Alpha Over, Under & Over Drop

Utilisation de l’alpha (canal de transparence), cet effet compose un résultat sur les zones transparentes de l’image dominante. Si vous utilisez une bande Scene, les zones de l’image où il n’y a rien de solide sont transparentes ; elles ont une valeur alpha de 0. Si vous avez une bande de film, ce film a une valeur alpha de 1 (totalement opaque).

So, you can use the Alpha Over / Alpha Under effect to composite the CGI Scene on top of your movie. The result is your model doing whatever as if it was part of the movie. The Adjust ‣ Compositing ‣ Opacity controls how much the foreground is mixed over the background, fading in the foreground on top of the background. The colors of transparent foreground image areas are ignored and do not change the color of the background.

Alpha Over

Avec Alpha Over

Avertissement

By clicking the Premultiply Alpha button in the Sidebar of the foreground strip, the alpha values of the two strips are not multiplied or added together. Use this effect when adding a foreground strip that has a variable alpha channel (some opaque areas, some transparent, some in between) over a strip that has a flat opaque (alpha=1.0 or greater) channel. If you notice a glow around your foreground objects, or strange transparent areas of your foreground object when using Alpha Over, enable Premultiply.

Example

../../../../_images/sequencer_sequencer_strips_effects_alpha-over-under-overdrop_example.png

Effet Alpha Over.

Alpha Under

Avec Alpha Under

Over Drop

Over Drop is between the two others: as with Alpha Under, the first selected strip will be the foreground, but as with Alpha Over, the Opacity controls the transparency of this foreground.

The Over Drop effect is much like the Cross, but puts preference to the top or second image, giving more of a gradual overlay effect than a blend like the Cross does. Of course, all of the Alpha effects respect the alpha (transparency) channel, whereas Cross does not.

The degree of Alpha applied, and thus color mixing, can be controlled by an F-curve. Creating a Sine wave could have the effect of the foreground fading in and out.