This blog entry is primarily aimed at game artists that work with Photoshop but it’s also useful for anyone working in sRGB and using alpha channels regularly!
It’s common in the games industry for artists to believe that a mid-grey alpha channel will equate to 50% opacity. If you fall into that category, let’s open Photoshop and do a sanity check. Create a new document, change your foreground colour to mid-grey (R128,G128,B128), switch to your document’s channels tab, create a new alpha channel and fill it.
Let’s examine this new alpha channel in a little more detail. Make sure your foreground colour is still mid-grey, select your new alpha channel and check out the information in your Color window. Why the hell is the black level showing 59%! It should be 50%, right? Hmm, let’s look at the Levels dialog just to make sure (Ctrl+L). The pixel information for the alpha channel clearly isn’t in the middle so there’s obviously a discrepancy between the RGB colour and alpha channel values.
Okay, it’s time to put you out of your misery! Without getting too technical, the reason for the difference is that Photoshop’s default colour profile is usually set to North American General Purpose 2. If you open up your Color Settings window from the Edit menu, you should see that Gray: is set to Dot Gain 20%, which is a greyscale profile primarily designed to work with CMYK/print.
Ideally, the greyscale profile should match the RGB working space profile which is sRGB. Luckily, it’s easy to create a new profile that’ll meet our requirements. Select More Options and then select the Gray: drop-down and choose sGray from the list. Save it as a custom profile and close the dialog.
As a final test, colour sample your mid-grey alpha channel with your new colour profile active and you should see that the RGB values are now R105,G105,B105! With this colour profile, a mid-grey colour of R128,G128,B128 really will equate to 50% opacity.
Granted, this isn’t the most exciting subject matter but it’s pretty fundamental to everything we do as digital artists. If you’ve found it informative and useful then please share the link. If you feel I’ve missed anything then please let me know! In the meantime, it’d be great if you’d check out our little debut game on the App Store!