Kuuvik Capture 3.2 : The Dual Histogram

The very first Kuuvik Capture release introduced RAW histograms in an attempt to provide a tool for judging exposure more precisely than what regular histograms are capable of. I even wrote an article on the merits of having a RAW histogram. The conclusion of that article was that despite you have a RAW histogram, white balancing could clip channels in the converted image even if everything was fine with the RAW; so you need to see both the RAW and the processed histograms (preferably in your RAW converter) for the final decision on your exposure.

While my former article revolved around the white balance issue, other image processing parameters, like picture style and color space, also have heavy influence on histogram precision and usability. Making to see both histogram types a fundamental need. Not to mention that launching a RAW converter is not always convenient to do.

So version 3.2 sports a new Dual Histogram tool to show Kuuvik Capture’s RAW histogram along the usual one generated from processed data.

For images, the processed part is based on the JPG preview that every RAW image contains (this is what Kuuvik Capture displays, and this is the source of the histogram shown on the camera’s LCD). This represents how the image was processed by the camera. Your RAW converter will almost certainly convert it in a different way, so the final word on exposure still belongs to the converter. But the camera’s interpretation gives a solid starting point.

The processed histogram also indicates the image’s color space. Different color spaces have different clipping points – more on this later.

You can clearly see on the example how white balancing influenced things. You’d be in trouble having made a decision solely based on the RAW histogram in this case – the blue channel would be almost completely clipped. The example belongs to the original of the following photo (that is, before contrast stretching and other adjustments).

Ice Abstract

For live view and movie recording the camera always serves video frames in sRGB – even if you set your camera to Adobe RGB. But why is that important? Well, it’s time to talk about the effect of color space choice on histograms.

Color spaces vs. histograms

I made two photos of a regular ColorChecker chart. One with setting the camera to sRGB and another with setting it to Adobe RGB. Lighting and exposure were the same.

As you can see, there’s absolutely no clipping in the RAW data. And there’s no clipping when converted to Adobe RGB. But in sRGB both highlights and shadows are clipped! So live view (which is always in sRGB) may show some clipping while Adobe RGB not. And even a histogram from an Adobe RGB conversion might show clipping while there’s absolutely no clipping in the image when converted to ProPhoto RGB in Capture One.

I’d recommend to treat these processed histogram clipping warnings as different levels. The sRGB warning in live view goes off first, this should ring a bell in your head to watch more closely after taking the image, as there may be a problem. After taking a picture, if Adobe RGB shows clipping, then it’s time to either check it in your RAW converter or back off a little bit.

But RAW histogram clipping warnings are always hard facts: indicating unrecoverable data loss.

The above example explains why I recommend to set your camera to Adobe RGB: to prevent premature clipping in histograms displayed on the camera’s LCD.

A few words on JPG support

JPG files slowly become a first-class citizen in Kuuvik Capture. The JPG processing engine in version 3.2 is up to 5x faster than previous releases. This speedup is what allows efficient histogram generation and made Dual Histograms possible. JPG histogram display was also a requirement for a JPG-only workflow, which was high on our feature request list. And now it’s fully supported as you’ll see in my upcoming post.

Kuuvik Capture 3.2 is available on the Mac App Store. It is a free update for existing Kuuvik Capture 2.x and 3.x users.

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