Publisher: What happens to RGB images placed in a CMYK document?

[user_0815]

What happens to a RGB image (sRGB) when I place and embed it into a Publisher document created as CMYK (ISO Coated v3 ECI)? Will it be converted to CMYK right away? Or will it remain RGB and conversion happens later at export?

[werfox]

Images with assigned/embedded ICC profiles such as sRGB remain so in your CMYK/8 document until you convert them to something different e.g. via AP or a third party program. You can check the profile / colour space with the Ressource manager in APub.

As for the export, It depends on the option you select:

• With "Convert image colour spaces" ticked: sRGB images get converted to CMYK with PSO Coated v3 (I assume you meant this profile name) profile assigned
• without the option above ticked: the images remain sRGB even if exported into a PDF-X/3 or PDF-X/4 (as those standards allow this)

Best regards

[lacerto]

One small addition to above: the placed (s)RGB images will stay in RGB color mode not just when exporting to PDF/X-3 or PDF/X-4 but whenever exporting with any other PDF method than PDF/X-1a, which specifically requires CMYK color mode, so unless you force conversion of image color space the placed RGB images will stay in RGB color mode.

Note though that when you e.g. paste raster images (including ones in RGB color space) via Clipboard, they will be converted to CMYK according to your document CMYK color space (you can check this using the Resource Manager), and that complex files, e.g. Affinity Photo files in RGB color space containing RGB defined vector objects (including text), or Pixel layers, will be converted to CMYK when exported, disregarding the value of "Convert Image Color Spaces" setting (as will all native objects like shapes and text objects defined in RGB color mode in your Affinity document, even if not necessitated by the export standard).

In addition, raster RGB images with adjustments applied will always be rasterized and therefore converted to CMYK when exported disregarding image color conversion setting), as will be ones with transparency (incl. blend modes) applied, whenever transparencies are not allowed in the export PDF method (in practice PDF/X-1a and PDF/X-3).

So it can become complex, and in practice you often need to have a utility that allows you examine the image color space in the exported PDF "item-wise". With some limitations, you can also open an exported PDF in an Affinity app and examine the image color spaces of exported objects.

[user_0815]

Thank you @werfox and @lacerto for this helpful explanation. Very interesting!