Transparent images in PDF

Question

This PDF is composed of several source pieces. Five of them are PNGs containing an alpha channel. One is a PNG with no alpha channel. And the last piece is a Photoshop PDF with transparency effects. The pieces were assembled using ABCpdf 9.1.

On Acrobat 9, Foxit Reader, and Chrome's in-browser PDF viewer, these display correctly, with the Photoshop background showing through the transparent part of the alpha channel PNGs, and the non-alpha PNG showing through the center of the Photoshop image.

On Acrobat X or Acrobat XI, anything that is meant to be a transparent area is instead solid white. If Edit/Preferences/Page Display/Show transparency grid is checked, then the solid white becomes the gray checkerboard. But the background never shows through.

(Yes, these screenshots are of different PDFs, and both are different from the PDF linked above. But the transparency problem affects all of them.)

Does anyone know why it would be doing this? Did we construct it wrong?

Answer 1

The one page in your document has a Group entry defining the attributes of the page (transparency) group:

/Group <<
/I true
/K true
/S /Transparency
/Type /Group
>> 

The K entry is specified as

K boolean (Optional) A flag specifying whether the transparency group is a knockout group (see “Knockout Groups”). If this flag is false, later objects within the group shall be composited with earlier ones with which they overlap; if true, they shall be composited with the group’s initial backdrop and shall overwrite (“knock out”) any earlier overlapping objects. Default value: false.

As the value is true, any earlier overlapping objects (including the objects you want to be visible by transparency) are knocked out and you only see the white page background.

Using /K false instead renders the image as desired:

Your original document (/K true) in Adobe Reader XI:

Your document with the knockout entry disabled (/K false) in Adobe Reader XI:

For details cf. the PDF specification ISO 32000-1

• table 30 Entries in a page object for the page dictionary Group entry;
• section 11.4.7 Page Group for a definition of the page group;
• section 11.6.6 Transparency Group XObjects for the entries of the page group dictionary.