I've been using InDesign for a long time, always doing everything "the right way", that means images always in CMYK and never in PNG... but, I am working on a project involving around 400 images given to me in PNG with transparency. My question is simple, since this material is supposed to be press printed, would it be ok if I use the PNG images as links in InDesign or would you recommend re-saving all of them as TIFF and changing the color mode to CMYK?
I do a lot of print and in the past, I would've resaved everything as CMYK and to a degree that might not be a bad idea if only to get a more accurate representation of "color". CMYK tends to shift/mute the colors slightly when converting from RGB. In today's world, having to convert the colors isn't so much of an issue and usually the printer handles all that on their end. But your best best is to speak with your printer and ask them what best practices they prefer. Depending on their equipment and workflow the answers might vary. Hope this helps.
The same profile that would be used in Photoshop will be used in inDesing when converting to CMYK. The only difference would be that you see the result working on the file. When you convert on the fly while doing PDF you would may want the client to sign it or print cromalin from few pages.
As a rule of thumb I don't like working with PNG as it may give other people some mislead to expected colors. The usual "on my screen it was green, the colors were not so washed out" and so on. It's better, for me, to present from beginning how the images may look like.
Do you know how to create a Photoshop Action? If so, record an Action that (1) converts Mode to CMYK; (2) do not flatten the conversion; and (3) saves as a Photoshop Layered File. This should not take more than a few minutes to run an Automate > Batch of your new Action onto your entire folder of 400 PNG images.
Lastly, using the freeware "A Better Finder Rename" change all 400 files to a PNG extension instead of the existing PSD extension. This should just take a few minutes as well.
InDesign will automatically update the old PNG files with the new pseudo PNG files (which are really CMYK layered files). InDesign Link Manager only cares about the name of the file, not what the content is (RGB vs CMYK).
To test this, print color separations from InDesign and see how the newley linked PNGs work as CMYK.
Is this a good idea, Paola, or is this too many steps?
I, personally, think quality requires time. So I would individually convert each image to CMYK, color adjust as necessary (because it often is) then resave as .psd for use in InDesign. You don't need a flattened .tiff. Just save as a .psd.
Yes there are shortcuts. Yes you can just link to the PNGs in InDesign, provided the PPI of the PNGs is high enough*, and allow a PDF/X export to change the color. Remember the RGB gamut is much wider than the CMYK gamut so there's a high probability color shifts may occur. You'll have to closely check each and every image to ensure color is correct. Incorrect color means you have to go back to convert manually and color correct anyway. And if you use shortcuts, will you remember what those shortcuts are should the design needs updating a year from now and you need to "match" some aspect?
For me, by taking the time to do things "right" from the start, I feel it greatly reduces issues due to overlooking some error introduced by an automated process. How many times have you seen a typo in a piece because everyone "forgot" or "didn't have time" to proofread? Basically, you run that same rick for images. I would be wholly UNexcited at the prospect of adjusting 400 images individually. But time is money and there's no substitute for quality work. So, it's all billable and the final results will generally be better in the end.
*My experience is that the PPI of PNG files is customarily insufficient for print production. If the PPI were inline with resolution standards, the image is most often provided as either .raw, .jpg, .tif, or .psd, rarely is it .png