Alpha在上,Alpha在下&向下覆盖

使用alpha（透明度通道），该效果将基于主图像的透明区域的结果进行合成。如果使用场景片段，图像中没有任何实体的区域是透明的;它们的alpha值为0.如果使用电影片段，该电影的alpha值为1（完全不透明）。

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 在上

使用 Alpha在上 时，片段按所选顺序分层; 所选的第一个片段是背景，第二个片段则在选择的第一个片段之 上。 不透明度 控制 前景 的透明度，即 不透明度 为0.0; 只会显示背景，而1.0的 不透明度 将前景将完全覆盖背景，（当然除了这个是透明区域情况！）

Warning

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

Alpha在上效果。

Alpha 在下

使用 Alpha在下 时，这是相反的：所选的第一个片段是前景，第二个片段是背景。此外，不透明度 控制 背景 的透明度，即0.0 的 不透明度 将仅显示前景（背景是完全透明的），1.0的 不透明度 将给出与 Alpha在上 相同的结果。

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.