TOOLS & TIPS (Chroma from Channels)

Chroma from Channels

Color information of each type are stored in as separate channels, which can be viewed in the Channels Tab. We can use the channel wise variations in color intensities for selecting objects in the image. This process is helpful only when background and foreground have separate color distribution in at least one channel.

Advanced Chroma

In the Histogram we can see that the blue channel is predominant in the image and it extends up to the highlights where other channels are very weak. Our subject here is the patch of vegetation in foreground.

Picking the Best Channel

A simple observation of the three channels tell us that the blue channel has the highest contrast between the Sky and Grass. We are going to exploit this property to separate the two. Duplicate (Command + J) the Layer and go to the Blue channel of the Copied layer. Click Image > Adjustments > Curves or Press Command + M to apply curves modification on the current channel.

Applying Curves

Select the point closer to the dark grays of the channel and drag it (2) forward in the x-axis (horizontally) keeping y-axis at 0 (shown by the small circle). This will make the darker pixels uniformly more darker. It shouldn't affect the background, drag it a little back to left if it does.

Next, select another point closer to the White Point, drag it (3) back towards the midtones keeping the y-axis(vertical) at maximum (shown by the small circle). What we are doing is saturating the dragged range of blue up to 100%. Make sure the whole background turns white or almost so, and there is a very high contrast between the Grass and Sky.

Channel as Mask

Command-click on the channel thumbnail to load the channel into the selection. Areas which are predominantly blue will be selected totally and the rest according to respective degrees. Areas which have no blue (the black parts) will be left from the selection. With the selection in place go to the Layers tab and click Add Layer Mask to save the selection as a Mask.

Inverting the Mask

Press Command + I to invert the Layer Mask.