Use a 2.5D LUT to quickly shift a set of image colors while keeping other colors unchanged.
A 2.5D LUT defines changes in a set of colors whose selection is based on their hue and saturation.
Changes keep pixel brightnesses invariant.
bLUe 2.5D LUTs can use the standard HSV color space, but better results are achieved with the HSpB space. It uses the so called perceptive brightness closer to human vision.
1 Load an image file and choose Layer New Adjustment Layer 2.5D LUT HSpB. The docked adjustment form opens in the right pane. It shows a color wheel. The centre of the wheel corresponds to saturation=0.
Sample the colors to be changed by a few clicks on the image. Selected (hue, sat) nodes are shown
as small dark circles on the color wheel.
When the option Select Neighbors is checked colors with neighbor hue and saturation are also selected to speed up the selection process.
To undo a selection check the option Remove Node and click again the corresponding image area.
Click on the color wheel and drag a rubber band around the nodes to select them.
Right click on any of the selected nodes and choose Group from the context menu.
The group is shown as a small white square.
Note. A group of nodes should correspond to "semantically similar" colors, for example skin, leaves, sky,...
- To save the LUT in format .cube, use the button Save LUT on the adjustment form.
- To load a .cube LUT use Layer Load 3D LUT from the main menu.
- Instead of applying changes to the entire image, you can draw a rectangular selection on the image with the marquee tool (left pane) and check the option Use Selection.