Compression, when to do it and when not?

[Dynamic Office]

Hi Guys,

Quick question about the value of compressing images within indesign, I’d designed an A4 2-page flyer ready for print and decided to play around with some setting to see the outcome,

Usually, I leave the compression as default which is ‘bicubic downsampling’ to 300 – 450 ppi which give me a 2.33MB file, then I decided to create the same file but I used ‘Do not downsample’ and had got a 724MB file.

Now with the file that was not downsampled, you can zoom right into the small photo’s on the cover and will it retain the native pixels and with the compressed version once zoomed in you’ve obviously got less pixels.

From the actual viewing distance, you can’t see the difference unless you digitally zoom in, so I know from experience that ‘bicubic downsampling’ to 300 – 450 ppi is fine for printing.

My question is, when is it good practice to use the ‘Do not downsample’ option? On a much bigger print like a motorway banner? Or have I been outputting incorrectly all these years and should be using this? Just need some context!

Finally, compression > Jpeg? Or none? I create all my images as CYMK .tiffs and as mentioned I usually leave compression as default on all options which is jpeg but had never really noticed you could change it so which is the best option for print?

Many thanks for any advise, the picture below is for context.

[SJRiegel]

I don't know when Do Not Downsample would normally be used, but the only time I have had the occasion was when standard downsampling resulting in a error, and a failure to produce the PDF. (Too many images being downsized too much - more than the system memory could handle). In that case, I exported a non-downsampled PDF, and then downsampled from within Acrobat.

[rob day]

300ppi is really an entrenched rule-of-thumb that is good enough for most continuous tone images that will be output to a (relatively coarse) halftone dot pattern. But it really is a magic number, the only benefit in downsampling at export is smaller file size—there's no quality improvement because it's always a down sample.

Usually line art needs considerably more resolution depending on what kind of line you are trying to resolve, so line art that needs color could benefit from more resolution—something like a fine woodcut or engraving with color applied.

The default Maximum JPEG compression will produce some compression artifacts but nothing a printing press is capable of reproducing, or even anything you will see on screen.

I create all my images as CYMK .tiffs

Unless you are making post CMYK conversion color corrections, there's no reason to convert images to CMYK in PS because you can place RGB and get the same CMYK conversion at export. And there are advantages in not making the conversion because you can wait until export to decide on the correct CMYK destination profile.

[Dynamic Office]

Thanks for getting back to me,

Actually, that makes a lot of sense - but would a big image say 30,000 x 30,000 px benefit from that too? since it's scaled so large.

And yes, I've read on this forum funnily enough about not needing to convert images to CMYK prior, I guess it's just habit at this point as whenever i work with an image i will always create two, RGB & CMYK using image processor pro.

Read this if anyone is interested;

https://indesignsecrets.com/import-rgb-images-indesign-convert-cmyk-export.php 

[rob day]

Actually, that makes a lot of sense - but would a big image say 30,000 x 30,000 px benefit from that too? since it's scaled so large.

Whether the Downsample rule kicks in or not depends on the placed image's Effective Resolution and not its Actual resolution. The default is anything over 450ppi gets downsampled, so all that matters is how your 30000x30000 pixel image is scaled in the layout and not its pixel dimensions. If it is placed and scaled to 100x100 inches there would be no downsample because its Effective res would be 300ppi (30000/100). If you placed it at 50x50 inches there would be a downsample because at that output dimension the Effective res is 600ppi—more than the 450ppi threshold.

[Danny Whitehead.]

In line with what Rob wrote about line art, my most frequent use of my PDF setting that turns off compression and downsampling, is when I'm exporting a PDF for output that contains raster logos. It's usually business stationery and the like, so there's no photographic imagery I have to worry about bloating my file size, but it would be great if InDesign let you specify downsampling and compression for individual images.