Processing and editing images in Lab mode

Geometry

The functions combined in the Geometry tab are like those for RAW converters. The five lens-related settings (Lens Parameters through Geometry Conversion ), and in a limited way also Defish , use the LensFun database [4]. These settings characterize lenses by specific parameters to correct distortion, vignetting, and color fringing. The Exif tags for the lens properties written into the image are thus combined with the parameters stored in the database.

Geometry Conversion can clean up the typical pillow-shape, barrel-shape, and other distortions. This tool provides good results in particular for architectural photos, but in other cases as well. If a parameter is unknown , no conversion takes place. Apply the LensFun-based filters in the order of the tabs for the best results.

Even Rotation is similar to many RAW converters. You can rotate images in 90-degree increments losslessly or at any angle. If you select a horizontal or vertical line, Photivo correspondingly corrects the image automatically. Cropping is done with the same tool. As in GIMP, you select a region and confirm it by double-clicking. Again, you have the option of optimally positioning the cutout with help from guide lines.

Content-aware resizing, known as Seam carving , is among the least known of the tools. It allows you to rescale and change the aspect ratio horizontally or vertically of your image while preserving important features.

GIMP has a corresponding "Liquid Rescale" function that is considerably more powerful. The filter uses relatively little processing effort in that it's limited to predefined image regions (Figure 4).

Figure 4: The result of content-aware resizing depends on the parameters (Energy method) used.

Many of the RGB functions are also known to common image editors and RAW converters. Some of the lesser known functions are nonetheless interesting: Reinhard brightening provides an effective method of correcting darker image regions without making broad changes to surrounding ones.

Sigmoidal contrast lays out an S-shaped curve whose course controls contrast in a way corresponding to normal viewing habits. Partial exposure recovery allows modification of light, shadow, and mid-tones independently. Both Black and white conversion and the RGB curve tool control the global brightness of an image.

The Block setting deactivates the computationally intensive tools in the other tabs so that you can quickly test the effect of a basic function. Functions under Camera , Geometry , and Output meanwhile always remain active.

Lab

Much more exciting than the RGB-based tools are the ones that apply to Lab mode (Table 1). Photivo collects these tools under the following three tabs: Lab Color and Contrast , Lab Sharpen and Noise , and Lab Eyecandy .

You first determine with Lab transformation how Photivo is to create the L channel. By default, it follows a well-known formula [5] for transformation from the XYZ color space. Photivo also provides other variations.

The functions in the first tab are like the ones in RGB ; they perform the same tasks but work differently. Thus, Shadows/Lights allows settings depending on detail size, which can lead to almost HDR effects, such as the luminance [7] that the Mantiuk 06 operator creates.

The Lab Color and Contrast also provides various sharpening and denoising filters. You can easily compare the effects. The preset and impressively fast Wiener filter brings sharpness to the image by largely removing the random noise. However, many very bright images with strong contrast might better require the Edges only option to minimize artifacts.

The Lab Eyecandy tab provides functions, on the one hand, to tone images after the fact, such as Tone curves or Adaptive saturation . On the other hand, the Edge avoiding wavelets filter can compensate for vignetting. Value lets you enter positive values – that create a vignette – or negative values that compensate for them. You can make exact modifications with parameters such as Softness , Inner radius , and Roundness .

Lab Eyecandy also includes the especially important color curves for L*, a*, and b*. The L* curve functions like the Value channel in GIMP's curve tool but allows for only a single control point by default. You can add more with a right mouse click.

This feature allows you to apply the much more exciting a* and b* curves, which control the red/green and yellow/blue color distributions, respectively, in the image. Application requires some careful finger work. Often, all the curves are shifted vertically or horizontally (Figure 5).

Figure 5: Color corrections with the a* and b* curves are tricky. The integrated color patterns help move the curve in the right direction.

Moving the middle control point often creates decolorization where it should normally remain unchanged. Lab Eyecandy contains the most important Photivo functions and requires the most time to master. The Eyecandy tab also provides toning functions, this time based on RGB. Here, you'll find Cross processing , which can produce subtle as well as spectacular color effects.

Another variation of Edge avoiding wavelets is there together with functions for Smoothing and Softglow/Orton , which produces a magical glow. In some situations, Color intensity can produce good results, such as enhancing water sparkling.