I found out a way to get the exact alpha value in Photoshop. The other solutions only get the closest integer percentage to the actual alpha.
Create a black background layer (#000000).
Duplicate the layer whose alpha you need, because we need to edit it. (Or if you need the composite alpha of several layers, duplicate them and merge them)
Hide the original layer, so that only the black background and the new duplicate is visible.
Click the new layer and then click "Lock transparent pixels" (The little checkerboard icon above the layers)
Fill the layer (Shift+Backspace) with White color.
Now you have a picture containing the alpha values of the original image, in greyscale. Just use the Color Picker to select any pixel in the image, and get the exact alpha value (for example #7f7f7f means an alpha of 0x7f, or 127).
use the PS info panel and set second color readout mode to opacity. turn off the visibility of all layers except the layer of the object you are analyzing (background should be transparent checkerboard) -info now gives original rgb value and correct opacity that can be converted into rgba.
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在 Photoshop 的信息面板中，您可以选择“不透明度”作为读出模式，尽管它会显示为百分比而不是真正的 Alpha 值。
要启用它，只需打开信息窗口，选择面板选项，然后将第二颜色读数模式设置为不透明度。
In Photoshop's Info panel, you can choose 'Opacity' as a readout mode, though it will show up as a percentage and not as a real alpha value.
To enable it, simply open the Info window, choose Panel Options and then set the Second Color Readout mode to Opacity.
使用 Gimp，您可以使用颜色选择器。
单击像素时按 shift。
（或在工具选项中勾选
使用信息窗口）单击某个像素后，
颜色选择器信息窗口将会弹出，向您显示包括 alpha 级别在内的详细信息：Using Gimp, you can use the Color picker.
Press shift while clicking on a pixel.
(or tick
Use info windowin the tool's options)Once you click on a pixel, the
Color Picker Information windowwill pop up, showing you details including the alpha level:我找到了一种在 Photoshop 中获取精确 alpha 值的方法。其他解决方案仅获得最接近实际 alpha 的整数百分比。
现在您有了一张包含原始图像的 alpha 值（灰度）的图片。只需使用颜色选择器选择图像中的任何像素，并获取准确的 alpha 值（例如#7f7f7f 表示 0x7f 或 127 的 alpha）。
I found out a way to get the exact alpha value in Photoshop. The other solutions only get the closest integer percentage to the actual alpha.
Now you have a picture containing the alpha values of the original image, in greyscale. Just use the Color Picker to select any pixel in the image, and get the exact alpha value (for example #7f7f7f means an alpha of 0x7f, or 127).
使用绝对颜色并在 Photoshop 中调整透明度，直到看起来正确，然后使用该百分比作为您的值。
use the absolute colour and adjust the transparency in photoshop until it looks right and then use that percentage as your value.
使用 PS 信息面板并将第二颜色读出模式设置为不透明度。关闭除正在分析的对象图层之外的所有图层的可见性（背景应该是透明棋盘） -info 现在提供原始 rgb 值和可以转换为 rgba 的正确不透明度。
use the PS info panel and set second color readout mode to opacity. turn off the visibility of all layers except the layer of the object you are analyzing (background should be transparent checkerboard) -info now gives original rgb value and correct opacity that can be converted into rgba.