert<Cmyk, Rgb>(source, destination);

I also found that it should be possible to convert the profile to sRGB during decoding. But when I tried with the images from here: https://displaycal.net/icc-color-management-test/ the pixels were not converted (Should I open an issue for that, or did I miss something ?)
EDIT: It seems IsLikelySrgb is true for those tests profiles
EDIT2: It works if I add return false in IsLikelySrgb()

Yes, your finding is correct. The existing method is not robust enough and we cannot rely on header checks alone. I've opened #3008 to address this.

[Socolin]

Hello,

Thanks for the anwsers.
About the color conversion, I saw the method Convert but it's in an internal class so I cannot access it

https://github.com/SixLabors/ImageSharp/blob/main/src/ImageSharp/ColorProfiles/ColorProfileConverterExtensionsCieLabCieLab.cs#L9C23-L9C66
https://github.com/SixLabors/ImageSharp/blob/main/src/ImageSharp/ColorProfiles/ColorProfileConverterExtensionsRgbCieXyz.cs#L9

Restore complete (0.2s)
  ImageSharpTesting failed with 1 error(s) (0.1s)
    Program.cs(10,11): error CS1061: 'ColorProfileConverter' does not contain a definition for 'Convert' and no accessible extension method 'Convert' accepting a first argument of type 'ColorProfileConverter' could be found (are you missing a using directive or an assembly reference?)

[JimBobSquarePants]

Ha! You're right! I was looking at the methods. It was all internal while I developed the API. I've updated that in the PR and added comprehensive intellisense documentation.

[Socolin]

Thanks !