16. 对 Alpha 应用 Logo
| 第 17 章 虑镜 |

## 16. 对 Alpha 应用 Logo

### 16.1. Introduction

图 17.369. The Alpha to Logo filters menu

These filters correspond to the logo-generating script-fu scripts. They add all kinds of special effects to the alpha channel of the active layer (that is, to the pixels with a non-zero alpha value).

| 注意
The menu items and the corresponding functions are enabled only if the active layer has an alpha channel. If you see that the menu items are grayed out, try to add an alpha channel.

The filter effect will always be applied according to the alpha values. The alpha of any pixel has a value ranging from 0 (transparent) to 255 (fully opaque). It is possible to apply a filter only partially to some (or all) pixels by using alpha values from 1 to 254.

You will notice that this is similar to selecting pixels partially. In fact, internally these filters always create a selection from the alpha channel by transferring the alpha values to the channel which represents the selection, and then work on the selection.

How to apply an “Alpha to Logo” filter to a selection?

1. If the active layer is the background layer, make sure that an alpha channel exists, otherwise add an alpha channel.

   | 提示
   If a layer name in the Layer Dialog is in bold, then this layer has no Alpha channel.

2. Invert the selection: Select → Invert.

3. Remove the (inverted) selection: Edit → Clear.

4. Apply the “Alpha to Logo” filter (the filters ignore the selection, you don't need to re-invert the selection).

15.8. Playback |  | 16.2. 3D Outline

Report an error in Bugzilla