I could have converted this underexposed color image into a correctly exposed B&W image using Adobe Camera RAW and Photoshop but it would have been a multi step process. After I converted the RAW data into a RGB image I would have had to imported it into Photoshop, converted it into a LAB image and then used a curve or level layer on the L channel to adjust the exposure. With RAW Therapee all than can be done in one simple step.
When I first started out I thought, like I suspect most user do, that Photoshop did its mathematical calculations in the RGB mode. But it starts out in the LAB mode. Instead of the familiar Red, Green and Blue channels LAB has, not surprising, L, A and B channels. The L or Luminosity channel has all the black and white picture information. The A channel has all the red and green color information. And the B channel has all the blue and yellow color information,
A major and practical advantage of this mode is you can apply a curve layer to the L channel and do what you want without touching the color information and introducing a color cast. You can also do the same thing to the A and B channels to correct a color cast. Or you can go hog wild and create colors that nature never intended.
Besides LAB curves there are several other things I like abut the exposure tab in RT. If you click on the + sign on the bottom you can pull up multiple viewing boxes. When I processed the image I monitored the eyes of the two girl, the shirt and the writing on the folder as I adjust the L channel. I've also ran the saturation down to zero for B&W. If I had wanted to change the intensities of the the various colors I could have adjusted the A&B channel. Or I could have gone to the color tab and used RT's channel mixer. There are often several ways to adjust a RAW image in RT.
(With more to come. Yesterday I saw postings about a new tone matching feature for HDR work on RAW data)
In Lab curves you can pull the the usual spot and parametric adjustments but I prefer using the control cage adjustment. This uses tangential Belzer controls. You don't have annoying dips at one end of a curve adjustment when you raise the other end. You also have a well designed histogram window up at the top where you need it instead having to call up a separate window.
A second feature you won't find anywhere except pricey astrophotography programs is Richardson-Lucy deconvolution. This algorithm was developed by NASA to deconvolute or enhance the images from the Hubble telescope.
Enhances Contrast Off
Enhanced Contrast On
I won't claim to understand the mathematics of the algorithm but the end result is you have four sliders that control the spacial frequencies in the image. The best example I've run across to explain this in a non mathematical way is to imagine you are taking a high iso image of a court yard filled with tables from the top of a high building. On each table there is an open laptop showing its keyboard.
The fine slider controls the information in the image about the keyboard, the second slider the laptop, the third the tables and the last the courtyard. You can multiply that information by a number that ranges from zero to four to control how much of that information appears in the reconstructed images. Or in other words you can use the sliders to enhance the contrast of the details and/or reduce the noise in the image.
Noise is usually randomly speckled pixels in the image. It is strongest in the fine area so I've run that slider down to zero. None of that information will appear in the final image. The other three sliders I've adjusted to create a pleasing image.
Compare the viewing boxes with the controls off and on. Notice how I brightened and brought out more detail in the girl's eyes, and sharpened the writing on the folder. This a simple and easy way to make adjustments that would require much more work in Photoshop. Great job RT folks.
So where does RAW Therapee stand now-Jan 8th 2011. The 770 build of Dec 29 is a big improvement. I can run the same program on both my 64bit AMD laptop and my older 32bit Intel desktop. But there are still memory management issues. So don't use the queue. Convert your images one at time using "Save Image." It also helps to end any unused programs. And if you have been wildly experimenting like I have been, use the excellent history panel to create a snapshot every now and then. Clicking on that reverts back to an earlier state and frees up more memory.
As usual you can find the window version of RT at http://www.rawtherapee.com/releases_head/windows/branch_3.0---stable/
to be continued