Determine pixel alpha value in photoshop

Roman Nurik provides the easy answer on Stackoverflow.

Note that it's usually easier to select the layers you are interested in and copy them over to a new document with no background (right click on the layers in the layer tab and pick Duplicate Layers, then choose, New Document), then:

Simply:

1. Open the info window (click F8)
2. Click the down triangle in the top right of the window and choose Panel Options
3. In the dropdown for Second Color Readout choose "Opacity" for the "Mode"

4. Now pick the eyedropper and hover over the image. You will see the color value and alpha value as a percent

   • To convert the percent to actual alpha value multiply by 255/100
   • For example a transparency of 80% is 80 * 255 / 100 = 204 in alpha.
   • To convert 204 (a decimal number) into regular alpha format (a hexadecimal number) use this tool, which for 204 gives cc (cc is 12*16 + 12 = 204 since a=10, b=11, c=12...)
   • so red with 80% alpha in AARRGGBB format (like Android uses) would be #CCFF0000

Well until now I thought the color picker tool (keyboard shortcut "I") would tell the alpha value, but it does not. It seems you need to use a workaround:

Select the complete content of your layer you want to inspect by ctrl-clicking the miniature in the layers palette. Transparent areas will not be selected. Create a new alpha channel from your selection. Then, in the channels palette, select your new channel. Now, select all (ctrl-a) and invert the color values. If you now use the color picker, you get values from 0% to 100% wich reflect the transparency of your original layer.