Alpha 通道

Alpha 通道以黑色显示在右侧

Alpha 是出现在 32 位位图文件中的一类数据，用于向图像中的像素指定透明度。

24 位真彩文件包含三种颜色信息通道：红、绿和蓝或 RGB。每个通道在各个像素上都拥有具体的强度或值。每个通道的强度决定图像中像素的颜色。

通过添加第四种 alpha 通道，文件可以指定每个像素的透明度或不透明度。alpha 的值为 0 表示透明，alpha 的值为 255 则表示不透明，在此范围之间的值表示半透明。透明度对于合成操作是至关重要的，如在 Video Post 中，位于各个层中的几个图像要混合在一起。

对于所渲染图像中对象的锯齿边缘周围部分透明的像素，alpha 通道特别有用。这些像素用于合成。如果生成 alpha 通道，并且与图像一同保存，那么类似于上述形式的图像可以平滑地合成到不同的背景中。

真彩位图文件的每个通道都定义为 8 位，可以提供 256 个级别的强度。因此，一个 RGB 文件是 24 位的，具有 256 个级别（都带有红、绿和蓝）。RGBA 文件（红、绿、蓝、alpha）是 32 位的，具有额外的 8 位 alpha，可以提供 256 个级别的透明度。

3ds max 可以在渲染时自动创建 alpha 通道。所渲染图像中的背景像素是完全透明的，alpha 通道也可以说明通过材料等对象创建的任何其他透明度。通过单击工具栏上的“显示 Alpha 通道”按钮，可以在渲染帧窗口中看到这种情况：在生成的显示中，黑色像素是完全透明的，白色像素是不透明的，而灰色像素则显示透明度。要返回到常规显示，再次单击“显示 Alpha 通道”即可。

要输出带有 alpha 的渲染图像，请以 alpha 兼容的格式进行保存，如 TIFF 或 Targa。如果使用 Targa，默认设置将包含保存 alpha；如果使用 TIFF，请确保打开“存储 Alpha 通道”复选框。

All Axialis Software products support images with Alpha Channel transparency. PNG, BMP and PSD file format actually support alpha channel. If you want to create a nice icon (using Axialis IconWorkshop™) or a screensaver with smooth sprites (using Axialis Professional Screen Saver Producer) we recommend you to use images with Alpha channel.

Alpha channels are masks through which you can display images. The alpha channel is an 8-bit channel, which means it has 256 levels of gray from 0 (black) to 255 (white). White acts as the visible area; black acts as the transparent area (you see the background behind the image when displayed). The level of gray in between determines the level of visibility. For example, 50 percent gray allows for 50 percent visibility. Alpha channels are usually used with 16.8M color RGB images. The resulting image is called RGBA (RGB+A, A means alpha channel).

In the illustration below you see how the alpha channel is applied to the original image to remove (visually) the orange gradient background. The result produces a 32 BPP (Bit Per Pixel) transparent image. The grey checkboard-like pattern shows the transparency of the image.

We'll learn how to create a transparent image with Alpha Channel from a standard image. Photoshop® contains powerful Alpha Channel manipulation features. We'll see here one of the simplest way to proceed: Extrude a part of the image and remove the existing background. Advanced techniques also exist to work with alpha channel. See Photoshop help to read more.

Create a Transparent Image using Adobe Photoshop®

1. Open your image in Photoshop® (we use version 7.0 in this tutorial). Select File/Open... Choose your file and click Open. The file open in a separate document window. You can download the original image we use in this tutorial here: logo.bmp (right-click and choose Save Target As).

2. If the file is not in RGB format (16.8 M colors), convert it: Choose Image/Mode/RGB Color.

3. Now select the background area of the image (the area you wish to remove). If the image background has a solid color (white in our example), select the Magic Wand Tool (W) or click in the toolbox. Be sure to specify Tolerance=10, Anti-aliased and Contiguous in the selection options (dialog box on top, under the menu). You may need to adjust the tolerance value (it depends on your image). When done, click in the background area of the image ():

4. A selection dashed-line appears around the logo and around the square border of the image. It means that the background is selected (not the logo). If the background is not made of a solid color, you can select it by using the Polygonal Lasso Tool (L) or click . It's a bit more difficult but that's the only way to proceed.

5. Now, we'll select the logo only. To do so, choose Select/Inverse (Shft+Ctrl+I). It's almost unchanged but the dashed-line around the square border of the image has disappeared.

6. Choose Edit/Copy (Ctrl+C), then Edit/Paste (Ctrl+V) to create a new layer with the "extruded" logo in it. The "layers" list has been updated (see below). The new layer, which is selected, appears at the top of the list (). The original image is present in the background (). Now, we'll hide the original image by clicking on the "eye" button of the background layer (). If you want, you can also delete it by drag-and-drop it on the trash (at the right-bottom corner of the list).

5. The image document now looks like shown below. The background area (which was white) has disappeared. Now you see a gray-checkboard-like texture around the logo () indicating that this area is in fact transparent.

6. We recommend to save the image to another file to keep the original one intact. Select "File/Save As...". In Format box, select "PNG" (our favorite format). Please note that BMP and PSD formats also support 32 BPP / Alpha Channel. Click Save. Select Interlace: "None", click OK.

7. You're done, the transparent version of your image is created. You can download the image we've created in this tutorial here: logo-ps.png (right-click and choose Save Target As).