Các kênh Alpha Trong phần trước, chúng tôi thảo luận lưu trữ một lựa chọn trong một kênh bổ sung được gọi là một kênh alpha. Các kênh alpha mô tả thông tin bổ sung được sử dụng khi xử lý dữ liệu điểm ảnh thực tế. Các kênh alpha không cung cấp bất kỳ dữ liệu về các nội dung màu sắc của điểm ảnh một mà là làm thế nào để xử lý dữ liệu. | Part III Selections Layers and Channels FIGURE The selections created have been added as alpha channels in the Channels panel. On the Web Site The project file used in the example of this section can be found on this book s Web site as Figure . You can open it in Photoshop and play around creating selections using different color channels. The Alpha Channel In the previous section we discussed storing a selection in an additional channel called an alpha channel. The alpha channel describes additional information used when processing the actual pixel data. The alpha channel does not provide any data about the color content of a pixel but rather how to process the data. The main purpose of the alpha channel is to provide transparency information that can be applied to an image. As with the other channels each pixel in the alpha channel can have a value from 0 to 255 where 0 represents that the pixel is 100 percent transparent and 255 represents that the pixel is 100 percent opaque. Another useful feature of alpha channels is that you can use them to store and retrieve selections for later use. The alpha channels allow you to have quick access to previous selections and even share them between documents by dragging the alpha channel from one document to another. Creating alpha channels Alpha channels are created from selections. The simplest way to create an alpha channel is to make a selection and then save the selection as an alpha channel using these steps 1. Open an image in Photoshop. 2. Select Window C- Channels from the main menu to open the Channels panel. 356 Chapter 11 Channels 3. Select an area of the photo that you want to use for alpha channel data as shown in Figure . 4. Click the Save Selection as Alpha Channel button in the Channels panel to convert the selection to an alpha channel. 5. An alpha channel is created containing the current selection. FIGURE Selecting the sky in an image and converting it to an alpha channel .
