Tonal depth is compromised when exporting PDF with 16-bit grayscale images

[icetype]

When I exported PDF files that contain 16-bit grayscale images, I found that the actual data are somehow compressed to 8 bit and have lost their original tonal depths, even though their apparent formats are kept as 16 bit; after PDF export, the upper 8 bit and the lower 8 bit of the images become identical as follows. This was reproduced in both macOS and Windows versions of Affinity Designer 1.10.5.

(771,771,771) #030303030303 grey1
(1285,1285,1285) #050505050505 grey2
(2056,2056,2056) #080808080808 grey3
(2570,2570,2570) #0A0A0A0A0A0A grey4

[NotMyFault]

This might be a misinterpretation, caused by how you measure the color values (a different bug), and another bug (Affinity unable to render 16 bit colors within the app)

• The Affinity color panel / studio does not show correct numbers in case of 16-bit values
• Can you please provide more evidence, and include screenshots / recordings of the measurement, and the actual test document?

There are methods to show the color values (lower 8 bit) correctly by transforming them into the higher 8 bit

1. use a procedural texture filter, and subtract the top 8 bit, multiply the remainder by 256
2. merge visible, export as 8-bit color, place 8-bit into file, set blend mode to subtract

converting to rgb/32 may help, too.

 

[icetype]

@NotMyFaultThank you for your suggestion! Attached please find the original 16-bit grayscale TIFF file, and three PDF files that were exported from Affinity Designer with three different document color modes. According to pdfimages, all the three PDF files contain 16 bit-formatted images. However, they seem to have only 8-bit tonal depth in practice, according to imagemagick. It might just be a limitation of the PDF format itself. Any ideas will be appreciated.

Gray16.pdf original image.tif RGB16.pdf RGB32.pdf

[NotMyFault]

I tried (on iPad) to export PDF, it always produced 8 bit images.

Edit:

I could not find any references if PDF supports more that 8 bit color depth. 

[User_783649]

@icetype Please, excuse my curiosity, but what is the purpose of making a PDF with 16bit grayscale image data?

[icetype]

@NotMyFault Thank you for your efforts! Indeed, it may just be a limitation of the PDF format itself. But, strangely, the images included in the PDF files exported from Affinity Designer are apparently formatted as 16 bit (although their internal data seem to be compressed to 8 bit).

@Alex M I use Affinity Design to make scientific figures, so it would be nice to be able to make a PDF with 16 bit color depth so the quantitative data of the images remain uncompromised.