カラーマネージメントの導入による変更点

Sirilは現在、カラーマネージメントを正しく行なっています。

つまり、以前は正しく処理されていなかったということです。この点において、Sirilは決して珍しくはありませんでした。多くのプログラムはカラーマネージメント機能を提供しておらず、すべてがsRGB色空間で行われているという前提に依存しています。通常はこれで問題ありませんが、Sirilと適切なカラーマネージメント機能を備えたグラフィックアプリケーションの間で画像をやり取りする際、問題が発生したという報告が寄せられていました。

一部のユーザーから、特定の状況下で、Sirilで保存した画像が他のソフトウェアではSirilとは異なって表示されるという報告がありました。新しいカラーマネージメント機能を備えたSirilからエクスポートされた画像については、この問題は解消されるはずです。正しいカラーマネジメントが実装されているすべてのソフトウェアにおいて、画像は一貫した外観で表示されるようになります。

However, if you have stretched images saved as FITS in a previous version of Siril and you open them in color-managed Siril, they may in some circumstances look pale and washed out. Don't panic! Here is an explanation of why it happens, and how it can very easily be avoided / fixed.

なぜそうなるのでしょうか？

When Siril reads a FITS image without an embedded ICC profile, if the "Auto assign ICC profile" preferences are set it tries to work out whether the image has been stretched or not. It can only do that by checking the HISTORY entries in the FITS header. If it can detect stretch functions have been applied, then it assumes the image has been stretched on a sRGB monitor and applies the relevant profile. However older versions of Siril didn't always add the history for every command, and other software may or may not add history but Siril can't recognise all the stretch keywords that might be used by all other software, so it may be unable to detect that an image has been stretched. The fallback assumption we choose to make is that a FITS image without an embedded ICC profile is linear data.

However, your stretched FITS isn't really linear data. It has been stretched, and you have in effect been stretching it so that it looks right in your monitor's color space, which is probably approximately sRGB.

When this has a linear ICC profile assigned to it, the display routines then apply a color space transform from linear space to the display profile, in effect stretching it a second time.

どうすれば防げるでしょうか？

This problem will only occur if you have the option "Auto assign preferred color space on load" enabled. If this option is disabled, Siril will not assign any color profile when the file is loaded. You can then assign one manually using the Color Management dialog.

どうすれば修正できますか？

If you loaded an image with the Auto assign option enabled and the wrong profile is assigned (or if the wrong profile is assigned for any other reason, for example if you loaded a fle saved by other software that has assigned a wrong profile) fixing the problem is also very easy. All you need to do is open the image, go to the Color Management dialog and select "sRGB (standard sRGB TRC)" from the drop down selection (or "Gray (sRGB TRC)" if the image is mono). Click the Assign button (not Convert to) and Siril will assign the selected color profile to your image.

That's it. The display will now look correct and when you save the image, and the appropriate color profile will be embedded.