6 Replies Latest reply on Jul 20, 2016 3:33 PM by Abambo

    Alpha channel rendering question

    joet082964 Level 3

      Not sure if this is an InDesign or Photoshop question - maybe both.


      In this screen grab, I have the exact same PSD placed in INDD over top of a medium blue box - with high resolution display turned on.

      Image C uses the alpha channel and image B uses the Photoshop path. You can see how imperfect the alpha channel is in a couple of areas.

      This is the Photoshop image - for clarity, I added a bright blue background so you can see that the alpha channel separation actually is clean.


      In the end, I created the path to make it perfect, but why the discrepancy in INDD between the two?

        • 1. Re: Alpha channel rendering question
          George_Salnik Adobe Community Professional

          This is only preview. If you not have problems in exported PDF - never mind. Preview always is not equal to original.

          • 2. Re: Alpha channel rendering question
            John Mensinger Adobe Community Professional & MVP

            When you say "Alpha Channel, I assume you mean that you chose Object > Clipping Path > Alpha Channel; and that your image actually includes an alpha channel.


            With InDesign's support for native Photoshop transparency, there is no need to set a "Clipping Path" at all, so without having done extensive experimenting in this area, I'd theorize the method you used approximates a path based on the alpha channel, rather than producing a direct-render actually showing the "alpha". I suspect this "detection-based approximation" is the reason for your discrepancy.


            You'd be better off just Placing the .psd with a transparent background. A non-destructive way to do that would be (in Photoshop) using the "alpha channel" to make a selection, then apply a Layer Mask based on the selection.


            PLUS, GeorgeS' point above is a good one. The image you see in the InDesign editing environment is not your image. It is a JPEG proxy image generated by InDesign, and should not be the basis for judgements of quality.

            • 3. Re: Alpha channel rendering question
              joet082964 Level 3

              Thanks for your feedback.  Can you explain  "you used approximates a path based on the alpha channel,"?I don't know this option, but it sure seems to be what InDesign is doing.


              I did solve by placing the PSD and using my PS path, but I'm curious about this approximation thing?



              • 4. Re: Alpha channel rendering question
                John Mensinger Adobe Community Professional & MVP

                Well, as your screen shot demonstrates quite well, InDesign's Clipping Path feature will produce and apply a clipping path based on an alpha channel in the image, approximating its (the path's) location based on the black/white edges in the channel, factored by the threshold and tolerance settings imposed by the user. This makes it adjustable, movable, variable, approximated, potentially inexact.


                As I already mentioned, there is really no need to use this, or any, path feature at all. Just mask, hide, or delete unwanted background in Photoshop; leaving the area transparent, then save as .psd, and Place in InDesign.

                • 5. Re: Alpha channel rendering question
                  Bo LeBeau Level 4

                  Are you sure the Photoshop version that you showed us is the same file you placed in InDesign?


                  The reason I asked is; look at the gray end plate, in your Photoshop doc the upper end of this part appears to be thin as paper.

                  Both of the images in Indesign have this upper part as having the same width as the lower part.



                  • 6. Re: Alpha channel rendering question
                    Abambo Adobe Community Professional

                    Indesign will create a perfect PDF output based on your Photoshop transparency. No need to add a path either in Indesign or Photoshop.

                    If however the layer mask as shown is not perfect, rework it in Photoshop.