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In Photoshop, when I use the eye dropper to read a value on an alpha channel on a RGB, 8 bit per channel file, the value is read in a 0%-100% scale, regardless it is, in reality, a 0-255 range grayscale image.

This scale is in both, the Inspector and the color panel.

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I can change the color window model to tone cube or brightness cube, but that is totally useless to read the values of a grayscale image...

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The RGB slider is blocked. (Yes guys, you will practice your Spanish language skills)

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Is there a way to read it on a 0-255 color range?

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Your info panel is showing grayscale and CMYK values. Photoshop shows 8-bit* grayscale, opacity and CMYK values in percentages.

If you switch it to show RGB values (hit the eyedropper icon), it will show you the correct 0–255 range 8-bit values... If the image is grayscale (or your viewing a channel; which is essentially the same thing) all the values will be the same, so that works just as well.

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Similarly with the color sliders, you can switch to the RGB sliders which will give you a 0–255 range (just set each slider to the same values again).


* Interestingly, Photoshop shows the correct 16 or 32-bit opacity and grayscale values, it's only 8-bit values that show as percentages...

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  • Oh. Changing to RGB on the left Info panel do the job, but I can not change the color slider, the RGB option is blocked :o( I will add that to the original question. – Rafael Sep 1 '17 at 2:16
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I'm not sure if it's wise to be measuring any pixel values of an RGB source in CMYK values, but somebody could correct me on that.

There doesn't appear to be any way to get 8-bit alpha values as a value between 0 and 255. I'm afraid you'll need to do some maths to get a value in a range that suits you. For 0-255 you could do:

PERCENTAGE * 255 / 100


Edit:

I'd originally answered this question mistaking that you were confusing the k value completely for an alpha value, rather than seeing you'd isolated your alpha channel and were taking independent samples from it. I've updated the answer to reflect this but thought a paragraph I removed may still be of use to some on this topic. Particularly if you want the alpha value of a pixel when you're not using an actual alpha channel:

You can get an alpha value in the same palette if you use the dropdown triangle in the info panel and change the mode to opacity in the panel options of any of the dropdowns.

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