Siril needs distortion correction in stacking

[flint]

@Entropy512

If that is the case, then linear is definitely the correct transfer function for the exported ICC, however it might be pure luck that some of those transforms make things look OK when misinterpreting as sRGB.

@lock042

I do agree but … All images exported from Siril, and opened as sRGB in GIMP or other software looks great… This is why I’ve used it.

I can explain this phenomenon.

The first problem is that Siril does not apply the appropriate display transform, which is typically the sRGB TRC in most cases, to its preview image. So, a raw (linear) image is rendered too dark in the preview window to begin with; that is, the raw linear image the user sees in Siril’s preview window is not linear data.

Next, the user processes the linear image based on what they see in the preview window. This usually involves applying non-linear operations like Asinh until the user is satisfied with the look of the preview. The processed image is then saved with the ICC sRGB profile but Siril does not apply the sRGB TRC. Basically, the saved image is stretched more than required in order to compensate for the missing display transform; that is, the image would look too bright (or washed out) if the equivalent operations were applied in color managed software.

Finally, the user opens the saved “sRGB” image in a color managed application (darktable, rawtherapee, photoshop etc). These software typically use an intermediate linear working space, like linear Rec2020, for processing. The software is expecting sRGB data and so applies the sRGB TRC accordingly to transform the image data into the working space before rendering the image in the preview window. Being color managed, unlike Siril, the software also applies the appropriate display transform to the preview image. These two operations, i.e converting from sRGB to the working space and then applying the display transform, will effectively cancel each other provided that the display profile is also sRGB. When that is the case, which is certainly true for the majority of users, then the image displayed by the color manged software will more-or-less match Siril’s preview image (discounting gamut mapping).

So, it’s the ubiquity of the sRGB color space rather than luck which explains why things look OK to most users. Unfortunately, this is rather a false sense of security because the reference for comparison, i.e Siril’s preview image, is incorrect to start with. I see two potential problems for users when the ICC profile is appended to the image file:

1. When the display media is not sRGB, e.g AdobeRGB or printer color space, the displayed image colors will not match Siril’s preview (remember the user bases the look on Siril’s preview image).

2. A linear image, say a stacked light calibrated with biases and flats, is transformed into non-linear data when it’s loaded into color managed software. Any operations intended for linear data are actually performed on non-linear data but the user is not really aware of this.