6 Replies Latest reply on Feb 1, 2010 11:08 AM by jbriant

    Where is alpha channel?


      Just got CS4 for OSX. Got a PNG RGB with 8bit, and it clearly has alpha, but the channel view doesn't show it. The color picker says my semi-transparent pixel is 255,255,255 but no alpha value.


      How do I show the alpha channel???




      jamie the noob.

        • 1. Re: Where is alpha channel?
          p_d_f Level 2

          As far as I can tell, png allows transparency but it does not allow an actual alpha channel. I does appear that the file format does indeed retain that information. Go to the Select menu and down to Load Selection, where you can load the selection of the transparency. So it's there in the file, you can access it, but just not saved as an Alpha channel. Interesting.

          • 2. Re: Where is alpha channel?
            jbriant Level 1

            No, PNG supports RGBA. My file is RGBA. But it shows up in photoshop with 3 channels, R, G and B, and these magically have transparency.


            Here are some sample files: http://www.libpng.org/pub/png/png-rgba32.html


            I'd really like to see the alpha value in a separate channel. Is there a way to do that?

            • 3. Re: Where is alpha channel?
              David A Rogers Level 3

              PNG files do not support Alpha Channels but they do support transparency.  If you Command or Control click on the Layer thumbnail to load the selection you can then add an Alpha Channel in the Channels Panel. That will allow you to "see" the Alpha transparency for what its worth. It will not be saved with the PNG.  Alternatively you can bring up the Save for Web dialog and as you mouse over the image areas you will get an Alpha indication in the bottom of the window.

              • 4. Re: Where is alpha channel?
                jbriant Level 1

                Whats the difference between alpha channel and transparency? Here's the thing. A "PNG-24 with alpha" stores four bytes per pixel: R, G, B and A. CS4 seems happy to show the R, G and B bytes as separate channels. How do I get it to display the A as a channel?


                If I do as you suggest, i.e. select the layer and then add a channel, it adds another channel called "Alpha 1", but this isn't the alpha of the image: because if I modify this new alpha channel and then save the file it doesn't get the alpha from this new channel. It gets it from the magic hidden alpha channel.


                So I guess I mispoke. I don't just want to see the alpha channel, I want to be able to edit it. In isolation. Does Photoshop just not do this anymore???? I haven't used it since V6, so perhaps much has changed.



                • 5. Re: Where is alpha channel?
                  David A Rogers Level 3

                  A lot has changed since V6 no doubt, including the disappearance of Image Ready. I have not worked extensively with PNGs in the manner you describe, however, after a short trip to Fireworks it seems that you will find all your needs there with respect to editing the Aplha objects.


                  I will presume your purpose is to create/modify graphics for either web or Flash. I believe PNG is the native file format of Fireworks but  I will say, aside form it being on my machine as part of the Design Suite, I rarely open it.


                  It is available as a stand alone application for $300 or $150 upgrade.

                  There is a Trial Version available which may be worth investigating if no one else chimes in with a more accurate answer.

                  Id be very interested to find out if this is in fact the solution you are looking for.




                  In this image you can see the A value. The object was pink and then converted to Alpha with a transparency of 80%. So, the Alpha object and properties are editable.

                  When opening the file in Photoshop you will see the same as you have been. 3 channels with transparency but no additional channel.

                  So to answer your ultimate question. Photoshop does not do what it apparently did for you in Version 6.


                  Message was edited by: David A Rogers. Added image with explanation

                  1 person found this helpful
                  • 6. Re: Where is alpha channel?
                    jbriant Level 1

                    Well I have Fireworks, but it seems a PITA to have to boot another program just to do what I could do 10 years ago in photoshop. Guess I'll have to find/write an extention :-)


                    I'm not an artist, btw, I'm a programmer, so I'm often looking at raw numbers and wanting to see them in photoshop to figure out if the problem is the artwork or my code. I forked out for photoshop because it used to be exactly what I needed. Seems like now its been dumbed down. Lucky I bought the Web Premium version and got Fireworks too, but Adobe, what were you thinking???


                    Thanks for your help David - at least now I can see the numbers. That's a big help.