The Alpha Over node is used to layer images on top of one another. Where the foreground image pixels have an alpha greater than 0, it will be overlaid over the background image.
控制前景图像的透明程度。小于 1 的参数将使前景图像变得更加透明。
Converts foreground image to Premultiplied Alpha format.
The Alpha Over node is designed to work with premultiplied alpha color format. Use this checkbox when you know that your image has Straight Alpha color values, to perform the correct over operation. Result will still be premultiplied alpha.
When set to 1, the foreground color values will be multiplied by alpha, i.e. premultiplied; this is equivalent to enabling Convert Premultiplied. When set to 0, color values does not change.
If Premultiply is not zero, Convert Premultiplied will be ignored.
在下图的节点树层次中，颜色渐变 节点被用于给黑白漩涡图像添加alpha通道。Alpha 上叠 节点被用于将其叠加在另一个图像之上。
In the next example, the Factor is used to make a "Fade In" effect. This effect can be animated by adding a Time node inputted in the Factor socket as shown below. Over the course of 30 frames, the Alpha Over node outputs an image that starts with the pure background image, and the title slowly appearing.
Note the Convert Premultiply checkbox is enabled, since as the foreground used a PNG image that has straight alpha.