Clipping Warnings in Lightroom

And why they deceive you

Adobe Photoshop Lightroom, like many other Raw converters, has a clipping warning.
The purpose of it is to give you a visual warning (apart from the histogram) of what parts of an image might be clipping.

What is clipping?

A pixel is clipping when it reaches a value of 0 or 255 in one or more channels, and “should have gone further”. Since it cannot go lower then 0 or go higher then 255, it remains at those values: Detail is lost if one or two color channels clip, part of the image is solid black or white if all 3 channels clip.

The effect of color space

As with anything in digital imaging, the color space used has a big influence: A wide gamut color space (such as ProPhotoRGB) will have lower values for the same color then for instance sRGB. So a color that is clipping in sRGB, need not be clipping in ProPhotoRGB!
This color for instance, has sRGB values of (250,40,30). The ProPhotoRGB values are (177,76,36) for the exact same color.

sRGB (250,40,30)

This means that you have a lot more “headroom” in ProPhotoRGB before you hit the “clipping wall”.

So what?

Lightroom uses MelissaRGB internally (ProPhoto RGB with sRGB Tone Response Curve).
The histogram in Lightroom is based on its internal working space. So when you are exporting images for a web gallery, the images might be clipping big time while Lightroom is not warning you!

An example

I opened a DNG file in Lightroom 2.4 and in ACR 5.4. These have basically the same raw conversion engine. The exact same settings were used in both Raw converters.

Here is the image, histogram and clipping warning in Lightroom. (click image to open bigger).
Almost no clipping indicated (It makes no difference what output color space you choose): Just a bit in the lower right that goes almost black, and absolutely no clipping highlights according to Lightroom:
Lightroom clipping warning
Here is the image in ACR 5.4. Output color space is ProPhotoRGB: About the same clipping warning Lightroom is giving.
ACR clipping warning; ProPhotoRGB output colorspace

Here is the same image in ACR 5.4. Output color space is sRGB: major clipping!
ACR clipping warning; sRGB output colorspace
For reference: Here is the image exported out of Lightroom: Clipping indeed:

sRGB image as exported from Lightroom

Clipping warning for highlights on exported sRGB image

Workaround

Is there a workaround? No (except using ACR that is).
Simply sad said, the only thing you can do is watch the histogram, guess, and use your eyes. If your screen has close to sRGB gamut, clipping in sRGB might also be visible on screen (as can be seen from the above screenshots in Lightroom).
If you use a wide gamut screen however, you might see quite a difference between the Lightroom “Develop” module and the actual exported image…

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2 Responses to “Clipping Warnings in Lightroom”

  1. Levina de Ruijter Says:
    October 26th, 2011 at 12:32

    Excellent article, René. Quite an eye opener. Thanks.

    So would you say that ACR is a better raw converter than Lightroom then?

  2. René Says:
    November 11th, 2011 at 17:00

    Oops, missed your comment…
    The Raw conversion is the same (if using the same generation), however the clipping warning in LR is pretty useless if you output AdobeRGB or sRGB (web!) images…

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