Alpha 是出現在 32 位位圖文件中的一類數據，用于向圖像中的像素指定透明度。
24 位真彩文件包含三種顏色信息通道：紅、綠和藍或 RGB。每個通道在各個像素上都擁有具體的強度或值。每個通道的強度決定圖像中像素的顏色。
通過添加第四種 alpha 通道，文件可以指定每個像素的透明度或不透明度。alpha 的值為 0 表示透明，alpha 的值為 255 則表示不透明，在此范圍之間的值表示半透明。透明度對于合成操作是至關重要的，如在 Video Post 中，位于各個層中的幾個圖像要混合在一起。
對于所渲染圖像中對象的鋸齒邊緣周圍部分透明的像素，alpha 通道特別有用。這些像素用于合成。如果生成 alpha 通道，并且與圖像一同保存，那么類似于上述形式的圖像可以平滑地合成到不同的背景中。
真彩位圖文件的每個通道都定義為 8 位，可以提供 256 個級別的強度。因此，一個 RGB 文件是 24 位的，具有 256 個級別（都帶有紅、綠和藍）。RGBA 文件（紅、綠、藍、alpha）是 32 位的，具有額外的 8 位 alpha，可以提供 256 個級別的透明度。
3ds max 可以在渲染時自動創建 alpha 通道。所渲染圖像中的背景像素是完全透明的，alpha 通道也可以說明通過材料等對象創建的任何其他透明度。通過單擊工具欄上的“顯示 Alpha 通道”按鈕，可以在渲染幀窗口中看到這種情況：在生成的顯示中，黑色像素是完全透明的，白色像素是不透明的，而灰色像素則顯示透明度。要返回到常規顯示，再次單擊“顯示 Alpha 通道”即可。
要輸出帶有 alpha 的渲染圖像，請以 alpha 兼容的格式進行保存，如 TIFF 或 Targa。如果使用 Targa，默認設置將包含保存 alpha；如果使用 TIFF，請確保打開“存儲 Alpha 通道”復選框。
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Create a Transparent Image with Adobe Photoshop®
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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).