Relight, a "digital fill-in flash" filter

This post an update about the work I am doing on shadows/highlights compression and tone mapping. It describes a new filter that works as a sort of "digital fill-in flash", that brightens the dark areas of an image while having a minor effect on the highlights.

Here is a preview of the filter in action (original image of the left, "relight" filter on the right with the corresponding tool dialog opened):

(image source: https://discuss.pixls.us/t/playraw-raw-challenge-backlit/1676)

New shadows/highlights and tone-mapping tools

This post is still work in progress!!!

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Introduction

This post describes the new shadows/highlights adjustment tool that I have introduced in PhotoFlow, and gives some implementation details in case someone would like to port the code to some other image editor.

The shadows/highlights adjustment can be split in the following steps:

1. the input pixels are converted from linear RGB to log grayscale, to create a log-luminance (LL) mask:
   LL = log10(lumi(RGB))
2. the LL mask is blurred with a guided filter pyramid (more on this below)
3. the blurred LL mask is tone-mapped with an analytical function that rises the shadows and lowers the highlights (more on this below); an anchor parameter allows to define the splitting point between shadows and highlights regions
4. the input RGB values are linearly scaled by the ratio of the blurred LL mask after/before the tone mapping

CSS image comparison slider test

Drag the black border ↔ pure Css

Credits:Original idea: solipsistaCP