Grayscale image gets converted to RGB when saved with default PDF/X-1a settings

[kimmohalonen]

I must be missing something really simple as I can't figure out why a grayscale image gets converted to RGB when the page is saved as a PDF/X-1a PDF. I am using InDesign CC 2019 and the default PDF/X-1a settings that come with it.

On the left the original image and on the right the same image opened from inside the newly created PDF.

What am I doing wrong?

Thanks,

-kimmo

 

[rob day · Accepted solution]

The PDF/X-1a standard doesn’t allow RGB images—all process color gets converted to CMYK and grayscale images are put on the CMYK black plate. A conversion is happening when you open the image into Photoshop.

Rather than checking the image in Photoshop, use AcrobatPro’s Output Preview>Object Inspector, or Preflight to check the image’s color mode. Here I’m running a List Page Objects preflight, which shows the image is a grayscale:

[kimmohalonen]

Thanks Rob for explaining why it behaves the way it does and showing me a right way to check it!