IT8.7 Profiling Tests — Continued

Although the slide is somewhat under-exposed, there are spots of this image that are really bright, so it would absolutely not be a conceivable solution to just crank up the lamp intensity to compensate for the under-exposure, since that would completely wipe out the highlights.

The real slide, when viewed on my light table, looks pretty OK. My light table has two sets of light tubes; even with just one of the sets turned on, it is not difficult at all to see some details inside the red rectangle. With both sets turned on, one can even see windows and some structure on the house in the center of the red rectangle, which in this image looks almost black! So subjectively, the applied curves makes the appearance of the image closer to the appearance of the real slide. (Curious to know where the picture is taken? It is shot from the Galata Tower in Istanbul 2003.)

As you can see in the large image, it is dark and noisy, but there are details there, or at least should be details! And for many of the details you know that they are white or various shades of grey, plus the reddish tiled roof.

I claim this is pretty close to the appearance of the real slide on the light table — at least concerning colors and contrast. However, the real slide has more details in the deep shadows if viewed with sufficiently much light.

The noise is caused by the CCD element in the scanner, despite this scanner is supposed to be one of the best you can get in this respect (apart from drum scanners of course)! Nevertheless, you can also see film grain in the shadows.

Note the extreme difference between this one and the corresponding SilverFast managed version!

As it turns out, it is actually very important which order you do color space conversion and apply Curves: In this image, both of the Curves applications were done before sRGB conversion, i.e while still in the input color space defined by the inCamera profile. Let's see what happens if the second Curve application is done after sRGB conversion:

The reason for the difference is partly that the gamma is different when Curves is applied the second time, partly because we have left the input space.

(The inCamera profile is assigned to an image that has a gamma of 1.0, and since profile assignments do not change any RGB values, the first application of Curves operated in a gamma 1.0 enviromnent, whereas after the sRGB conversion, we operate in a gamma 2.2 environment! But as mentioned, it only explains part of the difference.)

Overall, I think this image version is pretty close to the real slide if the slide is looked at using a strong light and a good loupe, except for the fact that there is some added scanner noise here. In the lower left area of the image, there is a tree, which is supposed to have green leaves. However, not even the real slide is able to reveal that — it is mainly just murky there, although I can understand that it is a tree, and there are some vague hints of foliage. So the scanner noise seems to add just slightly to the murkiness of the grainy film base.

Comparing this image with the SilverFast version where we applied Curves twice, it becomes just very apparent that SilverFast is a disaster in this respect!

We need to do one more experiment: What happens if we convert to a working space before any application of Curves? See next image:

Note the striking difference between the two images! The latter one (i.e this image) is not at all good — there is some nasty violet cast all-over, and the the grains in the transition zone has become blotchy, and the darkest shadows are blocked! The sheer magnitude of the difference is astonishing!

(The inCamera profile is assigned to an image that has a gamma of 1.0, and since profile assignments do not change any RGB values, the first inCamera version applied Curves while still operating in a gamma 1.0 enviromnent. In this image, however, we have converted to the Adobe RGB (1998) color space, which has a gamma of 2.2, so now the Curve is applied on a gamma 2.2 image! However, as mentioned above, it only explains part of the difference.)

In theory, one might think that one could simply modify the Curves, so that the result of applying it on a gamma 2.2 image was the same as the "standard" Curves application on a gamma 1.0 image. However, this is not possible! As an experiment, I tried to create a composite RGB curve that mimicked the first inCamera version as much as possible. After a fair amout of tricky tweaking, I could get close to the overall contrast, but colors were still quite off — for instance, the red tiled roof lost most of its color. Also, the blotchiness could not be completely avoided.

Also, I have made tests with inCamera profiles made for gamma 2.2 scans, with almost the same principal result. That shows it is not only a gamma issue — the input space has special behaviors, and right now I can unfortunately not explain the mechanism. If you want to experiment yourself, download this raw gamma 1.0 PNG image (4.9 MB), which has no profile and is totally unmodified. Then assign this gamma 1.0 inCamera profile (448 kB zip archive).

You can also compare the shown image with the first SilverFast version, since both applied Curves in the Adobe RGB (1998) color space. It is just very apparent how much the SilverFast profile lost and destroyed!

A conclusion to draw from this experiment is that: if one needs to apply significant (massive) amounts of Curves, to avoid getting nasty artifacts, it should be done before any conversions to (high-gamma) working spaces! This contradicts most work flow schemes, but it seems inevitable!

Overall, the two images are very similar — if not to say, extremely similar! However, there is a small advantage to inCamera. The comparison comes with a certain reservation, however, since the Scarse profile was generated a year ago from a different file than is used for inCamera. This is discussed in more detail in Comparisons using the IT8.7 target, where the conclusion is that a comparison is a valid one.

Although still very similar, the difference has increased, and it is not difficult to see that the inCamera version is slightly better.

In the original 2003 ariticle, I made a claim that the Scarse profile caused a blotchiness in the deep shadows. I revoke that claim now. It was caused by a combination of grain and scanner noise. I also made a claim about blotchiness when Curves are applied; but, as demonstrated above for the inCamera profile, it was caused by converting the image to a working space before applying Curves.

The Scarse profile did very well, but not quite as good as inCamera. It is worth a closer inspection, and it does certainly have potential.

The SilverFast profile is not recommended at all. As long as it is based on the preview of the scan, it does not have any potential. The HDR version of SilverFast, however, may have potential, since it works very differently.

The application of Curves to lighten up shadows can grossly amplify any imperfections in a profile — this can really make bad profiles destroy an image. The more you need to tweak your images, the better profile you need!

Perhaps somewhat surprising is the fact that the order in which you convert to a color space and apply Curves is very important! For strong lifting of shadows, it may be better to apply Curves (or similar tools) before any conversions to working spaces, such as Adobe RGB (1998), i.e while still in the input color space. For color balancing, however, it may be better to do that after you have converted to a working space, as was exemplified in a portrait.

Back to the beginning of the article.