BECASSO TUTORIAL 1


Black/White-to-Color Gradient


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.