This tutorial shows how to use the powerful Selection-to-Channel operations Becasso 1.1 offers, to yield an image that has a black-and-white background but with a smooth transition to a colorful region.

Note that this is an old tutorial, and the same effect can be achieved much easier now in Becasso 1.5 using the DuoTone add-on and a graded selection map. The tutorial is retained here for reference, and because it explains about pixel intensities and the "multiply" channel operation.

Step 1

Let's open a colorful image in Becasso. The sports car used in this example is mainly red. Simply replacing the saturation channel of the image with a gradient would throw away any saturation information already there, so we will first save this information by choosing Edit > Channel to Selection > Saturation. This copies the saturation channel into the selection map.

Step 2

Now, we add a second layer to the image. We want to 'extract' the selected saturation information into a channel of the newly created layer. Let's take the green channel as this is nice and contrasty. Selecting Edit > Selection to Channel > Green yields no visible result however, since the alpha channel of the new layer is still zero everywhere (i.e. the image is there, only completely transparent). To visualize what we are doing, do an Edit > Select All (Alt-A) and an Edit > Selection to Channel > Alpha to make the layer fully opaque. There's our saturation channel!

Step 3

Now, we add a third layer containing our actual gradient, using the Gradient Generator add-on. In this case, 'white' will correspond to maximum color later, and 'black' to 'black-and-white'.

We will now have to 'combine' the gradient and the original saturation information. This is simply done by setting the gradient layer channel operation to 'Multiply', and merging the top two layers. This works because multiplying with black yields zero, and multiplying with white yields the original information.

This results in a combined gradient ranging from the full original saturation information in the top left, to no saturation outwards.

Step 4

We will now copy this combined saturation information over the old saturation channel of the original image. To do this, we use Edit > Channel to Selection > Green. The selection map will then contain a simple copy of the saturation information in the layer we used as a 'scratch pad' for our multiplication.

Now we select the layer containing the original image. Selecting Edit > Selection to Channel > Saturation replaces the original saturation information with our new gradiented version, yielding the desired result.