Starting
from the RGB model and the three channels which compose it, a fourth channel,
that defines transparency, was created. Its name is Alfa.
The images'
edition programs, and only in some formats - TGA, PSD, TIF... but not JPG for
example -, show the four channels to define an image:
R (red), G
(green), B (blue), Alfa (transparency)
How to
create an alfa channel?
An alfa
channel is automatically created when we add a new layer on an image. In case
the alfa channel is not created, you can add it opening the layers menu and
clicking on the option "Añadir canal alfa".
The option
"Añadir canal alfa" appears also in the contextual menu when you
click on the dialogue window "layers".
What kind
of format does this channel admit?
When you
save an image with an alfa channel, you should use a PNG format file. Other
formats might not save the information of the alfa channel which will result in
a loss of transparency.
TGA format
file also admits alfa transparency, but it is not as common as the PNG.
EXERCISE
For this
exercise, we will use a photograh with the following specifications:
- The background has to be clearly different from the person.
- Lights and shadows should not change very suddenly.
- Man or woman's face in the photo should not contain marked shadows or extreme reflexes.
If the
image does not fulfill this requirements it will be necessary a previous work
so that we can apply the Andy Warhol effect successfully.
First, we
will cut out the image leaving the part of our interest, in this example, the
face.
Next, with
the tool "Selección difusa" (magical wand) we will select the black
background to finally invert the chosen area (SELECCIONAR > INVERTIR) so now
the face is selected.
We cut the
selection and we paste it as a new layer. We delete the layer with the white
outline ("SELECCIONAR TODO > SUPRIMIR").
Next, we
transform the photo to the grey scale (COLORES > DESARTURAR).
Once we
have done this, we will duplicate the layer to preserve copy of the photograph
in the grey scale.
We
transform the duplicated layer into an image of black pure & white pure. To
do this, we click over the COLOR menu and we use the tool "UMBRAL".
This tool allows us to define a threshold from which every color turns into
white or black depending if it is on the right or the left hand of the
threshold chosen. We do several tests until we obtain the desired result.
Then we go
to the "SELECCIONAR" menu clicking on the option "POR
COLOR", and finally we click again over the black area of the image to
select all the black pixels.
From the
"EDITAR" menu, we select "CORTAR (CTRL+X)”, and we paste the
black silhouette on another layer following the next steps "EDITAR >
PEGAR COMO > CAPA NUEVA".
We should
have four layers:
BLACK: This
layer contains the dark details (hair, eyes, shadows...).
WHITE: The
layer has to be totally white.
BACKGROUND:
A solid color layer.
DESATURATION:
The grey scale layer that, from this moment, it is not useful for our practice.
Now what we
have to do is:
- Paint the background layer with a color.
- Paint the black area with another color.
- Paint the white area with a different color.
Following
all these steps, we will finally obtain our Andy Warhol effect.
Repeating
the last step with other colors, we can create new images quite easily to elaborate
our composition.
One trick:
If in the
layers palette you click the option "BLOQUEAR EL CANAL ALFA" of the
black and white layers, you can paint them by using the paint pot or the spray
directly without the need of selecting the black and white areas respectively.
Final note:
My regards to juanpedrobn who conceived this exercise.
