Monday, January 14, 2013


If everything goes as planned and everything on the to-do list is checked off the  next stable build, RT 4.1, will be posted in early February. That build promises mucho minor bug fixes, welcome speedups and CIECAM02.

So what is RAWTherapee's latest and greatest addition.  If you google CIECAM02 the wiki entry pops up at the top of the list. If you understand what you read there, email me a translation in people talk. You obviously understand what going on far better than I do. Tis a confusing topic.

In the RAWTherapee implementation we are looking at 12 different sliders coupled with three curve sets and  five curve types. So I will state upfront that if you are looking for a blog about adjusting this slider to create exactly that effect look somewhere else.  I'm not even close to working out those details. But I will say by dragging  sliders around without knowing  exactly what will happen I've corrected many color problems. I've also seen interesting tone mapped HDR images in the developer's forum.  for examples go to

With a total of 12 sliders and 5 curves types  CIECAM02 corrects the subtle and not so subtle differences in how we humans see a scene or image and how cameras makes an image. The most obvious difference is chromatic constant or in photographer talk white balance. Another effect, seen by anyone who has thumbed through an optical illusion book, is what wiki calls the white illusion. Both grey patches are identical but they look different when displayed against white or black backgrounds

The  'scene condition' section deals with these effects. To quote wiki 'Though the human visual system generally does maintain constant perceived color under different lighting, there are situations where the relative brightness of two different stimuli will appear reversed at different illuminance levels. For example, the bright yellow petals of flowers will appear dark compared to the green leaves in dim light while the opposite is true during the day. This is known as the  Purkinje effect, and arises because the peak sensitivity of the human eye shifts toward the blue end of the spectrum at lower light levels.'

The top slider makes only minor adjustments to the histogram. I check the auto adjustment box in the corner.  With the dark surround--for example viewing a  framed photo hung in a dark hallway-- the change is more pronounced. To counter the Punkinje effect, it shifts the blue channel in the histogram. There are two algorithms for this, the first using only the normal RT adjustments with the second factoring in the CIECAM02 adjustments.

 In the next section there are 9 different sliders if I show them all.

Since I did the screen shots out of order, this is the photo I wanted to create. I started with the shot below, an ISO6400 image deliberately underexposed so I could shoot at 1/125 second when the girls were bowling.

I worked with the brightness and colorfulness algorithm. Instead of colorfulness I could have chosen chroma or saturation. To quote wiki  "colorfulnesschroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific colorColorfulness is the degree of difference between a color and grayChroma is the colorfulness relative to the brightness of another color that appears white under similar viewing conditions. Saturation is the colorfulness of a color relative to its own brightness.Perhaps there are technical advantages to using the other two choices, but I haven't found any yet.

After adjusting for exposure and white balance I did a curves adjustment on colorfulness to create a brightly saturated cake and table cloth along with neutral and slightly subdued skin tones. The dull slider made big changes, the neutral slider didn't do much. With the strong blues and purples in this image the pastel slider shifted the colors more than the saturation slider. When I switched from colorfulness to chroma or saturation the colors shifted slightly. In short with curves and the sliders  I could smoothly adjust the colors to just about anything I wanted. 

Pushing  matters further, with hue slider I could dress the girls in green without destroying the skin tones. 

Finally there is a set of adjustments to correct the differences between viewing an image on a monitor in a well lit room, on TV screen in a darker room or using a projector in a very dark room 

To sum up CIECAM02 is an excellent addition to RAWTherapee

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