-By Serge Birault
First, why working with opacity ? Well, it's a good and simple way to paint very smooth gradients and blending colors. Of course it's not the only way but if you want to achieve very clean results, I think you have to try this...
So what is opacity ? You can adjust the opacity of your brushes and the opacity of your layers, it's the same in every softwares. If you decrease the opacity of your layer, it will become more and more transparent. If the oacity of your layer is equal to zeo, you will no longer see it .... Simple.
If you work with a tablet, the opacity will depend on the pressure of the stylus. It's quite impossible to work well with the opacity by this way. Try to NOT USE the stylus sensitivity. Yes, I know, it sounds like hell for a lot of peoples but belive me, it's the good way to proceed.
In order to emulate a traditional airbrush, I use the soft round brush most of the time. The soft round brush is a standard brush, you can find it in PS, Painter, The Gimp.
The center of the soft round brus is more opaque than the edges. If I use the soft round brush with 50% of opacity, the opacity of the center of the brush is gonna be equal to 50% and the opacity od the edge is gonna be about 0% (transparent). Simple .
So in oder to work properly, you have to keep in my the center of your brush has to be outside your object.
It could be difficult to do a good gradient with 100% on one stroke. It's easier to decrease the opacity of your brush and to do several strokes.
Choose the size of your brush wisely. On big objects, work with big brush not with a small one ...
With a low opacity, one stroke is not enough ...
With several strokes, it will be good.
If you want to work with opacity, you have to think a little bit differently than usual. Indeed, you cannot paint with a static palette. I mean, you have to think like "I have to add a little bit of red or a little bit of brown to this color". Think like a painter, not like a digital painter ... Don't forget, msot of the time you have to use more than two colors to achieve an interesting gradient. Work with several layers, it's gonna be easier ...
: Cartoon nose
In this picture, I already created 3 layers. One for the sketch (on the top), one with my basic skin tone and one for my soft round brush strokes. The opacity of the layer of my sketch decreased litle by little.
I started witth a dark brown with very low opacity, about 10 %.
I added the shadow with a less red brown than the first one (opacity about 10%).
Then I painted the brightest part of the nose with a pale yellow (opacity about 10%).
I added the reflective part of the nose with pure white.
A little bit of red to emulate the scattering surface. Work carefully with very satured tones, look at the opacity .... 1% !
The eyedropper is very useful if you want very smooth gradient. Pick the color and do small strokes with very low opacity.
You can work with the opacity of the layers too. If you create several layers for each tones for example, you can adjust all the pat very quickly.
For example, If you want to add a little bit of texture on the skin (and a little bit of red in the same time), you can start with 100 % opacity brush ...
Then decrease the opacity of the layer. Sometimes, it's quicker like this ...
Keep in mind you can work in the same way with the eraser. Depending on the shapes you want to erase, using the soft round brush with opacity with the eraser could be a good solution.
I know my method could seem strange but I use it for ... a very long time (*cough*) and I think it's a good way to achieve very smooth results.
If you want to see the way I use to work, take a look on this:
As you can see, I spend my time changing the size and the opacity of my brush ... and I don't know any shortcuts :)