Introduction / Colorspaces
Short Version
FFDD6 has been re-designed from the ground up to provide in Photoshop the same Linear colorspace editing you get when using any Raw converter. This provides digital darkroom editing with no tone or color distortion when editing. FFDD6 will work with any colorspace which has Gamma set to 1.0, and also provides a Linear version of ProPhoto RGB in the ProPhotoRGB-Linear.icc file, which has the same huge Gamut as ProPhoto RGB (see the section on gamut for more info).
In Photoshop the default colorspace is set in the Color Settings dialog (choose Edit->Color Settings... from the main menu). Look in the section labeled Working Spaces then at the choice labeled RGB. The FFDD6 batch script by default embeds the ProPhotoRGB-Linear.icc colorspace into its output files, so as long as Preserve Embedded Profiles is selected as the RGB Color Management Policies option, there is no need to set a default colorspace for FFDD6.
Why Linear?
Linear (colorspace Gamma set to 1.0) simply means that the distribution of tonal values in the colorspace adds and subtracts like actual light would. In contrast a non-linear colorspace such as Adobe RGB 1998 (which has a Gamma of 2.2) has tonal values which add and subtract with a slight error. The image below shows how colors blend when working in a colorspace with each of the different gamma settings. Each square is a magenta->black gradient blended into a green->black gradient. The numerical values (or digital data) is the same for each square, only the colorspace is different. The gray square on the bottom shows the color formed half way between the brightest magenta and the brightest green. Notice that the linear square has an even lightness throughout the blend (as it should), and that the gamma squares do not (the center of the blend gets darker).
Those well versed in Adobe Photoshop know of the Blend RGB Colors Using Gamma: 1.00 option in the Color Settings dialog. While this setting will correct the blend errors between layers, non-linear color distortion errors still will crop up when processing an individual layer. So this is not a fix for the problems of editing in a non-linear colorspace.
How to Read a Linear Histogram
The diagram below shows the difference in tonal distribution between three different settings for Gamma. In the center there is a view of the histograms for the image on the left converted into the three colorspaces. The blue graph is the histogram, the different gradients mark zones 1-9 from the zone actions, and the red line marks the value for medium (18%) gray (which is preceived as half way between black and white). There are a few key notes below the diagram.
- In the linear mode, 18% of the values are darker than medium gray and 82% are lighter.
- The gamma 2.2 mode provides a more even split between darks and lights.
- Zone 1 (the left most zone) seems larger than it should be. This is due to a difference in how we preceive tonality near black compared with the rest of the tonal range.
Note that images in a Linear colorspace will always look overly left weighted in the histogram and might even appear to be clipping in the blacks, even when the image does not contain a true black (as can be see in the above example). So using the histogram to try and gage proper exposure is not a good idea in a linear colorspace (or even in a non-linear colorspace for that matter).
Historically Why Non-Linear?
The historical reason for having a colorspace with a Gamma tone distribution is because 8-bit color does not have enough tones in a Linear colorspace to be able to represent colors and shades without producing visible banding. For example, here is a sRGB (non-linear) colorspace 8-bit file,
And now how the same file would look like in a Linear 8-bit colorspace (without dithering),
However with 16-bits per color channel, banding is no longer an issue.
What About 16-bit Dynamic Range?
The difference between editing in Linear vs Non-Linear is about one stop less dynamic range in the darks for Linear. In Adobe Photoshop 16-bit color provides about 15 stops of dynamic range overall when working in a traditional Non-Linear colorspace. Linear provides 14 stops of range, which is more than enough for extreme adjustments. For example here is a very dark crop from an extended dynamic range 16-bit image,
Here is the same image with a massive exposure adjustment done in a Linear colorspace, effectively taking the darkest shadows and over-exposing them,
And the same adjustment done in a Non-Linear colorspace,
Sharpening in Linear Colorspaces
The standard Unsharp Mask and typical sharpening tools no longer work correctly in a Linear colorspace. For example, they tend to leave sharp black outlines,
Instead, use the FFDD sharp actions to sharpen the image without artifacts. Here is an example of FFDD denoise and sharp combind,
External References
- ColorFAQ - An excellent FAQ by Charles Poynton.
- GammaFAQ - Another excellent FAQ by Charles Poynton.
- Luminous Landscape : Understanding ProPhoto RGB - An article by Michael Reichmann.
- Bruce Lindbloom - Another good reference.