Processing and editing images in Lab mode

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Photivo closes the gap between RAW converters and easy image editing, thus making often required reworking in other programs unnecessary.

Many tools are available these days for image editing. In addition to common applications, such as GIMP, several tools exist for specific image processing scenarios. One of them is Photivo [1], which is based on the Lab function (see the "Lab Color Model" box). Because Photivo can read RAW files, its use normally precedes image processing (e.g., what you'd do with GIMP).

Lab Color Model

The Lab abstract color model has been around for 40 years as a way of defining colors independently of display devices and capturing the essence of human color perception. It presents colors so that equally perceived differences are described as similarly as possible. Lab also accounts for all theoretically possible colors, not only those perceivable by the human eye.

The "L" in Lab stands for luminance, the "a" is for the color components between red and green, and the "b" is for those between blue and yellow. Interestingly, this color models defines all the colors in the RGB and CMYK spectrum – and a whole lot more.

The special features of the Lab system lead to measures of brightness being totally independent of color components in the a and b channels. This provides a number of advantages over other color models. The independence of the b channel can be put to good use: For example, it helps in distinguishing the often problematic green tones of leaves without affecting any other color.

Photivo has many RGB functions and is suitable for processing 16-bit RAW files and bases all its capabilities on them. The program also supports formats like TIFF, PNG, and JPEG. If you've never worked with the Lab color model, then you should familiarize yourself with GIMP to start with.

First Steps

Photivo can be found in the repository of many Linux distributions, but with Ubuntu, you will have to add a specific ppa:

sudo add-apt-repository ppa:dhor/myway
sudo apt-get update

Then, you can install as normal.

To make sure Photivo works without a hitch, you may also have to upgrade GraphicsMagick :

sudo apt-get update graphicsmagick

Once installed, the program opens with a large, two-part window (Figure 1). On the right is the preview, and on the left are the tools and parameters.

Figure 1: Photivo's main window consists of a large preview window and a comprehensive tool list. The tools provide context menus for the most commonly used tasks.

Photivo also provides a variety of controls at various points. Below the histogram, you can set which tools you want handy. Below the tool list, you can specify the image size for the preview pane. The default is 1:4, which is usually a good compromise between accuracy and computation time. The manual setting next to it determines whether to change the preview automatically or explicitly by the user. You can set exactly what the preview shows with the RGB tab, where you can select the Structure (distribution of detail), the Gradient (luminance representing edges), or the individual Lab channels.

The next three buttons allow loading of templates, saving current views, and exporting images to a predefined image processor. The lower part of the main view serves as status bar and provides other control elements. When Photivo is processing an image, it shows the image in the preview window (Figure 2). You can set various controls by mouse click, mouse wheel, or held left mouse button. A right-click resets things to their default values.

Figure 2: Photivo performs most actions directly. Various messages show the current condition in the preview window.


Photivo differs from many RAW converters in that the functions in the Camera tab under Input only load one image at a time. You can then open an already existing editing profile, which can be a preset (via Open preset ) or one that you set yourself (via Open settings file ).

The first step is to define under Camera color space how Photivo interprets the image. Select an External profile that fits your camera, then set the Target of the profile [2]. As a whole, Photivo builds a workflow from the series of tabs defined.

Choosing the right profile here determines the desired results. For pure monitor display, sRGB would be most appropriate. BT-709 is fitting for HDTV videos; use Pure 2.2 with a standard gamma of 2.2 for all other output. The other settings on this tab are similar to other RAW converters, such as Darktable. However, Photivo has tools on other tabs that are closer to other image editing tools like GIMP.

The next tab is for Local Edit . The idea behind this is that not all image regions need to have the same editing treatment to optimize the results. Thus, it makes no sense to make "lights" (bright regions) even brighter, but perhaps shadows should be lightened.

The functions require a certain routine approach. Beginners should stick to editing functions that already have many separate edits for lights, mid-tones, and shadows.

Image processors like GIMP use selections, masks, and so forth for separate editing. In Photivo, rather, you define one or more image regions to work on with Local adjust . To determine regions, first click + and select Assign regions . Then, click the a spot on the image – the sky or shadows, for example – that particularly represents the region, which is like using the magic wand in GIMP to encompass an image area with similar colors and brightness.

Photivo creates a new "region" that adopts this name by default. You can have any number of regions, which you should give appropriate identifiers for a better overview. In the preview window, the current "mask" marks this region with a bright, cloud-like texture (Figure 3). The mask doesn't need to be too exact, because you can always adjust it later.

Figure 3: Photivo allows you to mark certain image regions for separate editing. The preview shows a "cloud" for the currently selected region.

Photivo provides three controls for Masks , along with a short help text if you hover your mouse over them. The luminance curve can also greatly affect the result. After defining the regions, you select the ones you want and remove the ones you don't want. Clicking the check box ends region definition mode and returns to the editing functions [3].

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