What does 8 bit alpha (PNG) mean?
I am trying to follow specs for iTunes app graphic.
The requirements state:
72ppi, rgb,flattended, NO transparency High qualtiy PNG plus 8-bit alpha.
Everything I read about alpha is WITH transparency. How do I meet their specs?
In this instance the "alpha" is referring to a mask channel, not directly meaning transparency in the image itself depending upon the software being used.
What they are actually asking for is a PNG32 format. A 24-bit PNG + 8 bits for transparency. Apple is trying to be diplomatic and state things in the lowest common denominator rather than referring to one specific piece of image software.
Within Photoshop, transparency adds the alpha to a PNG file. You can't directly save an independent alpha channel in the PNG format.
Using Photoshop, simply create an RGB image with transparency, then use Save for Web to save the file as a PNG24. This will result in a 24-bit PNG with 8-bits of alpha (32bit PNG).